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.

##