Account of the Skerryvore lighthouse

Transcriber’s Notes

Text transcribed between _underscores_ represents text printed in italics in the source doument. Small capitals have been replaced with ALL CAPITALS. ^{text} and _{text} represent superscript and subscript text resepecively. Text between ~tildes~ represents marginalia.

[Illustration: SKERRYVORE LIGHTHOUSE.]

ACCOUNT OF THE SKERRYVORE LIGHTHOUSE, WITH NOTES ON THE ILLUMINATION OF LIGHTHOUSES;

By

ALAN STEVENSON, LL.B., F.R.S.E., M.I.C.E., ENGINEER TO THE NORTHERN LIGHTHOUSE BOARD.

[Illustration: ·NORTHERN LIGHTHOUSES·

In Salutem Omnium.]

“ΥΠΕΡ · ΤΩΝ · ΠΛΩΙΖΟΜΕΝΩΝ” _Inscription on the Ancient Pharos of Alexandria._

By Order Of The Commissioners Of Northern Lighthouses.

Adam And Charles Black, North Bridge, Edinburgh. Longman And Co., London.

MDCCCXLVIII.

Printed By Neill And Company, Edinburgh.

Preface.

I am unwilling to dismiss the following pages from my hands without saying a few words in extenuation of the defects which they contain. My chief plea in defence is, that the preparation of this _Account of the Skerryvore Lighthouse_, and the _Notes on the Illumination of Lighthouses_ which follow it, was not chosen or assumed by me, but was a task imposed by the express desire of the Lighthouse Board, to whose enlightened and liberal views the Mariner owes the erection of the Lighthouse itself. My labours were also continually interrupted by the urgent calls of my official duties; and, on several occasions, I was forced to dismiss unfinished chapters from my mind for a period of several months--circumstances which, I hope, will in some measure account for the desultory character of the performance, the disproportion of some of its parts, and more especially for repetitions and perhaps omissions which would otherwise have been quite unpardonable.

Having said thus much by way of apology for this Volume, I must acknowledge my many and great obligations to my Father who preceded me as Engineer to the Board of Northern Lighthouses, and of whose experience, as the Architect of no fewer than twenty-five Lighthouses, including that of the Bell Rock, I had the full benefit during the erection of the Skerryvore Lighthouse. To the generosity of my esteemed friend, M. LEONOR FRESNEL, I owe all that I know of the Dioptric System of Illumination, invented by his late illustrious Brother; but this general acknowledgment will not supersede the necessity of frequent repetitions of my obligations to him, as occasion offers, in the course of these pages. I have also derived much assistance, as a careful reader will easily trace, from the valuable little work of M. PECLET, entitled _Traité de l’Eclairage_. There are, besides, many other obligations, which I cannot attempt to acknowledge individually, but which those, who kindly conferred them, well know how much I value.

* * * * *

In the _Account of the Skerryvore Lighthouse_, which forms the first part of this Volume, there is an important omission; and, in this short prefatory notice, I gladly embrace the only opportunity, which now remains, of supplying the defect. Although, in the course of the Narrative, I have occasionally noticed some special deliverances from danger, I have altogether neglected to record the remarkable fact, that, amidst our almost daily perils, during six seasons on the Skerryvore Rock, there was no loss of either life or limb amongst us. Those who best know the nature of the service in which we were engaged,--the daily jeopardy connected with landing weighty materials in a heavy surf and transporting the workmen in boats through a boisterous sea, the risks to so many men, involved in mining the foundations of the Tower in a space so limited, and above all, the destruction, in a single night by the violence of the waves, of our temporary barrack on the Rock, which had cost the toils of a whole season, will not wonder that I am anxious to express, what I know to have been a general feeling amongst those engaged in the work--that of heartfelt thankfulness to ALMIGHTY GOD for merciful preservation in danger, and for the final success which terminated our arduous and protracted labours.

EDINBURGH, _March 25, 1848_.

Contents.

Part I.

Account Of Skerryvore Lighthouse.

Page

Introduction -- Constitution of the Lighthouse Board -- Lights established since 1821 -- Improvements in the mode of illumination -- Dioptric Lights -- Beacons and Buoys, 9

Chapter I.

Topographic notice of the Skerryvore Rock, 19

Chapter II.

Preliminary arrangements and works, including survey of the rocks, and opening of quarries from 1834 to 1837 -- Survey of the Skerryvore rocks -- Disadvantages of Tyree -- Pier and workyard at Hynish, Tyree -- Quarries at Hynish -- Skerryvore Committee appointed, 37

Chapter III.

On the construction of Lighthouse Towers, 45

Chapter IV.

Operations Of 1838.

Temporary Barrack on Rock -- Tools and machinery -- Steam Tender for the works -- Employment and wages of workmen -- Progress of the outfit for the season’s operations -- Embark for Skerryvore -- Lay down moorings, and try to land on the Rock -- Driven to Mull -- First day’s work on the Rock -- Shipment of materials at Glasgow and Greenock -- Reach Tyree -- Driven to Mull -- Return to Tyree -- First good day’s work on the Rock -- Sudden gale and great peril to the vessels -- Reach Hynish in safety -- Detained by bad weather four days at Hynish -- Return to the Rock and have six days of good weather -- Erection of the pyramid of the wooden barrack -- Mode of determining the length of the beams, and the sites for their fixtures -- Pyramid completed -- Mode of living while erecting the barrack -- Shoals of Medusæ seen -- Driven by a gale to Mull -- Return to Hynish and are driven to Coll -- Return to the Rock -- Driven to Tyree -- Return to the Rock -- Horizontal braces fixed -- Driven to Mull -- Heavy gale -- Timber cast on Tyree -- Return to Rock and further progress of barrack -- Last day’s work on Rock this season -- Precaution for the benefit of shipwrecked seamen -- View from top of pyramid -- Destruction of the barrack during a gale -- Letter from Mr Hogben -- Proceed to Skerryvore -- State in which the Rock was found -- Cause of the destruction of the barrack -- Preparations for a new barrack -- Works at Hynish -- Hynish quarries, 71

Chapter V.

Operations Of 1839.

Shipping station and pier at Hynish -- Granite quarries in Mull -- Observations on the quarrying of granite -- Dressing of the Lighthouse blocks -- Excavation of foundation for the Lighthouse Tower on the Skerryvore Rock -- Fitting up of the second barrack on the Rock -- Sudden death of George Middlemiss -- Wharf and landing-place on the Rock -- Ring-bolts, water-tanks, and railways -- Incidents of the season -- Effects of a gale from the south-west -- Mutiny of the crew -- Near approach of the vessels to the Rock, and other circumstances shewing the importance of a Light on the Skerryvore, 107

Chapter VI.

Operations Of 1840.

Hynish workyard -- Hynish pier -- The Rock -- Life in the barrack -- Foundation-pit -- Landing of the materials on the Rock -- Laying the first stone, 140

Chapter VII.

Operations Of 1841.

Hynish workyard -- The Rock -- The waves -- Colours of the breaking waves -- The seals, 151

Chapter VIII.

Operations Of 1842.

State of the Rock in Spring of 1842 -- Commencement of Rock operations -- Last stone -- The Lantern, 163

Chapter IX.

Concluding Operations And Exhibition Of The Light.

Harbour works -- Bo Pheg beacon -- Light-keepers’ and seamen’s houses -- Concluding works on the Rock, such as pointing, &c. -- Interior fittings of the Tower -- Light-room apparatus, and first exhibition of the Light -- Removal of the barrack from the Rock -- Expense, 169

Part II.

NOTES ON THE ILLUMINATION OF LIGHTHOUSES, WITH SHORT NOTICES OF THEIR HISTORY, 181.

Early history, 181 -- Colossus of Rhodes, 182 -- Pharos of Alexandria, 183 -- Coruna Tower, 187 -- Lighthouse at the mouth of the Quadalquivir, 188 -- Ancient Phari in Britain, 188 -- Tour de Corduan, 189 -- Eddystone, 189 -- Bell Rock, 192 -- Carlingford, 194 -- Iron lighthouses, 194 -- Early modes of illumination, 195 -- Flame, 196 -- Drummond and Voltaic Lights, 199 -- Mr Gurney’s Lamp, 200 -- Argand Burners, 200.

Catoptric System Of Lights, 204.

Application of Paraboloidal Mirrors, 205 -- Reflection, 207 -- Paraboloidal Mirrors, 209 -- Divergence of Paraboloidal Mirrors, 212 -- Effect of Paraboloidal Mirrors, 217 -- Power of ditto, 218 -- Manufacture of reflectors, 218 -- Testing of mirrors, 219 -- Argand Lamps used in reflectors, 220 -- Arrangements for raising or lowering the Argand wick, 222 -- Flowing of the lamp, 223 -- Placing the lamp in the focus, 226 -- Distinctions of Catoptric Lights, 227 -- Colour as a distinction for lights, 229 -- Arrangement of reflectors on the frame, 230 -- Bordier Marcet’s reflectors, 232 -- Fanal sidéral, 232 -- Fanal à double effet, 234 -- Fanal à double face, 236 -- Mr Barlow’s spherical mirrors, 237 -- Captain Smith’s mirrors in the form of a parabolic spindle, 238.

Dioptric System Of Lights, 239.

Early history of Lighthouse lenses, Condorcet, Buffon, Brewster, Fresnel, 239 -- Refraction, 242 -- Lenses, 245 -- Spherical aberration, 248 -- Fresnel’s formulæ for annular lenses, 249 -- Testing lenses, 255 -- Divergence of lenses, 256 -- Illuminating power of lenses, 257 -- Arrangement of lenses in a Lighthouse, 258 -- Pyramidal lenses and mirrors, 259 -- Curved mirrors, 260 -- Cylindric refractors of fixed Lights, 263 -- Application of crossed prisms to cause flashes, 264 -- True cylindric form given to refractors, and other improvements in their construction, 264 -- Catadioptric zones, the mode of computing their elements, &c., 267 -- Testing of zones, 282 -- Framing of zones, 286 -- Mechanical lamp, 286 -- Height of flame of mechanical lamp, 289 -- Position of flame in reference to focus, 290 -- Working of the pumps, 291 -- Choice of focal point for various parts of apparatus, 292 -- Choice of a focal point for zones, 292 -- Application of spherical mirrors to fixed Dioptric Lights, 293 -- Arrangement of Dioptric apparatus, 293 -- Arrangement of Dioptric apparatus in Lightroom, 294 -- Power of Dioptric instruments, 298 -- Orders of French Lights, 298 -- Distinctions of Dioptric Lights, 299 -- Comparison of Dioptric and Catoptric apparatus for revolving lights, 301 -- Comparison for fixed lights, 303 -- Summary of views as to two systems for revolving lights, 306 -- Summary of views as to two systems for fixed lights, 306 -- Advantages and disadvantages of both systems under certain circumstances, 308 -- Distinctions of the Dioptric Lights and the application of coloured media, 311 -- Captain Basil Hall’s proposal for fixed lights, 313 -- Effects of rapid motion on the power of lights, 315 -- Connection of experiments with irradiation, 320.

Various General Considerations Connected With Lighthouses, 320.

Masking Lights, 320 -- Double Lights, 322 -- Leading Lights, 323 -- Distribution of Lights on a coast, 325 -- Height of Lighthouse Tower, and its relation to range of light, 328 -- Diagonal Lantern, 330 -- Glazing of Lantern, 331 -- Ventilation of Lanterns, 331 -- Arrangements and Management of a Lighthouse, 334 -- Cleansing of apparatus, 335 -- Mode of measuring relative power of lights, 336 -- More accurate comparison of intensity of lights, 338 -- Floating Lights, 346 -- Beacons and Buoys, 347.

Appendix.

1. Table of Co-ordinates of Hyperbolic Column.

2. Notes on the Making of Paraboloïdal Mirrors.

3. Notes on the Grinding and Polishing of Dioptric Instruments for Lighthouses.

4. Table of the Elements of the Catadioptric Zones for Lights of the first order in the system of Augustin Fresnel.

5. Notice to Mariners of the Exhibition of the Skerryvore Light.

6. Detailed Account of the Expense of the Skerryvore Lighthouse.

7. Excerpts from Account of Experiments on the Force of the Waves of the Atlantic and German Oceans, by Thomas Stevenson, F.R.S.E., Civil Engineer.

8. Annual List for 1848 of Lighthouses, Beacons, and Buoys, in the District of the Northern Lights Board.

9. Annual Report by the Secretary as to the Income and Expenditure of the Northern Lights Board for 1846.

10. Instructions to the Light-keepers in the Service of the Commissioners of the Northern Lighthouses.

Plates.

I. Chart shewing the situation of the Skerryvore Lighthouse.

II. Chart shewing the position of the Skerryvore Rocks and foul ground.

III. Plans of the Skerryvore Rock at high and low water of spring tides.

IV. Curves for Lighthouse Towers -- Marine Dynamometer.

V. Barrack of timber on the Skerryvore Rock.

VI. Details of fixtures of timbers at the top of the Timber Barrack.

VII. Elevation of Skerryvore Lighthouse.

VIII. Section of Skerryvore Lighthouse.

IX. Balance Crane used at Skerryvore.

X. Plan shewing the Lighthouse Establishment of Barracks, Harbour, &c., at Hynish, in the Island of Tyree.

XI. Elevation and Section entrance to Dock at Hynish.

XII. Plan and Section of Annular Lens.

XIII. Perspective Elevation of Revolving Dioptric Apparatus of First Order.

XIV. Plan of Revolving Dioptric Light of First Order, with Mirrors.

XV. Vertical Section of fixed Dioptric Lights of First Order, with Mirrors.

XVI. Elements of Concave Mirrors for Dioptric Lights of First Order, and arrangement on the Frame.

XVII. Perspective View of Fixed Dioptric Light of First Order, with Catadioptric Zones.

XVIII. Vertical Section of Fixed Dioptric Light of First Order, with Catadioptric Zones.

XIX. Vertical Section of Catadioptric Light of Fourth Order, with Framing.

XX. Elevation of Mechanical Lamp for Dioptric Lights of First Order.

XXI. Enlarged Views of Oil-Pumps of Mechanical Lamp.

XXII. Enlarged Views of Oil-Pumps of Mechanical Lamp.

XXIII. Details of Clock-work of Mechanical Lamp.

XXIV. Details of Clock-work of Mechanical Lamp.

XXV. Flame of Mechanical Lamp of First Order at full size.

XXVI. Elevation of Diagonal Lantern, and details of Astragals.

XXVII. Elevation of Ardnamurchan Lighthouse.

XXVIII. Plan of Ardnamurchan Lighthouse.

XXIX. Lines of a Floating Light Vessel belonging to the Corporation of Trinity House of Deptford Strond.

XXX. Elevation of Covesea Skerries Beacon.

XXXI. Details of Covesea Skerries Beacon.

XXXII. Elevations and Sections of Stone and Iron Beacons.

XXXIII. Elevations of Buoys.

Errata.

