Having now stated the result of the several objects for which our journey northward was undertaken, it may be proper to mention what occurs to us as the best mode of proceeding respecting the measures to be taken for the erection of the Light-house, with the construction that appears best suited to the stability of the work. The first object that naturally presents itself is the establishment of the Floating-Light, now in preparation, respecting which some correspondence has taken place between Mr Stevenson and Captain Huddert of the Trinity Board of London, which is now before you. Their information is very satisfactory, and from the measures taken in advising with the Trinity-House of Leith, it is hoped that every thing relative to the fitting out of the Prussian-dogger, purchased for this service, will be the most complete of the kind, and the sooner she can be got to her moorings the better, that the operations at the Rock may be commenced, and that the duties may be collected for paying the Treasury loan, and otherwise adding to the general fund.

The second object that comes under consideration is the inclosing the piece of ground at Arbroath for a work-yard. This may either be done by a ruble wall or a wooden fence, but the former is preferable, because several buildings must be made for stores, and for floors, to mark out the different courses with the dove-tails as they form the building, so that the ruble wall may form part of these buildings. Sheds must also be erected for the masons to work in.

Thirdly, Two vessels, of about 30 tons each, for conveying stones from Arbroath to the Rock, should, without delay, be contracted for. These vessels must be built strong, and of good materials, and calculated to work well at sea, that no opportunity of going to the Rock may be lost.

Fourthly, A crane for unloading the vessels should be established at Arbroath, and another at the Rock, for landing the materials, as also the necessary tools and triangles, with tackle for setting the stones in the work-yard, and at the Rock likewise, stone-carriages, jacks, &c.

Fifthly, Mooring-chains, anchors, buoys, with ropes, &c., to assist in landing the materials, and for enabling the men to leave the Rock when the tide prevents them from working.

Sixthly, A coffer-dam will be wanted, to the height of 4 or 5 feet, so as to enable the workmen to continue so much longer on the Rock than they could do, were the tide allowed to flow over the foundation when it rises above the level of the Rock.

Seventhly, Although we do not pretend to say that a stage, for the accommodation of the workmen, can with certainty be constructed on the Rock, yet it might be worth while to attempt the establishment of such a work, as, if it did answer, great facility in carrying forward the work would be obtained; and, should the project fail, the timber and iron can be applied to other uses, and the expence of the experiment cannot be great.

Eighthly, A variety of small tools, for cutting the foundation of the Rock, for dove-tailing the stones, and sundry other uses too tedious to mention, will also be wanted.

Ninthly, A quantity of Aberthaw lime should be procured, as well as other lime of the best quality which the country affords, and a small lime-kiln for burning it erected, and no more burned at once than can be used in a short time, that it may be always had as fresh as possible, as the less it is exposed to the air after being burned the better will it cement. Pozzolano or tarras should be got, if possible, to mix with the lime, these making the best cement that has yet been discovered; but, if it cannot be had, manganese, or even burnt ironstone, will form a good substitute. A quantity of Parker and Company’s Roman cement will be wanted, to keep the sea from washing out the other cement before it has time to harden in the joints.

Tenthly, Oak trenails will be required for fixing down the stones, while the building is in operation, and the several courses liable to be washed by the waves. Lead for running bolts into the Rock, &c. will also be necessary.

As to the construction of the Light-house itself, we are of opinion, that it is much better to follow what has been found to answer at the Edystone, a situation nearly similar to the present, than, in a work so important, to look for other methods that have not stood the test of trial. It must, however, be observed, that some alteration, suited to the difference of local circumstances, ought to be adopted. The Rock at the Edystone differs both in form and quality from the Bell Rock; it lies in a slanting direction, and the higher part was never covered by the tide. It is of a harder quality, and but small in size; nothing, therefore, was to be found in this case in regard to the wearing or wasting of the Rock; and from its limited size, there was no more space than what merely enabled the engineer to make the erection upon it.

The case at the Bell Rock is very different, the Rock being, on an average, not more than four feet higher than low-water of spring-tides. It is of a softer quality, but the dimensions are large. It therefore becomes necessary that the base should be more extended and flat where it is connected with the Rock, so that when the waves break upon it, they may spend their fury on the base of the Light-house, instead of the Rock itself, and thereby prevent it from wearing away. The tower should also be higher, as the tide rises more upon it than on the Edystone. These, and a few other alterations, will adapt the Edystone plan to the Bell Rock, unless in so far as the light itself is connected; but this need not now be a subject of inquiry. A Plan of the Light-house, nearly similar to what is proposed, is handed herewith, but no plan of the dove-tailing has been made, nor can it well be done at present, as the dove-tails must be suited to the different courses of stone as they come from the quarry; but, as the principle is similar to that of the Edystone, the design may be easily judged of.

