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.