Page 52, line 8, _for_ Redelet _read_ Rondelet ... 178, line 6, _for_ L.93,306:8:10 _read_ L.86,977:17:7 ... 292, line 30, _for_ radius rector _read_ radius vector ... 294, line 13, _for_ give _read_ gives ... 329, line 3, _for_ earth _read_ sea ... 347, line 29, _for_ Plate XXXII., _read_ Plate XXX.,

Part First.

Account Of The Skerryvore Lighthouse.

Introduction.

In the course of preparing the account of the building of the Skerryvore Lighthouse, it occurred to me, that a short Introduction should be prefixed, embracing a concise view of the constitution and acts of the Board of Commissioners of Northern Lights, more especially from 1824, when my Father’s work on the Bell Rock Lighthouse was published, up to the present time. This object will be best accomplished, by presenting to the reader, in the first place, an account of the constitution and powers of the Lighthouse Board, chiefly drawn from the “Introduction to the Bye-Laws, Rules, and Regulations of the Commissioners of Northern Lighthouses,” prepared by a Committee of their number; and by afterwards briefly noticing the principal works of the Board since 1824, and stating generally the nature of the changes and improvements made within that period on the mode of illumination, of which I propose, in a subsequent part of this volume, to give a somewhat detailed account.

* * * * *

~Constitution of the Lighthouse Board.~

The trade of Scotland had begun to increase very soon after the settlement of the civil war in 1745; but it was not till the year 1784 that the general establishment of Sea Lights upon the Coast appears to have been brought under the notice of the Legislature. In that year, the subject was first mentioned at a meeting of the Convention of the Royal Burghs of Scotland, by Mr DEMPSTER of Dunichen, M.P., the Provost of the burgh of Forfar; and, in the year 1786, that gentleman brought a bill into Parliament, and an Act was obtained establishing the present Board of Northern Lights.

This Act sets forth, that “it would conduce greatly to the security of navigation and the fisheries, if four Lighthouses were erected in the Northern parts of Great Britain, one on Kinnaird’s Head, Aberdeenshire, one in the North Isles of Orkney, one on the point of Scalpa, in the Island of Harris, and a fourth on the Mull of Kintyre, Argyllshire;” and it accordingly authorises the erection of those four Lighthouses. The Commissioners appointed for carrying this Act into execution were, the Lord Advocate and Solicitor-General of Scotland, the Lord Provost and first Bailie of Edinburgh, the Lord Provost and first Bailie of Glasgow, the Provosts of Aberdeen, Inverness, and Campbeltown, the Sheriffs of the counties of Edinburgh, Lanark, Renfrew, Bute, Argyll, Inverness, Ross, Orkney and Zetland, Caithness, and Aberdeen. An Act was subsequently passed, which authorised the Commissioners, when any new Lighthouse was established on any part of the coast of Scotland, to add to their number the Provost or Chief Magistrate of the nearest Royal Burgh, and also the Sheriff-Depute of the nearest county; and, by the exercise of this power of assumption, the board now includes the Sheriffs of the counties of Ayr, Fife, Forfar, Wigtown, Sutherland, Kincardine, and Kirkcudbright. To enable the Board to carry on the intended works and to provide the means of maintaining the Lights, those Acts gave power to the Commissioners to levy a duty of 1d. per ton on British vessels, and 2d. per ton on foreign vessels; and liability to pay this duty was incurred by all vessels passing any of the Lighthouses in the course of a voyage; but this single payment freed them from any farther exaction, although they should pass more than one Lighthouse in the course of the voyage. The Board held its first meeting at Edinburgh on 1st August 1786. A Secretary and Engineer were appointed, and a resolution was adopted to borrow L.1200. For this sum the Magistrates of the five Royal Burghs named in the Act interposed their security; and, at the same time, assigned, in farther security, the duties under the Act of Parliament. After appointing a Committee to prepare matters for a general meeting, they adjourned till the 23d of January 1787. Some inconvenience having been felt in conducting the business of the Board, particularly in the holding of stock and other property, by reason of its not being an incorporated body, an Act was obtained for erecting the Commissioners into a body politic, by the name of the “Commissioners of the Northern Lighthouses.” Several Acts have been subsequently passed, in order to facilitate the erection of particular Lighthouses, and for the purpose of granting duties for their support. All those duties, however, are now abolished, and others have been substituted, the collection of which is regulated by an Act, 6th and 7th William IV., cap. 79, intituled, “An Act for vesting Lighthouses, Lights, and Sea-marks on the Coasts of England, in the Corporation of Trinity-House of Deptford Strond, and for making provision respecting Lighthouses, Lights, Buoys, Beacons, and Sea-marks, and the Tolls and Duties payable in respect thereof.” This Act declares, “That from the first day of January one thousand eight hundred and thirty-seven, the tolls now payable by or in respect of vessels for or towards the maintenance of the several lighthouses at present under the management of the Commissioners of Northern Lighthouses shall cease to be payable, and that, in lieu thereof, there shall thenceforth for ever be paid to the said Commissioners of the Northern Lighthouses, for every vessel belonging to the United Kingdom of Great Britain and Ireland (the same not belonging to his Majesty, his heirs or successors, or being navigated wholly in ballast), and for every foreign vessel which, by any Act of Parliament, order in Council, convention, or treaty, shall be privileged to enter the ports of the said United Kingdom, upon paying the same duties of tonnage as are paid by British vessels (the same not being vessels navigated wholly in ballast), which shall pass any of the said lighthouses, or derive benefit thereby, the toll of one halfpenny per ton of the burden of every such vessel for each time of passing every such lighthouse, or deriving benefit thereby, and of one penny per ton for each time of passing the Bell Rock Lighthouse, and double the said tolls for every foreign vessel not so privileged.” And with regard to any new Lighthouses to be hereafter erected, it is provided, that there “shall be paid to the Commissioners by the owner, or other person having the command of any vessel not belonging to His Majesty, which shall pass such lighthouse, or derive benefit thereby, such reasonable toll as shall have been first approved in that behalf by His Majesty in Council.” Before the passing of this Act, the Commissioners had been uncontrolled in the selection of stations for Lighthouses, or in choosing the characteristic appearance for the Lights; but it being considered desirable to have a systematic arrangement in the three kingdoms, the Irish Lighthouse Board, as well as the Commissioners, are now required to give notice to the Corporation of the Trinity-House of Deptford Strond, before altering the character of any Light, or erecting any new Lighthouse; and that Corporation must, within the period of six months after receiving such notice, signify their opinion as to the propriety of the change, or the utility of any new Lighthouses submitted for their consideration. The Act, however, provides, that, if the Commissioners are dissatisfied with the opinion of the Trinity-House, they may appeal to the Privy Council, whose determination is final. By this Act, also, an important power is given to the Commissioners to control the exhibition of all harbour and local Lights, or other sea-marks, and to prevent the exhibition of any Lights or fires on the sea-coast, which might be mistaken for the regular Lights exhibited by the Board. In the Appendix I have given a copy of the Annual Statement of the Income and Expenditure of the Board for the year 1846, prepared by Mr ALEXANDER CUNINGHAM, the Secretary to the Commissioners.

* * * * *

~Lights established since 1821.~

Since the Sumburghhead Lighthouse in Zetland was lighted in the year 1821, with a notice of which the account of the Bell Rock Lighthouse concludes, the Commissioners have been engaged in the establishment of seventeen new Lighthouses, and the remodelling of several old ones; and they have, more particularly, effected important changes in the mode of illumination, and have begun to place Beacons and Buoys on the coast. They have, besides, executed several considerable improvements, for the purpose of facilitating the communication with the Lighthouses at Kintyre in Argyllshire, Cape Wrath in Sutherlandshire, and Dunnethead in the county of Caithness, by the establishment of landing-piers and the formation of roads, varying in length from three to ten miles, in connection with those Stations. Of those works, many interesting details might be given, were it not desirable that the introduction to an account of a single Lighthouse should be restricted within a very moderate compass; and I have, therefore, thought it sufficient to lay before the reader the most important circumstances of each Lighthouse Station belonging to the Board in a tabular form in the Appendix.

~Improvements in the mode of illumination.~

I shall not, in this place, enter on any exposition of the general principles which regulate the illumination of Lighthouses, and still less will it be proper to discuss the advantages of the different methods of illumination by Reflection and Refraction, as I shall, in the sequel, find a more convenient opportunity for speaking somewhat in detail on those subjects. It will be enough to present a very brief notice of the improvements in the mode of illuminating Lighthouses, which the Northern Lights Board have introduced since 1824, up to which time, as already mentioned, a sketch of their works is already before the public. One of the most important changes in Lighthouse apparatus was, unquestionably, the introduction of Revolving Lights at the Tour de Corduan about the year 1780, by which the means of distinguishing one light from another were greatly extended, and a marked difference in the appearance of contiguous lights was at once simply obtained. The mere variation of the velocity of the revolution is so simple as to afford an obvious source of distinction among lights; and yet it is remarkable, that it was only lately that one of its principal advantages was perceived by my Father, who first applied it in the year 1827 as a means of distinction for the Light of Buchanness. This distinction consists in giving the frame a greater number of sides or faces, and a more rapid revolution, so as to cause a flash in every five seconds of time, which produces an effect so marked and characteristic as to afford by far the most effective distinction which has been exhibited since the introduction of Revolving Lights. Under the auspices of the Board, this distinction has been since applied at the Rhinns of Islay Lighthouse, and has given much satisfaction wherever it has been tried. The late King of the Netherlands, a great patron of the useful arts, was so much pleased with this device that he presented the author of it with a splendid gold medal, in token of his approbation. The only other improvement on the Reflecting Lights, which I shall notice in this place, is that called the _intermittent light_, which is due to the same officer, and was by him introduced at the stations of Mull of Galloway, Tarbetness, and Barrahead. It consists of the apparatus of a fixed Light, in front of which two cylindric shades are alternately shut and opened by a vertical movement, so as to produce a sudden extinction and exhibition of the light, in a manner very difference from the gradual decline and growth of the flash, which is produced in revolving Lights by the attenuating effects of divergence on the penumbral portions of the light reflected from the mirror.

~Dioptric Lights.~

The introduction of lenticular apparatus into Lighthouses has been the last great improvement effected in their illumination. So far back as the year 1823, the attention of the Commissioners was first called by their Engineer to the invention of the late AUGUSTIN FRESNEL, who had succeeded in building polyzonal lenses of large dimensions, and in adapting to them a lamp of great power, having four concentric wicks supplied with oil by a clock-work movement like that of the Carcel lamp. A committee was appointed to consider this subject; and under its direction a long train of experiments was made with those instruments and with the paraboloidal mirrors which are generally used in British Lighthouses. The results of the experiments led the Board, in the summer of 1834, to send me on a mission to France, with instructions to report my opinion as to the comparative merits of the dioptric and catoptric apparatus for the illumination of Lighthouses. Through the kindness of my friend M. LEONOR FRESNEL, Secretary of the _Commission des Phares_, who in the most liberal manner put me in possession of all the information which I required, and afforded me an opportunity of visiting the most important Lighthouses on the French coast, I was enabled on my return to report very fully my views on the various topics whose investigation had been committed to me by the Lighthouse Board.

The characteristics of the two systems of illumination by Reflection and Refraction may be briefly described as follows: In the reflecting apparatus, the lamp is placed in _front_ of the mirror, whose surface is so formed that the rays which fall upon it, and are reflected from it, must afterwards move in lines parallel to the axis of the mirror; while in using Refracting instruments, the flame is placed _behind_ the lens, whose action is simply to bend the rays in their passage through it, in such a manner that they come out from its face parallel to a line drawn from the focus to the centre of the lens. In Revolving Lights, on the reflecting principle, the mirrors containing the lamps are placed on a frame which revolves on its centre, and carries them round in succession to the different points of the horizon, so that each mirror produces a bright flash when it crosses the line drawn from an observer’s eye to the centre of the Lighthouse; but in Refracting Lights, a single lamp of great power is fixed in the centre of the lightroom, while the lenses, placed on a revolving frame, intercept and modify the rays which fall upon them from the Lamp, as they pass in front of it, and thus produce successive flashes whenever the centre of the lens crosses the imaginary line already noticed, as joining the observer’s eye and the lightroom.

In Fixed Lights, on the Reflecting plan, the mirrors are ranged around a fixed chandelier in tiers, one above another, their centres being placed in spiral lines, so that each shall subtend an equal arc of the horizon, and thus distribute the light with as little inequality as is consistent with the application of such an instrument as the paraboloidal mirror to this purpose. This object of distributing the light equally over the horizon, which, next to intensity, is the main object of a fixed light, and ought, indeed, to be strictly co-ordinate with it, is much better effected by using dioptric instruments. That apparatus consists of successive rings or bent prisms arranged in the form of a hoop or belt, which may be described as a cylinder, generated by the revolution of the central section of a polyzonal lens about its focus as a vertical axis, and which consequently acts only in a vertical direction, leaving the natural horizontal divergence of the light unchanged, and thus distributing it with perfect equality in every direction.

Those two systems of illumination possess advantages and defects peculiar to each. The lenticular instruments insure greater intensity when applied to revolving lights; but this advantage is in part counterbalanced by the greater duration of the flash caused by the reflectors, whose divergence is greater; while in fixed lights, the refracting instruments not only produce at least an equal intensity of light, but, what is of the greatest importance, afford the same quantity of light in all directions, a property which fixed Lights on the reflecting principle employed in Britain cannot possess.

On my return from France I made a Report, which was printed by order of the Commissioners; and the views which I gave of the superiority of the refracting apparatus, led the Board to adopt the resolution of at once converting the revolving light of Inchkeith from the catoptric to the dioptric system, as its nearness to Edinburgh offered good opportunity of observation as to the effect of the change. In October 1835, the new light was exhibited to the public, and I was forthwith instructed to make a similar change on the fixed light of the Isle of May; but in carrying this into effect, I introduced an important modification of the form of the refracting part of the apparatus, with the view of obtaining a still nearer approach to the equal distribution of the light. The only other considerable change in the lightroom apparatus which has since been effected, is the substitution of catadioptric zones in room of the mirrors hitherto used in the subsidiary parts of the larger French lights, which, as will appear in the sequel, was suggested by me in 1841, and finally carried into effect in 1843, agreeably to the computations of M. LEONOR FRESNEL. A Table of the Elements of those zones computed by myself, and closely verifying M. FRESNEL’S results, will be found in the Appendix. The lenticular apparatus has been applied at the new Lighthouse stations of the Little Ross and the Skerryvore, and, still more recently, at Covesea Skerries, Cromarty Point, Chanonry Point, Loch Ryan, and Girdleness.