It is of much importance, that, in so hazardous a work as the present, recourse should speedily be had to advice when occasion requires; and, as a Quorum at all times cannot be got, it might be advisable to appoint a Sub-Committee of your number, to be called together when necessity may require.

(Signed) John Rennie.
              Robert Stevenson.

To the Honourable the Commissioners of the Northern Light-houses.

REPORT by Mr Rennie.

Edinburgh, October 29. 1807.

Gentlemen,

I went to the Bell Rock on the 5th instant, when I found Mr Stevenson with his workmen engaged in preparing the Rock to receive the foundation of the Light-house; also in erecting a Beacon on it, for the double purpose of pointing out to mariners the situation of the Rock more correctly than can be done by the Floating-Light, and as an asylum to the men employed on the Rock, in case they should be overtaken by the tide before the boats had time to take them to the vessel. Mr Stevenson had been constantly engaged in the different operations for about two months, during which time he had never been ashore; and it was fortunate he had taken the resolution of remaining constantly on board, for, without this, there is every reason to believe that the workmen could not have been prevailed on to continue at the work, they having previously shewn a disposition to leave it; but by perseverance, and by allowing them their provisions while on board the vessel and working at the Rock, they have not only been prevailed on to remain at the work, but they now seem to be better satisfied when at the Rock than on shore; and as no accident has happened, although the weather has several times been very stormy, there is great reason to believe that the workmen, being now accustomed to the work, and having gained entire confidence by the success of the past year, will return with redoubled vigour next season, and that the work will proceed with much spirit and regularity.

The work performed in the course of the present season, consists first of preparing the Rock for the foundation of the Light-house to a certain extent; but owing to that part of the Rock where the Light-house is to stand being very uneven in the surface, and some of it within 2½ feet of the level of the sea at low-water of spring-tides, the time at which the men can work is of course very limited; and as there would be great danger of shivering the Rock, if blasting or any other violent measure were resorted to for expediting the work, it has been thought better to proceed in the more secure mode of cutting the stone by picks and chisels, which, as the Rock is hard, and so consumes much time, considerable progress has been made towards this; and I hope, by the end of next June, the whole of the Rock will be completely ready to receive the foundation-course of the Light-house: perhaps it may even be ready sooner; but so much depends on weather, I do not venture to state particularly the time when this most essential object will be completed; but I have great confidence in the zeal Mr Stevenson has shewn in the past season, to prognosticate that he will do all that can be done in the next to expedite the work.

A large beacon, consisting of six main beams of timber, about 13 inches square, and 44 feet long, has been erected on the Rock; it forms a pyramid of about 42 feet high, and a base of the same extent. This is well secured to the Rock with dove-tail cramps, and with chains and braces. Near the top are cross-bars to stiffen the upper part of it, which have been found of great use, as the smith’s forge for sharping tools was erected there, and for the last fortnight the tools were sharped, and various other works done, which tended greatly to facilitate the operations at the Rock. The workmen continued there during the time the Rock was covered with the tide, which enabled them to begin working the moment the part was uncovered where they were employed, and it likewise enabled them to work till the last moment, when the tide began to cover the work. Had no such asylum been provided, the workmen would not only have lost much time in getting from the vessel to the Rock, but also in getting from it. This beacon is not yet entirely finished: a few good tides would enable the workmen to complete it, and when done, I have great hopes it will be found to resist the storms; indeed it has resisted several even in the unfinished state it is, and therefore is a favourable prognostication that it would be stable when completed.

There is, besides, a provision-chest intended to be placed on its top, and above this a ball and flag-staff, so that I hope it will be a very useful additional mark for mariners to avoid this dangerous Rock, and should be advertised as well as the Floating-Light.

The advanced state of the year renders it necessary that the operations at the Rock should stand until the spring of the year. This is proposed to be done whenever the beacon is completed; but during the winter every requisite measure should be taken to proceed with the work for the Rock as expeditiously as possible. For this purpose I must beg leave to recommend to the Commissioners that the remainder of the articles mentioned in our Report of the 26th December last, should be provided with as little delay as possible.

