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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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_.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.
* * * * *
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.