~Beacons and Buoys.~

The establishment of a system of Beacons and Buoys on the coast of Scotland for the purpose of affording additional facilities to navigation, had long been looked upon as a desirable extension of the operations of the Northern Lights Board; and the increase of the trade and shipping of the kingdom having, some years ago, directed particular attention to the subject, a committee was named, on the 12th January 1839, to take special superintendence of that department. In 1840, the Engineer reported to the committee upwards of fifty stations for Beacons, and nearly a hundred for Buoys, as auxiliaries to the navigation in situations where the establishment of a Lighthouse was either too expensive or not warranted by the wants of the district; and means were immediately taken for erecting three Beacons in the Frith of Forth, two in the Clyde, one in Loch Ryan, and two in Cambeltown Loch. Beacons were also erected on the Iron Rock or Skervuile in the Sound of Jura, and on the Covesea Skerries in Morayshire, in connection with the Lighthouse of that name. Those works, notwithstanding many obstacles arising from doubts as to the powers of the Board, have been carried on with great vigour. In the Appendix, I have given drawings of three of those Beacons, one being of masonry, and the other two of iron; and also Tables which shew the positions of the various Beacons and Buoys at present belonging to the Board.

Chapter I.

Topographic Notice Of The Skerryvore Rock.

From the great difficulty of access to the inhospitable rock of Skerryvore, which is exposed to the full fury of the Atlantic, and is surrounded by an almost perpetual surf, the erection of a Light Tower on its small and rugged surface has always been regarded as an undertaking of the most formidable kind. So discouraging was the consideration of expense, and the uncertainty of the final success of such a work, that the Commissioners of the Northern Lighthouses, after successfully completing the arduous and somewhat similar work on the Bell Rock, were induced to proceed with other operations of less magnitude, but probably, in some respects, of no less utility; and to delay the construction of the Skerryvore Lighthouse till the present time, although the Act of Parliament authorising its erection was obtained so long ago as 1814.

The cluster of Rocks, of which that called the Skerryvore is the largest, has ever been a just cause of terror to the mariner. Its dangers have long been known, and the means of removing these dangers, by converting its dark horrors into a cheering guide for the benighted mariner, have often occupied the attention of the Lighthouse Board, and especially of my predecessor in the office of their Engineer, with whom it was a constant subject of interest, from its similarity to his own work on the Bell Rock.

The first landing that my Father, in the course of his annual voyages round the coast, as Engineer of the Northern Lighthouse Board, effected on Skerryvore, was in the year 1804. In 1814, he visited it a second time, while accompanying a committee of the Commissioners on a tour of inspection to the Lighthouses all round the coast, from the Frith of Forth to the Clyde. On that occasion, Sir Walter Scott was of the party, and we find in his diary the following record of his impressions at the time, expressed in the terse and humorous language by which this interesting relic of the poet is characterised; and as the hasty observations of that great man seem worthy of a place in a work descriptive of the means which have been taken to obviate the dangers to which he refers, no apology seems necessary for introducing it in this place.

“Having crept upon deck about four in the morning,” says Sir Walter, “I find we are beating to windward off the Isle of Tyree, with the determination, on the part of Mr Stevenson, that his constituents should visit a reef of rocks called _Skerry Vhor_, where he thought it would be essential to have a Lighthouse. Loud remonstrances, on the part of the Commissioners, who, one and all, declare they will subscribe to his opinion, whatever it may be, rather than continue the infernal buffeting. Quiet perseverance on the part of Mr S., and great kicking, bouncing, and squabbling, upon that of the yacht, who seems to like the idea of Skerryvore as little as the Commissioners. At length, by dint of exertion, come in sight of this long ridge of rocks (chiefly under water) on which the tide breaks in a most tremendous style. There appear a few low broad rocks at one end of the reef, which is about a mile in length. These are never entirely under water, though the surf dashes over them. To go through all the forms, Hamilton, Duff,[1] and I, resolve to land upon these bare rocks, in company with Mr Stevenson. Pull through a very heavy swell with great difficulty, and approach a tremendous surf dashing over black pointed rocks. Our rowers, however, get the boat into a quiet creek between two rocks, where we contrive to land well wetted. I saw nothing remarkable in my way excepting several seals, which we might have shot, but, in the doubtful circumstances of the landing, we did not care to bring guns. We took possession of the rock in name of the Commissioners, and generously bestowed our own great names on its crags and creeks. The rock was carefully measured by Mr S. It will be a most desolate position for a Lighthouse--the Bell Rock and Eddystone a joke to it, for the nearest land is the wild island of Tyree, at fourteen miles distance. So much for the Skerry Vhor.”

[1] The Sheriffs-Depute of Lanark and Edinburgh.

Notwithstanding those occasional visits, however, it was not till the year 1834, that the Commissioners directed their Engineer to make a survey of the whole of this extensive reef, preparatory to taking measures for the erection of a Lighthouse on that part of it which might be found, after careful inspection, to afford the most suitable site; and, at the same time, the shores of part of the Island of Tyree were surveyed, with the view of establishing a Signal Tower for communicating with the Lighthouse, and of forming a small harbour, of shelter for the vessels to be employed in attending it. From these surveys the general view of the Reef which is given in Plate II., and the enlarged plan shewn in Plate III. of the Skerryvore or principal Rock, on which the Lighthouse has been built, were constructed.

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The Skerryvore or principal Rock of this remarkable group, is situated in North Lat. 56° 19′ 22″, and West Long. 7° 6′ 32″.[2] It is about 11 Nautic miles W.S.W ¹⁄₄ W. of the island of Tyree, which is the nearest land, 20 miles W.N.W ³⁄₄ N. of the island of Iona, 33 miles S. ¹⁄₄ E. of the Lighthouse of Barrahead, the most southern of the Hebrides, and 53¹⁄₂ miles N.E. by N. of Mallinhead, in the county of Donegal in Ireland. It may also be added, that the principal rock is about 50 miles from the nearest point of the main land of Scotland. The extent of the Reef, and its situation in reference to the general position of the coast, will be best understood by referring to Plate I., which is a small Map of the British Isles. From this it will be seen that it lies in an irregular semicircular sea, inclosed by the southern extremity of the Hebrides, the rugged shores of Argyllshire, and the northern coast of Ireland on the one side, but open on the other to the Atlantic.

[2] According to information for which I am indebted to Captain Yolland, R.E., of the Ordnance Survey.

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The importance of the Skerryvore as a station for a Lighthouse is so evident as to require but little comment. Although the smaller class of coasting vessels almost invariably sail through the sheltered Sounds of Mull, Loing, and Islay, to avoid the difficulties and dangers (Skerryvore among the number) of the rough navigation of the outward passage, yet these rocks lie much in the track of the larger vessels bound over seas round the North of Ireland from the Clyde and the Mersey. Government Cruisers and Ships of War are also necessarily often within a short distance of its dangers. But for homeward-bound vessels sailing for the Clyde, or for any of the Ports in the Irish sea, and directing their course for the North Irish Channel, the establishment of a light at this place is of the last importance. When such vessels happened to encounter bad weather before making land, and so had difficulty in ascertaining their true position in relation to the coast, they often, in the event of being driven so far north from their course, as to miss the lights of Ireland or that of Barrahead, continued their progress onwards in the direction of the Skerryvore Rocks; and thus, while running in apparent safety, and probably, from the state of the weather, not within sight of Tyree, which it is often difficult to see, they were very liable to encounter some of the many detached rocks and shoals which form this broken reef of nearly seven miles in extent.

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In estimating the risks to which vessels were exposed from this cause, the peculiarly insidious nature of the danger must be kept in view. A headland, or line of coast, which rises to some height above the surface of the sea can be seen in most states of the weather, at a sufficient distance, even during the night, to enable the seaman to avoid danger; but, in approaching a sunken reef or a low rock, in the dark, there is no object to warn the crew of their position, until their vessel gets unexpectedly among breakers, after which it is generally too late to bring her round again. And even the very knowledge of the existence of a reef, such as this, often causes the seaman, in ignorance of its exact position, to give it too wide a berth; in which case his ship is liable to be carried away by the force of tides or winds, perhaps on a lee shore, where, although the crew may be saved, the vessel generally goes to pieces.

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The exhibition of a Light, however, altogether changes the case. Instead of shunning as a danger those dreaded rocks, vessels will steer boldly on their course, until checked by the Light, availing themselves of which they will be enabled to _lie off-and-on_ during the night, and so wait the return of daylight, in perfect confidence as to their position, and without the necessity of endeavouring to avoid hidden dangers. Thus, that which was formerly an obstruction and a danger, is rendered an aid and a safety, to the navigation of the western coasts of our country.

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That this source of danger to shipping was by no means imaginary, and the consequent terror of mariners far from being ill founded, there is a too melancholy proof in the following list of disasters caused by the Skerryvore Rock, and the neighbouring dangers off the coast of Tyree:--

In 1790. The Ship Rebecca of 700 tons lost; crew saved.

1804. Ship Brigand of Nova Scotia, Wright, master, of 600 tons, lost off Hough, in Tyree; crew saved.

1804. _A Brig_, M‘Iver, master, lost off Hough; crew saved.

1806. Ellen of Bath, Paterson, master, of 90 tons, lost off Balaphuil, in Tyree; one man drowned.

1809. Brig Mary, Sanders, master, lost off Balaphuil; crew saved.

1813. Sloop, Penelope of Wick, 60 tons, lost at Gott Bay, Tyree; crew saved.

1810. A Brig from New York, Greenlees, master, lost off Hynish Point, Tyree; crew all drowned.

1813. _A Sloop_, Eugene M‘Intyre, master, lost off Balaphuil; one man drowned.

1814. Brig, Betsey of Leith, Ross, master, lost off Hough; crew saved.

1817. _A Brig_, of 400 tons, foundered off Kennavarah, Tyree; crew all drowned. Numerous casks of butter came ashore.

1818. Sloop, Benlomond of Greenock, M‘Lauchlan, master, lost off Balaphuil; crew all drowned.

1819. Sloop, Bee, Coice, master, of 60 tons, lost off Hough; crew saved.

1820. _A Sloop_, M‘Donald, master, of 50 tons, lost in Reef Bay, Tyree; crew saved.

1820. Ship, Masters, of Port-Glasgow, Martin, master, of 700 tons, foundered off Skerryvore Rocks, and came ashore at Clate Hynish, in Tyree; crew saved.

1821. Sloop, Catharine, M‘Rae, master lost; crew saved.

1821. _A Sloop_, of 60 tons, lost off Hough; master and three men drowned.

1825. Sloop, Dan of Campbelltown, M‘Innes, master, of 50 tons, lost; crew saved.

1828. Sloop, Delight, of 70 tons, Stevenson, Master, lost.

1828. _An Irish Schooner_ of 100 tons, Montgomery, master, lost off Hough; crew saved.

1828. Jane of Sligo, Collins, master, lost off Balaphuil.

1829. Van Scapan of Stockholm, Fisherton, master, of 700 tons, lost off Hough; fourteen people drowned.

1834. Confidence of Dundee, Wesley, master, lost off Hough; crew saved.

1834. _A Schooner_ of 70 tons, lost; three men drowned.

1835. Peggy, Bitters, master, of 500 tons, lost off Beist, Tyree; crew saved.

1841. April 2. Majestic of North Shields, Tait, master, of 400 tons, foundered _by a sea_ off Boinshly Rock, and came ashore at Gott Bay; captain and four men washed overboard and drowned, and the mate and one seaman had their legs broken when the vessel was struck by the sea.

1842. Fleurs of Liverpool, Thomson, master, of 300 tons, lost off Kennavarah; crew saved.

1842. March 14. Two deck beams, a knee, and some pieces of deck-plank of a _North American built vessel_, came ashore at Clate Hynish.

1842. _A Barra Boat_ wrecked, and four corpses washed ashore; two men, a woman and a child.

1842. Pieces of wreck were seen in the Sound of Coll, and at the same time the shores of Tyree were strewed with candles, mostly of wax, supposed to be altar candles for the West Indies.

1843. September 2. The Prussian Barque Formosa, of 326 tons, P. R. Reick, master, lost off Hough; two seamen drowned.

1844. December 1. The Hull of _a Sloop_ of about 70 tons, was washed ashore off Clate Hynish. The Hull was very much broken up by being in contact with the rocks; and one of the planks, apparently off the taffrail, had the words “Port of Dundee” lettered upon it; the crew supposed to be all drowned.

This list is made up chiefly from data kindly furnished to me by the Rev. Neil Maclean, the Minister of Tyree and Coll, whose long residence on the former island has afforded him ample opportunity for making observations on the subject. It is not to be imagined, however, that Mr Maclean’s list, which is made up from recollection, contains a full catalogue of the disasters caused by the Skerryvore, within the dates which it cites. Very many vessels were wrecked on this dangerous reef whose names could never be learned, and of which nothing but portions of the drift wood or cargo came ashore; and there have, no doubt, been many shipwrecks of which not a single trace has been left. Nothing, indeed, is more probable than that many of the foreign vessels whose course lay through the North Irish Channel, and whose fate has been briefly and vaguely described, as “foundered at sea,” have met their fate on the _infames scopuli_ of the Skerryvore. It is also well known that the Tyree Fishermen were in the constant practice of visiting the Skerryvore, after gales, in quest of wrecks and their produce, in finding which they were but too often successful.

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The natives of Tyree have many traditions of vessels having struck on the Skerryvore and gone to pieces; but, as might have been anticipated, few traces of this were to be found on the Rocks themselves, the breach of sea which sweeps over them during storms being sufficient to remove any heavy bodies which might be left there after a shipwreck. Some relics, however, were found during the progress of the works, and among the rest an anchor which was fished up close to the Rock, and which appeared to have belonged to a vessel of about 150 tons burden. It had been wasted to a perfect shadow by the action of the sea, and was covered with a thick coating of seaweed and barnacles. Although, however, the Rocks themselves do not retain the proofs of the disasters of which they have been the cause, the shores of the neighbouring Islands, during the progress of the works, were frequently strewed with drift wreck in such a manner as clearly to indicate what had taken place on the shoals round the Skerryvore.

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On examining Plate II., it will be seen that what I have hitherto denominated the Skerryvore Reef, is a tract of foul ground, consisting of various small rocks, some always above the level of the sea, others covered at high water, and exposed only at low water, and others, again, constantly under the surface, but on which the sea is often seen to break after heavy gales from the westward. This cluster of rocks extends from Tyree in a south-westerly direction, leaving, however, between that island and the rock called Boinshly, the first of the great Skerryvore cluster, a passage of about five miles in breadth, and having a depth of thirteen fathoms at low water of spring tides, but not without hidden dangers, which line the rugged shores of Tyree from Kennavarah to Ben Hynish, and some of which lie farther off than might be expected. This passage is called the passage of Tyree; but it is by no means safe during strong and long continued gales, as the sea which rises between Tyree and Skerryvore, is such that no vessel can _live_ in it. I have myself often seen it one field of white broken water, the whole way from Tyree to the Rock; and we know that the wreck of the Majestic, which occurred in 1841, during the progress of the works, was entirely caused by the heavy seas which she encountered off Boinshly.

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The principal rocks of the group, are called Boinshly, Bo-rhua, and Skerryvore, while those lying to the westward, which have been more recently laid down, have received the names of Mackenzie, Fresnel, and Stevenson.