The ground taken on lease from John Taylor, at Arbroath, for a wood-yard, has been covered in part with the necessary buildings and workshops, but sheds for the masons to work in during the winter season are not yet done. These may be got of the most temporary kind; if they just keep off the rain it will be enough, and no delay should take place in procuring them.

Several cargoes of Aberdeen and Dundee stones have been laid down and landed at the yard, part of which are worked, and the masons are engaged in working the remainder. I am sorry, however, to say, that neither the supply from Aberdeen nor Kingoodie has been so abundant as could be wished. Unless exertions far greater than has hitherto appeared be made, it will be impossible to proceed with the building on the Rock next season. I accompanied Mr Stevenson both to Aberdeen and Kingoodie. In viewing the former, it appears, that unless stones are purchased from such quarries near Aberdeen, as may turn them out, it is scarcely probable a sufficient quantity of large stones for the lower courses of the Light-House can be had in due time from the Rubieslaw quarries alone. It therefore will be advisable, that Mr Skene empower his agent to purchase from the other quarries whenever blocks of the requisite size can be found. As to the Kingoodie quarry, it only wants a little exertion on the part of Colonel Mylne’s agents, as the quantity wanted would be easily got in a few weeks, if exertion such as has been promised be made.

The Floating-Light was completed and moored within the distance of one mile from the Rock on the 9th July, and for some time the people employed at the Rock lodged in her; but the situation in which she was moored has been found not to answer, as, after the gale of the 6th September last, which she rode out with perfect safety, it was found that her cable had been cut by a piece of rock or wreck. She went adrift next day, luckily without receiving any injury, but it has been found that there is no clear ground, (at least that has yet been discovered), less than the distance of about two miles and a half from the Rock, which will render her of no use as a place to accommodate the workmen; another vessel of some sort must therefore be provided. The Yacht has been used since the Float was removed to such a distance, but as neither the manner she was rigged, nor her accommodation, is suitable, nor indeed can she well be spared from attending the other light-houses, she cannot be counted upon in future. In my opinion a proper vessel should be purchased, and fitted up for this purpose, and she will sell at little discount after the work is completed.

One vessel of the description mentioned in the second article of the Report of 26th December 1806, has been built and fitted out. She has been engaged in carrying materials to the Rock; in carrying stones to the yard; and sundry other matters. A second vessel of the like description should be got ready against the spring, to carry the stones and materials to the Rock, &c. Two cranes have been purchased for loading and unloading stones; one has been erected at Aberdeen, and the other at Arbroath; and they both answer the purpose very well. Triangles, and other necessary apparatus are preparing for the work on the Rock, and will be ready against the time they are wanted. Mooring-chains and buoys for the vessels employed about the Rock have also been prepared and laid down.

It was proposed, in the Report of December last, to have a coffer-dam on the Rock, for keeping out the water during a part of the time the tide is flowing; but it rises so quickly here, after having turned, that I do not think a coffer-dam will produce advantages adequate to the expence. It is therefore proposed to do the work without any coffer-dam at all: there has been sufficient trial already made, to satisfy us respecting its practicability without a coffer-dam. The beacon already erected, and which there is reason to believe will stand, renders a stage of the description formerly mentioned unnecessary. It is intended, should this beacon stand over the winter, to erect places for the accommodation of the workmen employed at the Rock on its top. This will greatly facilitate the operations, as before stated.

Most of the small tools mentioned in article 9th of Report of December last, have been procured. Some are still wanting; but these are few. No Aberthaw lime has been procured, nor any pozzolano. Mr Stevenson intends to send the Yacht for a cargo of Aberthaw limestone, as soon as she can be spared from the service of the Light-houses; and, if a vessel is purchased for the accommodation of the workmen at the Rock, she may also be sent for a cargo of Aberthaw lime. I have not been able to procure any pozzolano, although many trials have been made. I have some hopes that a small quantity may still be procured; but I cannot speak with certainty: if not, tarras, or burnt ironstone, must be used. No oak trenails for the stones have yet been got. Mr Stevenson, however, is about to order some from Southampton; and I hope they will be in time for the work. Lead is, at this time, very cheap. I submit it to the wisdom of the Commissioners, whether it might not be advisable to purchase now what may be wanted for the work.