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The rock called Boinshly lies about 3³⁄₄ miles from Skerryvore, and is of considerable extent. The origin of the names of the different rocks in the vicinity of Tyree is by no means clear, and very little assistance or information is to be obtained in this matter from the natives. The name of Boinshly is probably derived from the Gaelic words _boun_, signifying _bottom_, and _slighe_, _deceitful_, as indicative of the dangers of the place; but other interpretations have been put on it, and that which has been now given is by no means certain. In the course of the survey, several soundings were at considerable risk obtained, both upon this Rock itself, and in its immediate vicinity. The sea in that exposed situation is seldom so tranquil as to warrant an attempt to approach very near this Rock. The swell, which, in a greater or less degree, almost constantly prevails, is apt to impel, or seemingly draw the boat as by a kind of suction, upon the rock; and sometimes such accidents cannot be prevented, even although the greatest caution is used. Sudden _lifts_ of the sea, during an apparent calm, are common in all the more exposed parts of the coast, more especially in the Orkney and Zetland Isles, and on the shores of the most western of the Hebrides; and any one much accustomed to the use of boats on these shores, must have experienced the hazard of encountering such unexpected risings of the sea, more especially near shelving rocks, or in rapid tide-ways. In some places the boatmen apply the name of _lumps_ to these sudden waves. This effect is not felt to the same extent in attempting to reach a rock which is partially uncovered at low water, as a landing can, in such a case, often be effected on one side, at a time when the same rock on the opposite side, or a sunk rock just topping with the water, would, on every side, be quite unapproachable. From the soundings marked on the plan, it will be seen that shoal water extends all round Boinshly to distances varying from a quarter to half a mile. The sea breaks on the rock with great violence, and its position can easily be discovered from the island of Tyree by the white foam with which it is almost constantly surrounded, and which, in the heavy swells which sometimes accompany a dead calm, before or after a heavy gale of wind, rises to a prodigious height in a column or jet, resembling, at a distance, the play of a gigantic fountain. So high, indeed, does the sea rise on this shoal after heavy gales, that it often quite obscures the larger and more distant object of the Rock and Tower of the Skerryvore, even when viewed from the top of Ben Hynish in Tyree. The wooden barrack erected on the Skerryvore for the use of the workmen during the progress of the operations, although about sixty feet in height, was often lost sight of at Tyree by the uprising of the sea on Boinshly, and could be seen only during the calm that intervenes between returning waves.

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The next Rock that occurs is Bo-rhua, a name derived from the Celtic, and signifying, according to the natives, Red Rock. It lies about 2³⁄₄ miles from Boinshly, and about one mile from the Skerryvore. The passage between it and Boinshly is clear, and has a depth of about fourteen fathoms; but it is too narrow to be safely navigated except by daylight, even under the most favourable circumstances, and then no mariner would run the risk of taking such a passage, but would prefer, even at some sacrifice of time, the fairway of the passage of Tyree. Bo-rhua is completely covered at high, but is dry at low water. The extent of rock uncovered is about forty feet by twenty feet, and the highest point of it is about six feet above low water level of spring tides. A small outlying pinnacle, about ten feet square, is also uncovered at low water. The depth immediately round Bo-rhua is considerable, from three to seven fathoms being found within fifty feet of it; and in this respect it differs from Boinshly, which, as already mentioned, is surrounded by shoal water for some distance. Between Bo-rhua and Skerryvore, however, which is a distance of about a mile, there cannot properly be said to be any clear navigable channel, as will be distinctly seen by referring to the plan. The whole of this tract may, in fact, be termed _foul ground_.

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The Skerryvore or chief rock, and the detached rocks immediately surrounding it, were surveyed with greater minuteness than the others, as it was at once apparent, that on this part of the reef alone could a suitable site for a lighthouse be found. The name is derived from the Gaelic, and signifies the Great Rock. It is very much wasted and cut up; the number of detached rocks, sunk and exposed, in its immediate neighbourhood, whose positions were determined during the survey, amounting to no fewer than 130. The depth of water between those different detached fragments, which extend over a surface of about a mile in length, by half a mile in breadth, is considerable, varying from 2¹⁄₂ to 8¹⁄₂ fathoms at low water of spring tides.

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The surface of the main or principal rock, on which the Lighthouse has been placed, measures, at the lowest tides, about 280 feet square. It is extremely irregular, and is intersected by many gullies or fissures, of considerable breadth, and of unlooked for depth, and which leave it solid only to the extent of 160 feet by 70 feet. The extremity of one of these gullies, at the south-east corner of the rock, forms the landing-creek, which is a narrow track of 30 feet wide, having deep water; and, with the help of some artificial clearing and dressing, which was executed with much difficulty, by blasting under water, while the other works were in progress, its sides and bottom are now comparatively smooth. At this place a landing can often be effected when the rock is unapproachable from any other quarter, although great inconvenience is felt from the surge, which finds its way from the opposite side of the rock, through the westward opening of the gulley in which the landing-place is situated.

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Another of the gullies, immediately to the south-east of the Lighthouse, was found, on examination, to undermine the rock to the extent of eight or ten feet, and to terminate in a hollow submarine chamber, which threw up a spout or jet of water about twenty feet high, resembling in appearance the Geyser of Iceland, and accompanied by a loud sound like the snorting of some sea monster. The effect of this marine _jet d’eau_ was at times extremely beautiful, the water being so much broken as to form a snow-white and opaque pillar, surrounded by a fine vapour, in which, during sunshine, beautiful rainbows were observed. But its beauties by no means reconciled us to the inconvenience and discomfort it occasioned, by drenching us whenever our work carried us near it. One calm day I contrived, at a very low tide, by means of ropes and a ladder, to explore the interior of the cavern, from which this fountain rose, and found it to terminate in a polished spherical chamber, about seven feet in diameter, its floor filled with boulders, whose incessant play had hollowed it out of the veined rock, and rendered its interior beautifully smooth and glassy. As I considered that this curious cavern penetrated too far, and came too close to what I had selected as the best foundation, I changed the site of the tower, so as to avoid any chance of its being undermined. I also deemed it prudent to fill up the cavity, to prevent its further extension, and at the same time to rid ourselves of the discomfort of being drenched by the column of water which spouted up from it, even during fine weather, when the sea was apparently calm. This gulley affords a good example of the power of pebbles kept constantly in play by the waves to wear down the hardest rock, and shews what extensive effects so insignificant an agent may effect in the course of time.

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Before the excavation for the foundation of the tower was made, a single conical loaf of rock, about five feet in diameter, rose to the height of eighteen feet above the level of high water, the greater part of the rest of its surface being about six feet above the tide mark.

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In addition to its shattered and disjointed appearance, the Skerryvore Rock presents, in another respect, a striking example of the action of the sea, which no one, on first landing on the rock, can fail to perceive. I allude to the glassy smoothness of its surface, a feature that existed to so remarkable an extent as to have proved throughout the whole duration of the work, but more especially at its commencement, a serious obstacle and hindrance to the operations. It may, at first sight, appear strange that this grievance should have been so much felt; but, when I mention that the landings were often made in very bad weather, it will be obvious that there was considerable danger in springing ashore from a boat in a heavy surf upon an irregular mass of rock as smooth and slippery as ice. The workmen were, in that respect, often sorely tried, and many inconvenient accidents occurred from falls. It was after one of these trials of patience, that the foreman of the masons was heard very graphically to describe a landing on the rock as “like climbing up the side of a bottle.” Instead of a weather-beaten rock, whitened by the dung of sea-fowls, and with marine crustacea adhering to it, the surface of the Skerryvore is smoothly polished by the action of the waves, every projecting angle or point is worn down, and the whole presents more the appearance of a mass of dark-coloured glass than a reef of gneiss-rock. Excepting in some of the more sheltered crevices, no marine crustacea find shelter; but different kinds of sea-plants grow upon it, in great abundance, at and below the low water mark. These plants are, doubtless, enabled to resist the action of the waves in the same way as the sapling, yielding to the blast, is preserved during the storm that uproots the aged and more stubborn oak.

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The rocks of Skerryvore have the same characteristics as those of the neighbourhood of Tyree, being what we may, perhaps, call a syenitic gneiss, as it consists of quartz, felspar, hornblende, and also mica. It will be seen, from the narrative of the progress of the works, that this rock was, from its hardness, exceedingly difficult and tedious to excavate. The only variation in the geology of the Skerryvore, is the presence of a trap rock, in the form of a dyke of basalt, which intersects the strata, and exhibits a fine specimen of the intrusion of igneous rocks. It is shewn in Plate, No. III., by a thick black line.

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Connected with this general view of the appearance and geology of the rock, it may be interesting also to notice, that a considerable mass of foreign matter, somewhat resembling, in its structure, a deposit of lime, was found in different places resting in horizontal layers of various thickness and size. This substance was found in pools or sheltered parts of the rock, about the level of high water mark, and, in some cases, even a little below it. It was so hard as to admit of a pretty high degree of polish; and emitted an offensive odour on being burned in the fire, or rubbed on a stone with water. It gave other clear indications of containing animal matter, and in other respects resembled the bergmeal and guano. To account for its presence in such a situation, seems rather a difficult problem. On sending a specimen of this material to my friend the Rev. Dr Fleming, Professor of Natural Philosophy in King’s College, Aberdeen, I received from him an analysis of the substance, and a concurrence in the opinion I had formed as to its containing animal matter; and Dr Fleming, indeed, expressed his belief that the matter in question is the indurated soil of birds, and had been deposited when the reef was more extensive, and the resort, and probably the breeding-place of sea-fowls.[3] How this singular formation should be found on the verge of the ocean, and even within the high water mark, in spite of winds and waves, or how it should have assumed the stratified structure which seems to indicate the depositation of successive layers in still water, are matters very difficult to be explained, without coming to the conclusion, that the uncovered surface of Skerryvore Rock must at some distant period have been much more extensive than at present, so as to permit the deposit to go on in an interior basin or lagoon, sheltered from the waves, and somewhat similar to those which Dr Darwin has described as characteristic of the Coral Isles of the Pacific. This supposition seems not at all improbable, as it does not require a great stretch of fancy to conceive, that at some period, the whole of the rocks in the immediate vicinity of Skerryvore, and extending perhaps even so far as Bo-rhua, may have been connected by a matrix of softer materials, which have gradually yielded to the action of the sea, leaving the harder portions to be smoothed and polished by the waves, and to assume the characteristic features of permanent rocks and sunk reefs which they now possess. There is also some countenance to such a view to be derived from the features of the neighbouring Island of Tyree, which contains numerous small lagoons, in which such deposits might be formed by the flocks of sea fowl which frequent them. Some of these pools are so near the shore, as to make it no difficult matter to conceive that their walls might be broken by the sea, and that they might eventually become part of it, and thus exhibit the phenomenon of deposits apparently lacustrine within the verge of the ocean.

[3] Dr Fleming has since obtained from Ichaboe indurated bird-soil or guano, closely resembling that from the Skerryvore.

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Another remarkable feature which I observed in the Skerryvore Rock, was a deposit of gravel in the narrow crevices of the rock, which run nearly from north-east to south-west, dipping at an angle of 80° to the westward. In almost all of the fissures we found great quantities of small water worn boulders, less in size than a horsebean, and generally of the same materials as the rock itself. The boulders bore the appearance of having been forced into the fissures of the rock by some very powerful pressure, and were wedged hard into the crevices. In some cases a considerable quantity of softer matter containing iron was found, and in it the pebbles were imbedded. In the upper parts of the rock the crevices swarmed with centipedes of a reddish-brown colour. The rock was covered with sea fowl when first visited, and during heavy gales seals resorted to it.

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About three miles to the westward of Skerryvore lie Mackenzie’s Rock, Fresnel’s Rock, and Stevenson’s Rock, which, as will be seen from Plate II., are connected by a tract of foul ground of about a mile and a quarter in length. Those rocks are the western limit of what we have already denominated the Skerryvore Reef. The passage between them and the Skerryvore or main rock is clear, and has a depth of water varying from eleven to twenty-eight fathoms.

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Mackenzie’s Rock, which derives its name from the celebrated Marine Surveyor, is uncovered, at low water, to the extent of about forty yards, and consists of scattered patches of rock, one of which, at its highest part, rises about ten feet above high water mark of spring-tides. Fresnel’s and Stevenson’s Rocks are always under water; but the sea is often seen to break violently over them, as well as over the whole stretch of the sunken reefs which extend between them. The first of those rocks is indebted for its name to the great optical philosopher, who so greatly improved lighthouses; and the second bears the name of the surveyor who first laid down the rock,--the late Engineer of the Northern Lights Board.

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During the progress of the survey, a register of the rise and fall of the tides was regularly kept at Hynish on the neighbouring Island of Tyree; and from those observations it was determined, that the rise at that place is between twelve and thirteen feet at high spring tides, and three feet at dead low neap tides; and observations subsequently made while the works were in progress, gave the same results at the Rock of Skerryvore. It is high water at the Rock at full and change of the moon, at five hours and twenty-five minutes. The tides round the Skerryvore are not remarkable for their rapidity. In spring-tides the velocity is between four and five miles, and in neap-tides between two and three miles an hour. The flood sets to the N.N.E., and the ebb to the S.S.W.

Chapter II.

PRELIMINARY ARRANGEMENTS AND WORKS, INCLUDING SURVEY OF THE ROCKS, AND OPENING OF QUARRIES, FROM 1834 to 1837.

~Survey of the Skerryvore Rocks.~

In this chapter I shall very briefly notice those preliminary arrangements which may be said to have been in a great measure preparatory to the commencement of the work itself. It has been already stated, that the erection of the Lighthouse was provided for in the Act of 1814; but so formidable did this work appear, that although it was repeatedly under consideration, it was not until the General Meeting of the Board, on the 8th July 1834, that any measures were taken to carry into effect the provisions of the Act. On that occasion it was moved by the late Mr MACONOCHIE, Sheriff of Orkney and Zetland, that the Engineer should be instructed to make the necessary survey, and to report as to the expense of erecting the Lighthouse. In terms of this remit, the survey of the Rocks was commenced in the autumn of 1834; but from the broken state of the weather, little was effected at that time beyond making the triangulation; and it was not until the summer of 1835 that the survey was completed from which the Chart, Plate No. II. was constructed. This survey was attended with much more labour than its extent would lead one to suppose, in consequence of its embracing the entire range of operations required in a more extensive nautical survey, and combining with the ordinary details required for a Chart, the minute accuracy in regard to surface and levels, which are always necessary for the purposes of the Engineer.

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The first step was the measurement of a base line in the low lands of the adjoining Island of Tyree, which, owing to the distance and disadvantageous position of that island, could not be satisfactorily extended to the Rock without fixing stations in some of the more distant islands; and in the course of the work not fewer than twenty land triangles were measured. The calculations of the distances founded on this triangulation agreed with those afterwards obtained from the data of the Trigonometrical Survey, which were kindly furnished to me by Captain Yolland of the Royal Engineers, in 1843. For the purpose of making the soundings and laying down the sunken rocks, an entirely separate triangulation, based upon and connected with that which has already been noticed, became necessary, as the land objects were too distant, and their relative positions were such as to render it difficult by observations from them alone to determine any stations on the sea. Buoys were therefore moored at convenient points, and their positions determined by a subsidiary triangulation, so as to form a net-work of triangles between the shore and the Skerryvore Rock. The distances between these buoys were afterwards used as the bases of imaginary triangles, having points of sounding or shoals in their apex; and the angles subtended by those distances being measured by the sextant, the positions of the shoals or soundings were thence easily deduced and protracted on the Chart.[4] In connection also with the soundings whose positions were determined in the way above described, a complete set of tide observations was made, extending over a period of about six weeks. Those tide observations were connected in point of time, with the soundings, and were employed as the means of correcting the observed depths taken with the sounding-line, so as to give the true depth in reference to the high or low water of a given tide. Accurate measurements, and minute sections, were also made of the rocks in reference to the tide-level, and more especially of the main rock, on which alone it was obvious, from the first inspection, that the Lighthouse Tower could be erected. In the course of this survey, the positions of upwards of 140 rocks were determined, and laid down on the Chart, and 500 soundings were taken, and their positions protracted. An interesting fact was also noticed regarding the mean level of all the tides which had been watched during the period of about six weeks, as above noticed, viz., that the point half way between the high and low water of every tide is on _one and the same level_. This fact regarding the tides was, it is believed, first detected by my Father, in the course of some tidal observations which he made in the Dornoch Frith in 1830, and has since been observed in the Frith of Forth in 1833, and again on the shores of the Isle of Man, and at Liverpool. The agreement of so many observations by various persons at places on the opposite shores of the Kingdom, seems to imply the universality of this phenomenon in the British Seas; and the position of Skerryvore would lead to the belief, that it is not confined to narrow seas, but that it exists in the ocean. I cannot dismiss the subject of the survey, without mentioning the late Mr James Ritson, who acted as principal assistant surveyor, and to whose zeal and intelligence so much of its accuracy is to be attributed. The deep gulley which intersects the main Rock from N.E. to S.W., and across which he one day sprang while it was filled with a breaking wave, bears his name, as a memorial of his activity and perseverance. At the close of the survey in 1835, the station-pole was left wedged and batted into one of the fissures or crevices of the Rock, and a cask of water was firmly _lashed_ to ring-bolts in a cleft of the highest part of the Rock, in the hope that it might possibly prove useful to some shipwrecked seamen.

[4] _Vide_ Stevenson’s Marine Surveying and Hydrometry. Edinburgh, 1842, p. 144.

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For the purposes of navigation generally, a survey merely indicating the position and extent of the foul ground would have been sufficient. But in connection with the work which was about to be commenced, it was particularly desirable to have exact details of the depths, rocks, and shallows of the surrounding sea, with the nature of the bottom, accurately laid down; and our experience during the course of the work, more than once shewed how essential was the possession of minute topographic information to the safety of the shipping attending the works; more especially as some of the vessels lay very near the rocks, and were frequently driven, by a sudden change of wind, to seek shelter, during the darkest nights, among the neighbouring islands.

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Until this time the greatest ignorance prevailed amongst seamen as to the extent of the Reef, which had never before been minutely surveyed. Of this some proofs occurred even during the progress of the survey; for several vessels came so near the Rocks as to cause, in the minds of the surveyors, who witnessed their temerity, serious fears for their safety. On one occasion, in particular, a large vessel belonging to Yarmouth, with a cargo of timber, was actually boarded between Mackenzie’s Rock and the main Rock of Skerryvore by the surveyors, who warned the master of his danger in having so nearly approached these rocks, of the existence of which his chart gave no indication. On another occasion, a vessel belonging to Newcastle was boarded while passing between Bo-Rhua and the main Rock; and so little, indeed, had the master (whose chart terminated with the main Rock, and shewed nothing of Bo-Rhua) been dreaming of danger, or fancying that he was within a cable’s length of the reef, that he was found lying at ease on the companion, enjoying his pipe, with his wife sitting beside him knitting stockings.

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~Disadvantages of Tyree.~

Much preliminary investigation was necessarily occasioned by the difficulties and disadvantages arising from the remote situation of the island in which a great part of the works was to be carried on. Not only is the Rock itself often inaccessible and dangerous, being surrounded by numerous shoals, and visited by the heaviest seas of the Atlantic; but what gave rise to no small part of the difficulties which attended this work, was the nature of the neighbouring Island of Tyree. This island is unhappily destitute of any shelter for shipping, a fact which was noticed as a hinderance to its improvement, upwards of 140 years ago, by Martin, in his well-known description of the Western Islands.[5] Nor is its interior more attractive; for although some parts of the soil when cultivated are excellent, the greater part of its surface is composed of sand. It was therefore obvious, at a glance, that Tyree was one of those places to which every thing must be brought; and this is not much to be wondered at, as the population, who, on a surface not exceeding 27 square miles, amounted in 1841 to 4687 souls, labour under all the disadvantages of remoteness from markets, inaccessible shores and stormy seas, and the oft-recurring toil of seeking fuel (of which Tyree itself is destitute) from the Island of Mull, nearly 30 miles distant, through a stormy sea. It is said that this total absence of fuel in Tyree is the result of the reckless manner in which it was wasted, in former days, in the preparation of whisky; but, however this may be, certain it is that the want of fuel greatly depresses the condition of the people. For our works, therefore, craftsmen of every sort were to be transported, houses were to be built for their reception, provisions and fuel were to be imported, and tools and implements of every kind were to be made.

[5] A Description of the Western Islands of Scotland, &c., by M. Martin, Gent. London, 1703. _Vide_ 2d Edition of 1716, p. 267.

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~Pier and workyard at Hynish Tyree.~

In the course of the survey, much attention had been bestowed upon the selection of a convenient place for a workyard in Tyree for the preparation of materials, and in examining its rugged shores in quest of the best site for a Harbour, for the shipment of the building materials for the Rock, and for the all-important purpose of enabling the future attending vessel to lie in safety within sight of signals from the Rock, when the Light should come to be exhibited to the public. The point chosen for this establishment was Hynish, which, though twelve miles distant, is, nevertheless, the nearest creek to the Skerryvore Rock, and which, however exposed it may be, if compared with creeks elsewhere dignified with the name of Harbour, certainly affords as good prospect of shelter as any other part of the Island of Tyree, and is, in this respect, greatly to be preferred to any other place within sight of the Rock. A deputation of the Commissioners visited the Skerryvore in the month of July 1836, and concurred with the Engineer in regard to his choice of Hynish as a site for the Harbour and establishment.

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~Quarries at Hynish.~

Another most important point of inquiry was regarding the materials for building the Lighthouse; and on this subject the suggestions in a Report by the Engineer, of the 31st December 1835, were followed, which proposed the opening of quarries among the gneiss rocks around Hynish. Much facility was afforded by the liberality of the late Duke of Argyll, the proprietor of Tyree, who granted to the Commissioners free permission “to quarry materials for the purpose of the Lighthouse, on any part of the Argyll estates.” This freedom was generously continued by the present Duke, who has all along taken a lively interest in the success of the works. In terms, therefore, of the Engineer’s recommendations contained in the above noticed Report, Mr James Scott and fourteen quarriers were employed, during the summers of 1836 and 1837, in opening quarries, with very promising appearances of final success, among the gneiss rocks near Hynish Point. In the summer of 1837, Mr Scott and his party turned out about 3800 cubic feet of rock, capable of being applied to the purposes of squared masonry, and a very large quantity of stones fit for rubble work. This produce, although small, if contrasted with that of established quarries, is by no means despicable, when the _force_ employed and all the disadvantages of the situation are considered; and if the nature of the material, which is full of rents and fissures (technically called _dries_ and _cutters_), the frequent deceptions attending the opening of new quarries, the excessive hardness and unworkable nature of the rock, the quality and size of the blocks required to entitle them to claim a place in a marine tower, and the great loss of time, caused by the badness of the weather, be considered, it will not appear that Mr Scott and his party had been eating the bread of idleness.

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In the mean time, measures had been taken for obtaining from his Grace the Duke of Argyll a feu of fifteen acres of ground at Hynish, for carrying on the works, with a view to its being finally occupied as an establishment for the crew of the vessel which was to attend the Lighthouse, and the families of the four lightkeepers, as well as for the site of the harbour. To this was added a lease of thirty acres, for the various purposes connected with a workyard, and such an establishment as seemed necessary for carrying on the work. A subject of anxious deliberation with the Board, was the construction of the harbour at Hynish for the vessels engaged in the service of the work; and the Commissioners, on the 24th May 1837, authorized the Engineer to make arrangements for commencing the formation of the Pier. The work was, accordingly, undertaken, in terms of his Reports of the 31st December 1835, and 27th February 1836; and the summer of 1837 was chiefly occupied in preparing a wharf, mostly composed of _pierres-perdues_,[6] and in the opening of the quarry already noticed. Such may serve as a brief and somewhat desultory notice of the work during the seasons of 1836 and 1837, after which it appeared to the Board that the operations must soon assume such an aspect as to require the superintendence of a committee of their number, as well as that of an Engineer specially entrusted with the management of the work.

[6] Blocks rough from the quarry, which are dropt or thrown promiscuously into the sea.

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~Skerryvore Committee appointed.~

At the meeting of the Board, on the 8th December 1837, a Committee of their number was accordingly named, to superintend the erection of the Lighthouse. This Committee consisted of--ROBERT BRUCE, Esq., Sheriff of Argyllshire; ANDREW MURRAY, Esq., Sheriff of Aberdeenshire; ROBERT THOMSON, Esq., Sheriff of Caithness; and the late JAMES MACONOCHIE, Esq., Sheriff of Orkney and Zetland; and, shortly after its appointment, the Committee, on the motion of Mr BRUCE, the Chairman, appointed me Engineer for the work.

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Among the first matters which engaged the attention of this Committee, was a Report from the Engineer, dated the 30th January 1838, in which the necessity of erecting a wooden barrack, as a place of shelter for the workmen on the Rock, was pointed out; the general arrangements for carrying on the operations were described; and the building of a steam-tender, to act as a towing vessel for the stone lighters between the workyard in Tyree and the Rock, was also recommended. The Report was accompanied by a detailed requisition or estimate for the operations of the ensuing season, amounting to L.15,000 : 3 : 3; of which sum it was proposed to expend about one-third in building a steam-tender, and the rest in erecting the wooden barrack on the Rock, and in providing tools and materials for the work, as well as in the wages of men to be employed in preparing the foundation of the Lighthouse Tower, and in building the Pier, and dressing stones at Hynish.

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The Committee, after considerable deliberation, sanctioned the various items of the estimate, but hesitated to embark in the expense of building a steamer, until a fruitless correspondence with various ports of the kingdom, with the view of purchasing an old vessel, satisfied them of the necessity of building a tender expressly for the purpose.

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Offers were immediately received from various parties at Greenock for the preparation of the wooden barrack, which was soon afterwards commenced by the late Mr John Fleming, house-carpenter, who was the successful competitor.

Chapter III.

On The Construction Of Lighthouse Towers.

In this chapter I purpose, in the first place, to make a few observations regarding the construction of Lighthouse Towers in situations which are exposed to the assault of the waves, and afterwards to give a short notice of the design which I adopted for the Tower on the Skerryvore Rock. In making a design for a Lighthouse Tower in an exposed situation, numerous considerations at once present themselves to the Engineer; and it is difficult to assign to any one of them a priority in the train of thought which eventually conducts him to the formation of his plan. These considerations, however, may be conveniently divided into two classes:--_1st_, Those which refer to elements common to Lighthouses in all situations, and differ only in amount, such as the height of the Tower necessary for commanding a given visible horizon, and the accommodation required for the Lightkeepers and the Stores; and, _2d_, Those which are peculiar to Towers in exposed situations, and which refer solely to their fitness to resist the force of the waves which tend to destroy them. The first class of considerations is so extremely simple, as to require few remarks in this place. The distance at which it is desirable that a light should be visible being ascertained, with reference to the nature of the surrounding seas and the extent to which any dangerous or foul ground lies seaward of the proposed Lighthouse, the height of the Tower is at once determined by means of the known relations which subsist between the spheroidicity of the earth, the effects of atmospheric refraction, and the height required for an object which is to be seen from a given distance. The question regarding the space to be provided in the interior of the Tower, can only be properly answered by a person who has a minute practical acquaintance with the peculiar wants and the internal economy of Lighthouses. The accommodation required for Lighthouses in exposed situations must, in a considerable degree, depend upon the greater or less facility of access to them, and the opportunities for replenishing the stores of all kinds which are in daily consumption. In such places, also, the risk of accidents naturally leads to the precaution of retaining additional Lightkeepers, and of having duplicates or even triplets of those parts of the apparatus that are liable to be injured. Of such circumstances, corresponding extension of the space devoted to the reception of Stores and the accommodation of the Lightkeepers, is the necessary consequence. In the long nights of a Scotch winter, when the lamps are kept burning for about seventeen hours, during which time they are never left for a moment without the superintendence of at least one Keeper, the care of the light, even in the most favourable situations, necessarily occupies at least two persons; but in places like the Eddystone, the Bell Rock, and the Skerryvore, where it sometimes happens that six or eight weeks elapse without its being possible to effect a landing, it has been thought necessary that there should never be fewer than three Keepers on duty. This addition to the ordinary establishment of a Lighthouse calls for a greater number of sleeping-cabins, and, at the same time, involves a corresponding increase in the supply of water, fuel and other provisions, requiring much additional stowage. So far, therefore, a Light Tower in an exposed situation, differs from one on shore only in the extent of its internal accommodation.

The second class of considerations, which must guide the Engineer in framing a design for a Light Tower which is exposed to the force of the waves, refers solely to the stability of the building.