As to the construction of the Light-house, I submitted a plan to your consideration in the month of February last, (see Plate VII.) According to this plan, the works are proceeding; plans of each course of stone have been made; the whole is dove-tailed, but somewhat different from the mode pursued at the Edystone; they are less in length on the outside, but deeper in the direction of the radius of the Light-house, which will render the structure, on the whole, stronger than the Edystone plan. The extension of the base of the building is also much greater; and the base is considerably different. By this means, not only the impulse of the waves on the building will be less, but their action on the part of the rock adjoining the foundation will be much easier. The Rock is softer than that on which the Edystone is built, but it is harder than I imagined when last here. On the whole, I feel confident, that this work will be brought to a successful termination, within a reasonable period. The knowledge which has been acquired by the operations of the last season, impresses me with additional confidence in the practicability of the work, although confident, from the commencement, that, with proper care and attention, such a work might, with certainty, be completed.

The necessity of Mr Stevenson’s remaining much at the Rock while the operations are going forward, render it necessary that some person, by way of agent, should be appointed to pay money at Arbroath, and to credit and settle accounts. As to this the Commissioners will order as they think right. I am, honourable gentlemen, your most obedient servant,

(Signed) JOHN RENNIE.

To the Honourable the Commissioners of the Northern Light-houses.

REPORT by Mr RENNIE.

Edinburgh, 12th December 1808.

Gentlemen,

I visited the Bell Rock on the 25th ult. The weather was favourable, but the tide did not ebb so low as to allow me to examine the foundation of the masonry so completely as I could have wished; but as there was little swell, I was enabled to form a tolerable judgment concerning the perfection with which it is executed, and this, I have the pleasure to say, is very favourable.

The Rock was nearly all cut last season, but several parts in the interior was hollow, and others appearing unsound, they could not be cut out without a serious expence in money, as well as a great loss of time. It was therefore judged preferable to make up these places with stone, and, accordingly, 17 stones have been let into different parts of the foundation, which brought the whole on which the Tower stands to a level; on which level, the first course of 12 inches was laid in August, and the outer-edge margin of this course, was nicely fitted all round to the rock, which, in every place except one, as I have been told by Mr Stevenson, projects higher than the upper part of it, by which, the base may be said to be completely indented into the Rock. Above this 12 inch course, two others, of 18 inches, have been laid, and it gives me great satisfaction to inform you, that they are done in a very complete manner, and the mortar, in all the outside joints, has not been at all affected by the sea beating upon it. A very small quantity was out of some of the inside joints, which, I am told, had arisen from the sea catching it (the weather being unfavourable) while the course was laying; this has been directed to be made up, and covered with Roman cement, which will render it as perfect as if no such thing had happened.

It is here proper to remark, that although the pozzolano mortar forms the best of all cements I have seen, yet it does not acquire a sufficient degree of induration to resist the effect of heavy waves for several weeks after it is used. The Roman, or Parker’s cement, acquires almost an immediate induration, capable of resisting the heavy waves; but it never acquires so great a degree of induration, nor does it bind the stones so well together as the pozzolano mortar ultimately does. Its use, therefore, is to guard or preserve the pozzolano mortar until it has time to harden, which it does most effectually, and therefore every joint should be plastered over with this cement as soon as laid.

The place where the lower or 12 inch course is above the Rock, should be made up with stone of the same quality as the Rock itself. These stones should be dove-tailed into the rock, and laid with pozzolano mortar, secured by the Roman cement, the same as the other; for if done with harder stone (should the rock waste at all) it would resist the waves better than the soft, and occasion a more speedy waste of the other parts of the Rock than the Tower. All the protuberances of the Rock round the base of the Tower, should likewise be cut off, and smoothed, so as to form a regular curve at the base of the Tower, by this means there will be no place for the waves to catch, and their action will be regular on the whole, and I have no doubt, when so done, there will be little or no wasting in the rock itself.

The operations in the work-yard seem to be proceeding with much regularity, and there is a good stock of materials now there. Two courses are nearly completed on the platform, and as the diameter of the Tower diminishes fast, I have little doubt that there will be several other courses ready by the season that the work can with safety be resumed on the Rock; so that if the next season is at all favourable, I would fain hope the whole solid part of the Tower may be completed; every course of stone will raise it so much above the rock, and, of course, prolong the time of working a tide, until it is above the top of the springs, and then, if materials can be landed at the rock in sufficient quantity, the laying of them on the Tower will proceed with little or no interruption.