The first observation which must occur to any one who considers the subject is, that we know little of the nature, amount and modifications of the forces, on the proper investigation of which the application of the principle which regulates the construction must be based. When it is recollected, that, so far from possessing any accurate information regarding the momentum of the waves, we have little more than conjecture to guide us, it will be obvious, that we are not in a situation to estimate the power or intensity of those shocks to which Sea Towers are subject; and much less can we pretend to deal with the variations of these forces which shoals and obstructing rocks produce, or to determine the power of the waves as destructive agents. No systematic or intelligible attempt has been made practically to measure the force of the waves, so as to furnish the Engineer with a _constant_ to guide him in his attempts to oppose the inroads of the ocean. The only experiments, indeed, on the subject, with which I am acquainted, are those of Mr Thomas Stevenson, Civil-Engineer, who had long entertained the idea of registering the force of the impulse of the waves, and lately contrived an instrument for the purpose, which he has applied at various parts of the coast. I therefore gladly avail myself of the present opportunity, to give a brief statement of the results indicated by it, as contained in a paper by the inventor, which appeared in the Transactions of the Royal Society of Edinburgh of 20th January 1845, and of which a digest will be found in the Appendix, as any attempt to throw light upon this most obscure, but highly important subject, cannot fail to be interesting, not merely to the philosopher, but to the Marine Architect. It would naturally be expected, that the force of the waves should vary according to the season of the year, and the nature of the exposure, and this expectation is fully justified by the indications of the Marine Dynamometer. Thus it appears, that during five summer months of 1843 and 1844, the average indications registered at different places near Tyree and Skerryvore, gave 611 lb. of pressure per square foot of surface exposed to the waves; while the average for the winter months for the same places during those two years, gave 2086 lb. per square foot, or upwards of _three times_ that of the summer months. It also appears, that the greatest result as yet obtained at Skerryvore Rock was 4335 lb. per square foot; while that observed on the Bell Rock was 3013 lb., or _one-fourth part_ less than that of Skerryvore. But these experiments have not been continued long enough as yet to render them available for the Engineer. In the present state of our information, therefore, we cannot be said to possess the elements of exact investigation, and must consequently be guided chiefly by the results of those numerous cases which observation collects, and which reason arranges, in the form which constitutes what is called _professional experience_. This kind of experience can only be acquired by long habit in carefully observing the appearance and effects of waves in different situations, and under various circumstances. We must attend to their magnitude and velocity, their level in regard to the rocks on which they break, the height of the spray caused by their collision against the shore, the masses of rock which they have been able to move, and those which have successfully resisted their assault; as also, where such exist, the slopes of the shores produced by the waves, viewed in connection with the nature of the materials composing the beach, with many other transient features which an experienced eye seizes and fixes in the mind as elements of primary importance in determining the power of the sea to produce certain effects. Such phenomena, with all their features and circumstances, we may carry in our recollection; and by comparing them with what has been observed at places where we know that artificial works have resisted the shocks of the waves, we may in some cases successfully arrive at a conclusion as to what works will, at all events, be within the bounds of safety. We must not, however, in any case, venture to approach too near the limit of stability, so long as we continue to labour under our present disadvantages of defective information on some of the most important elements in the inquiry. If it be asked, therefore, how the size and form of buildings exposed to the shock of the waves are to be determined, the answer must be, that, in any given case, the problem is to be solved chiefly by the union of an extensive knowledge of what the sea has done against man, and how, and to what extent, man has succeeded in controlling the sea; together with a cautious comparison of the circumstances which modify and affect _any given case_ which has not been the object of direct experience; nor does it seem possible as yet to found the art of Engineering, in so far as it refers to this class of works, upon any more exact basis. The uncertainty which must ever attend such reasoning can only, it is obvious, be dispelled by actual experience of the result; and time only can test the success of our schemes in cases of difficulty.

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A primary inquiry, in regard to Towers in an exposed situation, is the question, whether their stability should depend upon their _strength_ or their _weight_; or, in other words, on their _cohesion_, or their _inertia_? In preferring _weight_ to _strength_, we more closely follow the course pointed out by the analogy of nature; and this must not be regarded as a mere notional advantage, for the more close the analogy between nature and our works, the less difficulty we shall experience in passing from nature to art, and the more directly will our observations on natural phenomena bear upon the artificial project. If, for example, we make a series of observations on the force of the sea, as exerted on masses of rock, and endeavour to draw from these observations some conclusions as to the amount and direction of that force, as exhibited by the masses of rock which resist it successfully and the forms which these masses assume, we shall pass naturally to the determination of the _mass_ and _form_ of a building which may be capable of opposing similar forces, as we conclude, with some reason, that the mass and form of the natural rock are exponents of the amount and direction of the forces they have so long continued to resist. It will readily be perceived, that we are in a very different and less advantageous position when we attempt, from such observations of natural phenomena, in which _weight_ is solely concerned, to deduce the _strength_ of an artificial fabric capable of resisting the same forces; for we must at once pass from one category to another, and endeavour to determine the _strength_ of a comparatively _light_ object which shall be able to sustain the same shock, which we know, by direct experience, may be resisted by a given _weight_. Another very obvious reason why we should prefer _mass_ and _weight_ to _strength_, as a source of stability, is, that the effect of mere _inertia_ is constant and unchangeable in its nature; while the _strength_ which results, even from the most judiciously disposed and well executed fixtures of a comparatively light fabric, is constantly subject to be impaired by the loosening of such fixtures, occasioned by the almost incessant tremor to which structures of this kind must be subject, from the beating of the waves.[7] Mass, therefore, seems to be a source of stability, the effect of which is at once apprehended by the mind, as more in harmony with the conservative principles of nature, and unquestionably less liable to be deteriorated than the _strength_, which depends upon the careful proportion and adjustment of parts.

[7] It was chiefly on these grounds that the Commissioners of Northern Lights, after consulting a Committee of the Royal Society of Edinburgh, and Messrs Cubitt and Rennie, Civil Engineers, rejected the design of Captain Sir Samuel Brown, R. N., who volunteered a proposal to build an Iron Pillar at the time that the erection of the Skerryvore Lighthouse was determined on in 1835.

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Having satisfied himself that _weight_ is the most eligible source of stability, the next step of the Engineer is to inquire what quantity of matter is necessary to produce stability, and what is the most advantageous form for its arrangement in a tower. The first question, which respects the mass to be employed, is, as already stated, one of the utmost difficulty, and can be solved by experience alone, directed by that natural sagacity which Smeaton, in his account of his own thoughts on the subject, with much _naïveté_, terms ‘_feelings_,’ in contradistinction to that more accurate process of deduction which he calls ‘_calculation_.’ It is very difficult, for example, to conceive that the waves could displace a cylindric block of granite, 25 feet in diameter and 10 feet high, which would contain about 380 tons, and we almost _feel_ that they could not do so. If, in order to test the soundness of this expectation, we appeal to such experience as we possess, and apply to the _largest vertical section_ of such a solid, the greatest force yet indicated by my brother’s Marine Dynamometer, which, as already stated, is 4335 lb. per square foot, we shall obtain a pressure of 484 tons, which, being reduced by _one-half_[8] for the loss of force occasioned by the convexity of the opposing cylindric surface, gives 242 tons, as the greatest force of the waves tending to displace the cylinder. But in the extreme case we have now supposed the solid will be entirely immersed in the water, and its efficient weight will thus be reduced by 140 tons, or the weight of an equal bulk of sea-water; and the remaining weight of 240 tons, by which it will resist the force of the waves, will be almost exactly equal to the pressure which they exert. This imaginary cylinder may, however, be regarded as still within the limits of safety, because the waves could not overturn it, unless their pressure exceeded the weight of the block in a ratio greater than that of its diameter to its height, which in this case is that of 25 to 10, or 2¹⁄₂ times. In order, therefore, to endanger the stability of the solid by overturning it, the pressure, instead of being 240 tons, must be 600 tons.[9] We have thus seen, that the cylinder is secure from the chance of being overturned; but we have yet to consider how far it is exempt from the risk of being displaced by the pressure of the waves, causing it to slide along the surface of the Rock, owing to deficiency of friction between the two surfaces in contact. The block, for our present purposes, may be regarded as _monolithic_, either being really so or as a mass composed of parts so united by joggles, treenails and mortar, as to be free from any tendency to disintegration by the force of the waves; and in this case the stability of the cylinder will depend upon the amount of friction opposing the pressure of the waves which tends to produce a sliding movement. It appears, by some experiments of M. Rondelet,[10] that the friction of a block of stone sliding on a chiselled floor of rock is equal to ⁷⁄₁₀ths of its own weight; and we should thus obtain in the present instance 168 tons, as the amount of friction tending to resist the pressure of the waves, which would therefore exert a power superior to that resistance by 74 tons.[11] But this excess of force would be easily neutralized by the adhesion of the mortar and the abutment of the block against the sides of the foundation pit into which Lighthouse Towers in such exposed places are generally sunk in the solid rock. When, in addition to these considerations, we learn that the solid frustum, or lower part of the Eddystone Tower, which has weathered so many storms for the last ninety years, does not greatly exceed in mass the imaginary cylindric block which I have spoken of, our confidence in the stability of the cylinder is greatly increased. Our belief receives a still farther confirmation from the fact, that the strongest instance recorded of the power of the waves, falls considerably short of the case which we have just imagined. The instance alluded to is given in Mr Lyell’s Geology, on the authority of the Reverend George Low, of Fetlar, in Zetland, who mentions, that a block, whose dimensions seem to give us reason to estimate its weight at nearly 300 tons (or about _one-fifth_ less than that of the cylinder), was moved over a point, and thrown into the sea; and it must be remembered, that the form of this block, which was only 5 feet thick and about 40 feet long, rendered it very susceptible of a sliding motion, and must have greatly aided its transport. We may therefore not unreasonably conclude, that, in designing such a tower, it is safe to assume a mass which our own judgment and recorded facts seem to concur in pronouncing beyond the power of the greatest waves, as fixing the _lowest_ limit to which the contents of the proposed edifice may be reduced.

[8] This reduction seems to be warranted by the results of some experiments of Bossut.

[9] This is the product of 240 tons, by the ratio of 2·5.

[10] L’art de bâtir.

[11] The number 168 is ⁷⁄₁₀ths of 240, which is the weight of the cylinder, reduced by the weight of an equal bulk of salt water; and 74 is the excess of 242 tons, the pressure of the waves, above 168, the amount of friction.

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There are several circumstances, however, which tend to increase or diminish the stability of the same mass exposed to the same forces. Of these a very prominent one is the _form_ of the mass, which may be so modified as to offer more or less resistance to the forces which assault the building. Thus a parallelopiped would be a much less suitable form for a sea tower than a cylinder, and so proportionally of all the polygonal prisms which may occur between these two extremes. I remember having heard it proposed, in the course of conversation, by a non-professional friend, that Lighthouse Towers might be formed in such a manner, that each horizontal section should be a wedge with its narrow end directed to the greatest assaulting force. This notion is in itself not destitute of ingenuity; for, if the circumstances to which it is to be adapted were constant, we should thereby present the form of least resistance, and, at the same time, the greatest depth and strength of the building to the line of greatest impulse. But the notion is wholly impracticable, because the direction of the winds and waves is so variable, as to render it almost certain that a Tower so constructed would, on some occasion, be assaulted in the line of its thinnest section; and thus, what might in one case be an advantage, would, in the event of such a change in the point of attack, become a great source of weakness, as the flat side of the wedge would then be opposed to the force, thereby presenting to the direct assault of the waves the largest surface, with, at the same time, the most disadvantageous disposition of the resisting matter. There seems little reason, therefore, for any doubt as to the circular section being practically the most suitable for a Tower exposed in every direction to the force of the waves.

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Next to this, and hardly to be separated from it, inasmuch as it involves the question regarding the form of the Tower, is the position of the centre of gravity. The stability of any solid will, in general, greatly depend upon its centre of gravity being placed as low as possible; and the general sectional form which this notion of stability indicates is that of a triangle. This figure revolving on its vertical axis, must, of course, generate a cone as the solid, which has its centre of gravity most advantageously placed, while its rounded contour would oppose the _least_ resistance which is attainable in _every_ direction. Whether, therefore, we make _strength_ or _weight_ the source of stability, the conic frustum seems, abstractly speaking, the most advantageous form for a high Tower. But there are various considerations which concur to modify this general conclusion, and, in practice, to render the conical form less eligible than might at first be imagined. Of these considerations, the most prominent theoretically, although, I must confess, not the most influential in guiding our practice, is, that the base of the cone must in many cases meet the foundation on which the Tower is to stand, in such a manner, as to form an angular space in which the waves may break with violence. The second objection is more considerable in practice, and is founded on the disadvantageous arrangement of the materials, which would take place in a conic frustum carried to the great height which Lighthouse Towers must generally attain, in order to render them useful as sea-marks. Towards its top, the Tower cannot be assaulted with so great a force as at the base, or, rather, its top is entirely above the shock of heavy waves; and, as the conoidal solid should be prolate in proportion to the intensity of the shock which it must resist, it follows that, if the base be constructed as a frustum of a given cone, the top part ought to be formed of successive frusta of other cones, gradually less prolate than that of the base. But it is obvious, that the union of frusta of different cones, independently of the objection which might be urged against the _sudden_ change of direction at their junction, as affording the waves a point for advantageous assault, would form a figure of inharmonious and unpleasing contour, circumstances which necessarily lead to the adoption of a curve osculating the outline of the successive frusta composing the Tower; and hence, we can hardly doubt, has really arisen in the mind of Smeaton the beautiful form which his genius invented for the Lighthouse Tower of the Eddystone, and which subsequent Engineers have contented themselves to copy, as the general outline which meets all the conditions of the problem which they have to solve. And here I cannot help observing, as an interesting, and by no means unusual, psychological fact, that men sometimes appear to be conducted to a right conclusion by an erroneous train of reasoning; and such, from his “Narrative,” we are led to believe, must have been the case with Smeaton in his own conception of the form most suitable for his great work. In that “Narrative” (§ 81), he seems to imply, that the trunk of an oak was the counterpart or antitype of that form which his (§ 246) “feelings, rather than _calculations_,” led him to prefer. Now, there is no analogy between the case of the tree and that of the Lighthouse, the tree being assaulted at the top, and the Lighthouse at the base; and although Smeaton goes on, in the course of the paragraph above alluded to, to suppose the branches to be cut off, and water to wash round the base of the oak, it is to be feared the analogy is not thereby strengthened; as the _materials_ composing the tree and the tower are so different, that it is impossible to imagine that the same opposing forces can be resisted by similar properties in both. It is obvious, indeed, that Smeaton has unconsciously contrived to obscure his own clear conceptions in his attempt to connect them with a fancied natural analogy between a tree which is shaken by the _wind_ acting on its _bushy top_, and which resists its enemy by the _strength_ of its fibrous texture and wide-spreading ligamentous roots, and a tower of masonry, whose _weight_ and _friction_ alone enable it to meet the assault of the _waves_ which wash round its _base_; and it is very singular, that, throughout his reasonings on this subject, he does not appear to have regarded those properties of the tree which he has most fitly characterized as “its elasticity,” and the “coherence of its parts.” One is tempted to conclude that Smeaton had, in the first place, reasoned quite soundly, and arrived by a perfectly legitimate process at his true conclusion; and that it was only in the vain attempt to justify these conclusions to others, and convey to them conceptions which a large class of minds can never receive, that he has misrepresented his own mode of reasoning. In the paragraph preceding that which refers to the tree (§ 80), he has, in point of fact, clearly developed the true views of the subject; and, with the single exception of the allusion to the oak, he has discussed the question throughout in a masterly style.

* * * * *

In a word, then, the sum of our knowledge appears to be contained in this proposition--_That, as the stability of a sea-tower depends_, cæteris paribus, _on the lowness of its centre of gravity, the general notion of its form is that of a cone; but that, as the forces to which its several horizontal sections are opposed decrease towards its top in a rapid ratio, the solid should be generated by the revolution of some curve line convex to the axis of the tower, and gradually approaching to parallelism with it_. And this is, in fact, a general description of the Eddystone Tower devised by Smeaton.

* * * * *

[Illustration: No. 1.]

It is deserving of notice, as one of the many proofs which the records of antiquity afford of the similarity of the results of human thought in all ages, and of the truth of the Wise Man’s saying, that “there is nothing new under the sun,” that the ancient Egyptians appear to have had the same conceptions of the solid of stability that were present to the mind of the modern Engineer of the Eddystone Lighthouse. In the admirable work recently published by Sir J. Gardner Wilkinson on the Manners and Customs of the Ancient Egyptians, he gives, in the first volume of his second series, at page 253, a wood-cut, shewing the figure of the deity Pthah, under the symbol of stability, according to Egyptian conceptions. This symbol so closely and strikingly resembles the general appearance of the Eddystone, that I willingly give it a place in the text, (No. 1) denuded, however, of the arms and head-dress of the deity whom it shrouds.

* * * * *

In applying these general notions to the design of a Tower for the Skerryvore Rock, I was, of course, guided by numerous circumstances, which modified my views and produced the individual form of Tower which I have adopted. Since the days of Smeaton, when his magnificent Tower was lighted by common candles, the application of optical apparatus to Lighthouses has greatly altered the state of the case; and the improvement of the system in modern times has, in most instances, rendered a greater altitude of Tower desirable, in order to extend, as much as possible, the benefit of a system capable of illuminating the visible horizon of any Tower which human art can reasonably hope to construct. In the particular case of the Skerryvore, also, the great distance of the outlying rocks (some of which, as will be seen from the chart, are 3 miles right seaward of the Lighthouse) concurs with the improvement of the Lights, in making it desirable that the Tower should be of considerable height, and that the light should command an extensive range. It was, therefore, from the first consideration of the subject, determined that the Light should be elevated about 150 feet above high water of spring tides, so as to illuminate a visible horizon of not less than 18 miles of radius; and, after much deliberation, and a full consideration of the infrequency of communication with the proposed Lighthouse from the great difficulty of landing on the Rock, and the consequent uncertainty of keeping up the supplies, I found that, for the convenient accommodation of the Lightkeepers and the suitable stowage of the stores, a void space of about 13,000 cubic feet would be required. These elements being fixed, the general proportions of the Tower came next to be considered.

* * * * *

In the Eddystone the radius of the base, at the level of high water of spring tides, is somewhat less than _one-fifth_ of the height of the Tower above that level; while in the Bell Rock, at the same level, it is little more than _one-seventh_ of the height. If, again, we suppose the curve of the Eddystone to be continued downwards to the level of low water, the radius (in so far as we may judge from sketching the continuation of a curve undefined by any geometrical property) would be rather more than _one-fourth_ of the whole height above that level; while in the Bell Rock the proportion, in reference to the same level, is a little more than _one-fifth_. Viewing the whole height of the Skerryvore Tower above _high water_ of spring tides as equal to 142 feet, and finding that, in the cases of the Eddystone and the Bell Rock, the radius of the horizontal section at that level is respectively _one-fifth_ and _one-seventh_ of the whole height; and again, viewing the extreme height of the Skerryvore Tower above _low water_ of spring tides as equal to about 155 feet, and considering the proportionate radii of the Bell Rock and Eddystone (in so far as the latter is ascertainable) as respectively _one-fifth_ and _one-fourth_ of the heights of the top of the masonry above the level of low water, I finally decided upon giving the Tower at the Skerryvore such dimensions as would not be widely discordant with these general proportions. In this view, I determined that the radius of the base should not exceed 22 feet, on the level of about 4 feet above the high water mark, where I expected to obtain a solid foundation--a base which bears to the whole height of the Tower a proportion somewhat _less_ than that of the Bell Rock, which is _one-fifth_. It so happens, that the diameter adopted is nearly the greatest which the Rock affords; for, although a glance at the accompanying plan of the Rock at high water (Plate, No. III.) would lead one to suppose that a more extended base might have been obtained, I found, after many careful examinations of the gullies and fissures which intersect it, that some of the concealed fissures run much farther into the Rock than might at first be imagined. The adoption of a much larger base, even had it been otherwise advisable, would therefore have involved some risk of the external ring of stones of the lowest course giving way by the yielding of an unsound part of the outer portion of the Rock to the pressure of the superincumbent mass, and might eventually have led to the destruction of the Tower.

[Illustration: No. 2.]

The height of the Pillar having been finally fixed at 138·5 feet, and the radius of the base, at the level of about 4 feet above high water, at 21 feet, I next proceeded to consider the details of its proportions. Of the whole height of 138·5 feet, 18 were to be absorbed in a suitable capital for the Pillar, consisting of a parapet for the Lantern, an abacus, a cavetto, and a belt separating these from the shaft. The internal void I determined should be 12 feet in diameter, as the size most suitable for the reception of the lantern and apparatus; and this, combined with the choice of about 13,000 cubic feet of void already mentioned, fixed the height of the solid frustum at the base of the Tower at about 26 feet above the foundation. Having farther decided that the thinnest part of the walls, immediately under the belt-course which separates the capital from the shaft, should not be less than 2 feet thick, as necessary to give due solidity and strength to the walls, and prevent, by the breadth of the joints, the percolation through the walls of the water which might be furiously dashed against them in storms, I had nothing farther to do but to determine the nature of the line which should connect the extremities of the top and bottom radii of the Pillar. As I had already concluded that this line must, as in the Eddystone and Bell Rock, be a curve line, concave to the sea, I next proceeded to try the effects of various curves traced between these points, in giving a convenient and advantageous disposition of the materials, with regard to both the thickness of the walls and the mass of the solid frustum at the base of the Tower. These two points, as will be better understood by means of the accompanying diagram (No. 2), are separated from each other vertically 120·25 feet, and are horizontally distant from each other 13 feet, which is the excess of the bottom radius over that of the top of the shaft, or the consequent amount of what may be called the _aggregate slope_ of the wall. The solid generated by the revolution of some curve line about the vertical axis of the building then becomes the shaft of the pillar. For this purpose I tried four different curves, the Parabola, Logarithmic, Hyperbola, and Conchoid, figures of which, upon the same scale, will be found in Plate, No. IV., with the position of the centre of gravity, which was carefully calculated, marked on each. The logarithmic curve I at once rejected, from its too near approach to a conic frustum, and the excessive thickness of the walls which such a figure would produce, where the hollow cylindric space for the internal accommodation commences at the level of 26 feet above the base. The parabolic form displeased my eye by the too rapid change of its slope near the base; and I had some difficulty in reconciling myself to the condition of the exterior ring of stones at the base, too much of the outer portion of each stone being left without the advantage of direct pressure from the superincumbent mass of the wall above. The two remaining pillars, derived from the hyperbolic and conchoidal[12] frusta, are nearly identical in form; and of these two curves I preferred the former, which gives the most advantageous arrangement of materials, in regard to stability, of all the four forms. This quality of advantageous proportion exists in these forms, in the ratio of the numbers in the last column of the following table:[13] which shews a slight superiority of the Hyperbolic over any of the other forms.

+------------+------+-----------+--------+---------+-----+----------+ | | | | Volume | Distance| | | | |Height| Diameter | of | of | | | | |of the| | | solid |centre of| | Economic | | | Tower| at | at | Tower | Gravity | |Advantage.| | | in | Base| Top |in cubic| from | H | G·M. | |Hypothetical| feet.| in | in | feet. | Base. | - | ------ | | Towers. | (H.)|feet.|feet.| M. | G. | G | G′·M′. | +------------+------+-----+-----+--------+---------+-----+----------+ |Hyperbolic, | 120 | 42 | 16 | 62,915 | 41·227 |2·911| 1·00000 | |Conchoidal, | 120 | 42 | 16 | 62,984 | 41·336 |2·903| 0·99627 | |Parabolic, | 120 | 42 | 16 | 63,605 | 43·400 |2·765| 0·93963 | |Logarithmic,| 120 | 42 | 16 | 74,742 | 42·460 |2·826| 0·81608 | |Conical, | 120 | 42 | 16 | 84,737 | 43·280 |2·773| 0·70725 | +------------+------+-----+-----+--------+---------+-----+----------+

[12] The solid, in this case, would have been formed by the revolution of the interior conchoid of Nicomedes about its directrix; and its co-ordinates were kindly calculated for me by my late revered preceptor, Dr WALLACE, Professor of Mathematics in the University of Edinburgh, who employed so many hours of his latter years in labours of kindness among his friends. This act of the Professor was the result of a conversation I had with him on the subject. Before I received his friendly communication, however, I had resolved to adopt the rectangular hyperbola, whose co-ordinates I had myself determined with this view some time before; and when I found that the conchoid and the hyperbola, traced between the two fixed points by means of the calculated co-ordinates, were so nearly coincident, that it was difficult to prevent their running into each other, even when drawn out on a large scale, I determined to adhere to my original purpose of adopting the latter curve as my guide.

[13] The last column of this table is derived as follows:--Assuming that the economic advantage of any proposed tower of given height and diameter at base and top, is _inversely_ as the mass and the height of the centre of gravity above the base, and denoting these quantities by M and G respectively, the fraction

1 --- G·M

may be taken as an indication of the economic advantage of the proposed tower. Let

1 ----- G′·M′

express the economic advantage of another tower; then the advantage of the second tower, compared to that of the first, taken as unity, will be

G·M -----, G′·M′

by which expression, the last column in the table was calculated.

The shaft of the Skerryvore Pillar, accordingly, is a solid, generated by the revolution of a rectangular hyperbola about its asymptote as a vertical axis. Its exact height is 120·25 feet, and its diameter at the base 42 feet, and at the top 16 feet. The ordinates of the curve, at every foot of the height of the column, were carefully determined in feet to three places of decimals; and the Appendix contains a tabular view of the co-ordinates from which the working drawings were made at full size. The first 26 feet of height is a solid frustum, containing about 27,110 cubic feet, and weighing about 1990 tons.[14] Immediately above this level the walls are 9·58 feet thick, whence they gradually decrease throughout the whole height of the shaft, until at the belt they are reduced to 2 feet in thickness. Above the shaft rests a cylindric belt 18 inches deep; and this is surmounted by a cavetto 6 feet high, and having 3 feet of projection. The contour of this cavetto is that resulting from a quadrant of an ellipse revolving about the centre of the tower, with a radius of 8 feet on the level of its transverse axis; and the moulds for this curve were drawn at full size from co-ordinates calculated for the purpose. The cavetto supports an abacus 3 feet deep, the upper surface of which forms the balcony of the tower, and above it rest the parapet-wall and lantern.

[14] At the rate of 13·62 cubic feet of granite to a ton.

It may, perhaps, be not uninteresting to the reader to examine the woodcuts (No. 3), which shew, on one scale, the elevations of the Lighthouses of the Eddystone, the Bell Rock, and the Skerryvore, and exhibit the level of their foundations in relation to high water. They will also serve to give some idea of the proportionate masses of the three buildings. The position of the centre of gravity, as calculated from measurements of the solids, is also marked by a round black dot on each tower; and in the table following, I have given the cubic contents of each of these towers, the height of the centre of gravity above the base and the ratio of that quantity to the height of the tower.

[Illustration: No. 3.

Eddystone.

Skerryvore.

Bell Rock.]

+-----------+-------+--------+----------+----------+----+ | | Height| | | | | | | of | | | Distance | | | | Tower | | | of | | | | above | | |centre of | | | | first | | | gravity | | | | entire|Contents| Diameter | in feet | H | | |course.| of | at | at |from Base.| - | |Lighthouse.| (H) | Tower. |Base.|Top.| (G) | G | +-----------+-------+--------+-----+----+----------+----+ |Eddystone, | 68 | 13,343 | 26 | 15 | 15·92 |4·27| |Bell Rock, | 100 | 28,530 | 42 | 15 | 23·59 |4·24| |Skerryvore,| 138·5 | 58,580 | 42 | 16 | 34·95 |3·96| +-----------+-------+--------+-----+----+----------+----+

I come now to notice the few subordinate points in which the design of the Skerryvore Tower may be regarded as differing from those of the Eddystone and the Bell Rock. In glancing at the contrasted figures of the three buildings, it will be at once observed that the outline of the Skerryvore approaches more nearly to that of a conic frustum than the other two. To the adoption of this form, various considerations induced me; and these I shall very briefly detail. In the first place, it seemed to me that, in both the Bell Rock and the Eddystone, the thickness of the walls had been reduced to the lowest limits of safety towards the top; and the effects of the sea and wind acting upon a heavy cornice, cause a degree of tremor which I felt satisfied would not occur in a building with thicker walls. The effect of thickening the walls at the top, is, of course, _cæteris paribus_, to diminish the projection of the base, and thus to produce less concavity of figure, and consequently a nearer approximation to the contour of a conic frustum. I have already stated, that this excess of the bottom radius over that of the top, is in the Skerryvore Tower 13 feet, and that the height of the shaft is 120·25 feet. The quotient resulting from the division of the height by the excess of bottom radius over that at the top is 9·27; and, if the figure had been conical, this number would have given a measure of the slope of the walls throughout. There can be little doubt that the more nearly we approach to the perpendicular, the more fully do the stones at the base receive the effect of the pressure of the superincumbent mass as a means of retaining them in their places, and the more perfectly does this pressure act as a bond of union among the parts of the Tower. This consideration materially weighed with me in making a more near approach to the conic frustum, which, next to the perpendicular wall, must, other circumstances being equal, possess the property of pressing the mass below with a greater weight, and in a more advantageous manner, than a curved outline in which the stones at the base are necessarily farther removed from the line of the vertical pressure of the mass at the top.[15] This vertical pressure operates in preventing any stone being withdrawn from the wall in a manner which, to my mind, is much more satisfactory than an excessive refinement in _dovetailing_ and _joggling_, which I consider as chiefly useful in the early stages of the progress of a work, when it is exposed to storms, and before the superstructure is raised to such a height as to prevent seas from breaking right over it.

[15] It is most satisfactory to find that the views expressed above, regarding the eligibility of the conical form, seem to have the sanction of the late Dr Thomas Young, who appears to have connected his preference of this form with its greater efficiency as a source of friction among the parts of a building. In his syllabus of Lectures, under the section “Architecture,” he thus speaks: “For a Lighthouse where a great force of wind and water was to be resisted, Mr Smeaton chose a curve convex to the axis. In such a case, the strength depends more on weight than on cohesion, and also in a considerable degree on the friction which is the effect of that weight. Perhaps a cone would be an eligible form.”