To secure a regular supply, another Praam or Stone-lighter should be got ready by the end of April next. Mr Stevenson has already received proposals for the building of one, and the sooner it is set about the better. It will also be necessary to build or hire another vessel, to assist the Smeaton in carrying out the stones from the work-yard; I apprehend it will be best to hire a vessel for that purpose, as the services required of her will be of short duration. About 10 tons more of pozzolano will probably be wanted, and this I have been able to procure through the friendship of the Hull Dock Company; it is already ground, and now lying there to be shipped. A small additional quantity of Aberthaw limestone is likewise to be procured, by the kelp-vessels that trade to Bristol, as a return freight.

It happens, rather unfortunately, that some of the granite quarries about Aberdeen have veins of a soft sort, which I have reason to believe decomposes with the sea and weather. Some of these stones have, unluckily, been sent to the work-yard. My decided opinion is, that all of this kind should be laid aside for the out-side work, and others of a good quality procured in their place. They can be substituted for the Dundee stone, and, therefore, although some loss will by this means be sustained, it will, on the whole, be trivial. I am, Gentlemen, your most humble servant,

(Signed) JOHN RENNIE.

To the Honourable the Commissioners of the Northern Light-houses.

No. V.
REMARKS relative to the Ground-swells of the Sea.

In the Memoirs of the Wernerian Natural History Society, Vol. III. p. 814., and in the Edinburgh Philosophical Journal, Vol. III. p. 42., the subject of the Wasting Effects of the Sea upon the firm ground, is variously treated by the writer; and is here alluded to, in reference to the production of the appearance termed a Ground-swell, so often observed on our shores. In these papers it has been assumed, that the German Ocean, or North Sea, bounded partly by the coasts of Great Britain, Norway, Denmark, Germany, Holland, and France, as will be seen from Plate III., is almost every where wasting and extending its margin, upon the principle of its forming the great receptacle for all the debris of the surrounding countries above mentioned, which have a direct tendency to lessen its capacity for containing the waters which flow into it from the Atlantic, by the apertures between the shores of Scotland and Norway, towards the north, and between Dover and Calais, to the south. These passages may respectively be conceived as admitting a constant quantity, while this great basin is continually receiving the debris of the extensive tracts of country which surround it, through the medium of the innumerable rivers and streams that fall into it.

Upon similar principles, the extensive lakes of interior countries become so many receptacles for the debris of their respective districts; and must in the lapse of time, contain less water, in proportion to the cubical contents of solid matters carried into them. So it is upon the great scale with the basin of the North Sea, from the extensive banks deposited therein, which lessen its capacity, and thereby give a tendency to the water which flows into it to overflow and waste its margin, producing, under various modifications, those heavy swells in the Sea to which we now allude. With this state of things in view, we may now inquire into the effects of the storms which agitate the waters of the Atlantic, and reach our shores only in the form of what has been significantly termed a Ground-swell; which may partly be illustrated from the storm described in page 312. of this work. We accordingly find the waters of the North Sea often in an agitated state, without any apparent cause, though no doubt proceeding from distant storms at sea. Hence, a continued gale from the westward, passing along the Atlantic Ocean, overcharges all the seas and inlets communicating immediately with it. Under these circumstances, although the surface of the North Sea be comparatively smooth, yet having acquired motion to a considerable depth from the distant storm, it continues to produce the effect of a Ground-swell for a considerable period. A swell of this description is almost constantly observable in the Bay of Biscay, owing to its exposure to the Atlantic, and from the configuration of the bounding land between Cape Finesterre in Spain, Ushant in France, and the relative position of the British Isles. Here the Ground-swells, proceeding from the same cause as those of the North Sea, are either the precursors of a gale of wind, or the effects of one which does not reach us.

Similar, though less evident effects, are produced by gales of wind which may be conceived to be confined to the boundary of the North Sea itself. Hence the heavy waves which often break upon the eastern shores of Great Britain, without any apparent cause at the time, are afterwards found to arise from storms at sea. But the heaviest Ground-swells of the North Sea are produced by gales from the north-east, which proceed in a progressive manner along the inclined plane of its bed, until the waves, by impinging upon the bottom and sides of the firm ground, are so disturbed that they ultimately cease, after wasting the whole extent of the shores, and sometimes causing the most destructive effects upon the coast of Holland, forming the southern extremity of this great basin.

No. VI.

SCHEDULE, containing an Enumeration of the MATERIALS used in the Erection of the Bell Rock Light-House.