If these views be substantially correct, it may not, perhaps, be altogether inadmissible (without, however, venturing to enunciate any general law) to conclude, that, in the three Lighthouses of the Eddystone, the Bell Rock, and the Skerryvore, this source of union among the outer stones of the lower courses must bear some proportion to the numbers 753, 659, and 927, which are the quotients of the height of the column, divided by the difference of the top and bottom radii of the shaft in each case respectively. This consideration seems too important to be entirely overlooked; and I conceive that, by following out this view, I have been enabled to depart with perfect safety from the intricate and elaborate work required for the connection of the materials by means of dovetailing and joggling, which the adoption of a more concave outline (in which the vertical pressure could not have been so advantageously transmitted to the outer stones of the base), would perhaps have rendered advisable. In the case of the Bell Rock, however, whose construction, in regard to this property, is the least advantageous of the three buildings, it must be borne in mind that the Tower is covered to the depth of 15 feet at spring tides, and that this principle of vertical pressure could not have been safely appealed to during the whole time which intervened between the commencement of the building and the attainment of a height sufficient to render it available, which, in a Tower having so great a part submerged, was of necessity much prolonged. The stones were thus exposed to the full effect of heavy seas, at all levels, during two entire winters, and could not therefore have been safely left, without being kept together by numerous ties and dovetails. It also seemed important, in designing that Tower, with reference to the rise of tide, to give its lower part a sloping form, as the least likely to obstruct the free passage of the waves. The outer stones of the lower courses were also selected of unusual length _inwards_, so as to bring them more under the influence of the vertical pressure of the upper wall.

Before leaving this subject, I may remark, that it is quite possible to construct a Tower of a curved form, in such a manner, that the pressure of the upper part of the pillar shall be distributed to the greatest advantage on every stone, by building the outer walls as inverted arches, so that the section of each stone shall be that of a voussoir, with joints perpendicular to the successive tangents of the curve. This arrangement of the stones is, in fact, practised in sea walls of various kinds, and has even been recommended for circular Towers in an ingenious paper in the Transactions of the Royal Scottish Society of Arts. But in many situations, and at Skerryvore in particular, this mode of transmitting the pressure, so as to throw it perpendicular to the beds of the stones, is inadmissible, as conducing to or involving a greater evil. The evil has already been noticed, and consists in the thrust of the lowest stone (which is of course inclined to the horizon) having a tendency to push out the sides of the Rock on which the Tower is built. This fear, where the Towers are to be placed on small steep rocks or pinnacles, and more especially when these Rocks are traversed by veins nearly vertical, is by no means visionary; and there is good reason to apprehend, that the pressure thus resulting in a line considerably inclined to the plane of _cleavage_, might throw outwards a thin portion of rock, which, under the more conservative influence of a vertical pressure, might continue to retain its connection with the rest of the Rock unimpaired for ages.

Another method of, in some degree, increasing the resistance of a Sea Tower to a horizontal thrust, if such aid be required, is to give the line of courses a continuous spiral form, instead of building them in successive horizontal layers. Were there reason to fear that the entire dislocation of the building might take place in a plane nearly horizontal, this method seems more calculated to counteract the danger than the use of dowels or joggles passing from the course below to the course above; but, as this is one of the accidents least to be apprehended, there does not seem any good ground for resorting to a mode of structure which would lead to considerable intricacy of workmanship, and would, in practice, be attended with difficulty in obtaining a proper vertical bond or union among the several stones.

The only remaining point, in which the example furnished by the Eddystone and Bell Rock Lighthouses has been at all materially departed from, is (as has already been hinted at by an unavoidable anticipation) the mode of uniting the different parts of the masonry together. In both these Towers the stones were dovetailed throughout the buildings, chiefly (at least in the case of the Bell Rock where the foundation was so much below the tide) with the view of preventing the sea from washing away the courses which might be left exposed to the winter storms before the weight of the superstructure had been brought to bear upon them. In the upper part of the Bell Rock my father also introduced a kind of band joggle, which consists of a flat ribband of stone raised upon the upper bed of one course, and fitting into a corresponding groove cut in the under bed of the course above; and this system of tying the adjoining courses together also forms a chief feature in his design for a Lighthouse on the Wolf Rock.[16] When the great pressure of the superstructure of these Towers, however, and the effect of the mortar are considered, there seems little probability of one course being dislocated, in defiance of the friction resulting from the weight of the column. An impulse sufficient to produce such an effect would tend to overset the whole superstructure from mere deficiency in weight, and in this case the joggle would have little effect. But if joggles be thought necessary for this purpose, the ribband form certainly produces a better arrangement than that of the cubic joggles employed by Smeaton for connecting the adjoining courses of his building together, as the sectional strength of these scattered square joggles is very small compared to the effect of a shock which could be supposed capable of moving the whole mass of a Tower. In the lower parts of the Skerryvore Tower, I entirely dispensed with dovetailing and _joggles between the courses_, and thus avoided much expensive dressing of materials. The stones were retained in their places during the early progress of the work, chiefly by common diamond joggles, and the courses were temporarily united to each other by wooden treenails, like those used in the Eddystone and Bell Rock. These treenails had split ends, with small wedges of hardwood loosely inserted, which being forced against the bottom of the holes in the course below, into which the treenails were driven, expanded their lower ends until they pressed against the sides of the holes; while their tops were made tight by similar wedges driven into them with a mallet. I have, however, adopted the ribband-joggle in the higher part of the Tower, where the walls begin to get thin in the very same manner as at the Bell Rock, where it was used, partly that it might counteract any tendency to a _spreading_ outwards of the stones, and partly that it might operate as a kind of _false joint_ to exclude the water which, when pressed with great violence against the Tower, is apt to be forced through a straight or plain joint. The stones in the higher courses throughout each ring are also connected at the ends by double dovetailed joggles, which unite the two adjoining stones; and the walls are, besides, tied together at various points by means of the floor stones, which are all connected by dovetails let into large circular stones which form the centres of the floors. I also ventured to leave out the metallic ties at the cornice, which consisted, at the Eddystone, of chains, and, at the Bell Rock, of copper rings. The reasons which induced me to adopt this change I need not here enlarge upon. It is sufficient to state, that I believe I have nearly balanced the forces which would have tended to throw the cornice outwards, had a greater disproportion existed in the weight of the outer and inner parts of the cavetto, and to point out (Plate VII.) that the Lightroom or highest floor occurs, at such a level, as of itself to answer all the ends which metallic ties could have served.

[16] Account of the Bell Rock Lighthouse, Plate XXI.

Chapter IV.

Operations Of 1838.

~Temporary Barrack on Rock.~

The hazardous nature of the anchorage, and the consequent difficulty of mooring a vessel in the neighbourhood of the Skerryvore Rock, induced me, from the first, to consider it as a matter of great importance, even at a large expenditure of time and money, to erect some temporary dwelling on the Rock for the accommodation of the people engaged in the work, with the view of rendering the operations less dependent on the state of the sea, which varied with every wind. So important, indeed, did this object appear to me, that I was at times apt to look upon it as an indispensable step towards ultimate success. That opinion was amply confirmed during our first season’s operations, by the experience of the oft-recurring difficulty of returning to the moorings when driven away by stress of weather, together with the daily risk and loss of time in landing the workmen in small boats, even in weather when they could be profitably occupied if once placed on this small _terra firma_. With this view, I naturally turned to the same plan which had been adopted at the Bell Rock, where the temporary barrack stood the test of five winters. That structure, which is represented in Plate No. V., and is particularly described in the Appendix to my father’s Account of the Bell Rock Lighthouse, consisted of an open framework of six logs, about 47 feet long and 13 inches square, assembled in such a manner as to form by their union a hexagonal pyramid, on the top of which rested a wooden turret; the whole erection rising to the height of about 60 feet above the rock. This pyramidal framework was strongly trussed and tied; and, being open at the lower part, offered little resistance to the waves. The upper part contained a gallery for keeping various stores and such materials as could not be safely left on the Rock, even in the finest weather; but it was framed of lighter materials, so as to admit of its yielding easily to any extraordinary waves, without involving injury to the principal part of the structure, by offering great resistance to the sea. The turret on the top was in the form of a twelve-sided prism, 12 feet in diameter, and 30 feet high, and was securely attached, by means of the ties and braces shewn in the drawing, to the apex of the pyramid, which entered into the lower part of it. The small space which the turret afforded was, with the utmost economy of room, divided into three storeys, of which the lower was entirely taken up by the kitchen and the bread-store, a great deal of room being occupied by the main beams of the pyramid which passed through its centre. The next storey was subdivided into two chambers, of which one was appropriated to the foreman of the works and the landing-master, while the other was set apart for myself; and the top storey, which was surmounted by a small lantern and ventilator, formed a barrack room, capable of containing 30 people. Of the comforts and discomforts of this habitation I shall at some future time have occasion to speak. I merely draw attention to its erection at present, as an operation, which it was most desirable should precede every other work on the Rock. One of the first proceedings, therefore, was to obtain estimates for the preparation of this log-house, which, in order to avoid loss of time in making adjustments on the Rock, was to be carefully fitted up in the workyard of the contractor before being shipped. Drawings and a specification were accordingly prepared, and submitted to several carpenters in Greenock, who gave in offers for the work; and it was finally commenced in the month of March, by the late Mr John Fleming, who was the successful offerer.

~Tools and Machinery.~

It was also necessary to provide a large assortment of quarriers’ and masons’ tools of every kind; and many cranes, crabs, anchors, mooring buoys and other implements were ordered, according to detailed specifications and drawings. These preparations necessarily occupied the early part of the year 1838.

~Steam Tender for the Works.~

From the extent of the foul ground round the Skerryvore, and the absence of good harbours in the neighbourhood, it was foreseen at the outset that the operation of landing about 6000 tons of materials on the Rock could not be accomplished by means of sailing vessels with that degree of certainty or regularity which was desirable, in order to obtain the full benefit of the short working season which the climate of the Western Hebrides affords; and the necessity for providing a steam tender was, therefore, generally admitted. It has already been stated, that, in order to avoid the expense attending the building of a vessel for this purpose, application was made at the principal ports of the kingdom, with the view of purchasing a suitable vessel; but, although twenty-four vessels of nearly the required dimensions were offered for sale, not one of them was considered fit for such a service, the great majority being light craft, such as are generally used in river and port navigation. It was therefore found necessary to build a steamer; for which purpose, specifications and drawings were prepared, and after receiving various tenders from respectable parties, a contract was entered into with Messrs Menzies and Sons, shipbuilders, and Messrs J. B. Maxton and Co., engineers, both of Leith, for building a steamer of 150 tons, with two engines of 30 horse power each.

The use of a steamer, at the very outset of the works, would doubtless have proved of the greatest service in the erection of the barrack on the Rock, and would have materially lightened our cares and toils; but I am not sure that I should have acquired so thorough an acquaintance with the difficulties and dangers of the Skerryvore, or that I should have been so well prepared for all the obstacles that presented themselves in the after parts of the work, had the first season’s operations been conducted under those advantages which are always derived from the use of steam-power. As it was, we had much to bear from the smallness of the Lighthouse Tender, named the _Pharos_, a vessel of 36 tons, new register, which was all the regular shipping attendance we possessed during this first season; and the inconvenience arising from her heavy pitching, was, to landsmen, by no means the least evil to be endured. But the frequent loss of opportunities, of which we might easily have availed ourselves, if we had possessed the command of steam-power, and the danger and difficulty of managing a sailing vessel in the foul ground near the Rock, and between it and Tyree, were, perhaps, even more felt by the seamen than by the landsmen; and if the experience of a single year’s work can form any ground for an estimate of the length of time required for building the Skerryvore Lighthouse, with a sailing vessel, I should say, we must still (even in 1845) have been engaged in the masonry part of the work, which was finished on the 25th July 1842.

~Employment and Wages of Workmen.~

About the middle of April, arrangements were made with Mr Charles Neilson, a builder in Aberdeen, to select granite masons for the works at the Skerryvore, as it was expected that the operation of dressing stones for the Tower would be begun in the ensuing summer; and it was also obvious, that their services would be required in excavating seats for the supports of the Barrack-house on the Rock. Masons were accordingly selected, and engaged on the terms stated in the following letter to Mr James Scott, the Foreman, who was sent to Aberdeen to assist in choosing the men:--“Although it is difficult to fix the precise number of men who may be required, during the progress of the works, as this must, in some measure, depend upon the produce of the quarries at Hynish, and of those to be opened in Mull, you may, in the mean time, engage thirty masons or stone-cutters, twelve quarriers, and three or four smiths, for two years of certain employment. With regard to the rate of wages to be paid to the men, this will, in some measure, depend upon the demand for the season at Aberdeen; it is, at all events, expected, that they will on no account exceed the rate of 3s. 10d. per day for masons, and 2s. 6d. per day for quarriers, as paid last season during the long day, or from the 1st of February till 31st of October; and for the short day during the remaining three months, 3s. for the masons, and 2s. for the quarriers, from 1st November till 31st January.

“It is intended that subsistence money shall be paid to such of the families or relatives of the workmen as may require it; and that their wages shall be fully settled monthly, deducting the subsistence money advanced to their relatives. A Store will be kept at the works by the Lighthouse Board, from which provisions will be served out at stated periods, to be fixed by the storekeeper; and these provisions shall be sold to the workmen at the cost prices at which such stores are laid in. Barrack accommodation or lodgings, with cooking, will also, as formerly, be allowed to the men free of expense.”

~Progress of the outfit for the season’s operations.~

Early in the month of May the preparation of the wooden barrack for the Rock had been completed, and the whole had been set up in the workyard at Greenock; and when I visited it for the last time about the 5th of that month, I found it all ready for shipment, excepting some additional iron ties, which I ordered for securing the turret to the top of the pyramid, which were to be applied at the level of the floor of the upper or barrack-room storey. I also found that the moorings, including the mushroom anchors and chains, and the workyard materials, consisting of several cranes, trucks, a janker for the transport of timber, and a Woolwich sling-cart for carrying stones to the various sheds, were in the course of preparation. A large assortment of masons’ and quarriers’ tools was at the same time ready for shipment at Aberdeen. Early in June, a vessel called the Duke of Montrose was chartered to carry coals to Tyree, both for household purposes and for the work; and two small portable smiths’ forges were prepared for use on the Rock.

~Embark for Skerryvore.~

In providing the means of efficiently carrying on so many complicated operations in a situation so difficult and remote, it is impossible, even with the greatest foresight, to avoid omissions; while delay of a most injurious kind may result from very trivial wants. Even the omission of a handful of sand, or a piece of clay, might effectually stop for a season the progress of plans, in the maturing of which hundreds of pounds had been expended. Accordingly, although I had bestowed all the forethought which I could give to the various details of the preparation for the season (of which I found it absolutely indispensable to be personally aware, even to the extent of the cooking dishes), new wants were continually springing up, and new delays occasioned, so that it was not until the evening of the 23d of June that I could embark at Tobermory in the _Pharos_ Lighthouse Tender, commanded by Mr Thomas Macurich, with all the requisites on board for commencing the season’s operations. Next morning we moored off Hynish Point about three o’clock, and, from the roughness of the passage, were not unwilling to land at that early hour. Here I found that Mr Scott, the foreman of the workyard, had, notwithstanding the unworkable nature of the Rock, more particularly afterwards noticed, procured about sixty fine blocks of gneiss, as the produce of the Tyree quarries, which had been wrought for upwards of 15 months; and had at the same time completed the masonry of a range of buildings for stores and barracks, capable of containing upwards of 100 men, and had built about 100 feet in length of a landing-pier, reaching nearly to low-water mark. A magazine for gunpowder, of which a considerable stock was required for quarrying purposes, had also been built; and a piece of garden ground had been inclosed and stocked for t