[15]It is most satisfactory to find that the views expressed above, regarding the eligibility of the conical form, seem to have the sanction of the late Dr Thomas Young, who appears to have connected his preference of this form with its greater efficiency as a source of friction among the parts of a building. In his syllabus of Lectures, under the section “Architecture,” he thus speaks: “For a Lighthouse where a great force of wind and water was to be resisted, Mr Smeaton chose a curve convex to the axis. In such a case, the strength depends more on weight than on cohesion, and also in a considerable degree on the friction which is the effect of that weight. Perhaps a cone would be an eligible form.”
[15]It is most satisfactory to find that the views expressed above, regarding the eligibility of the conical form, seem to have the sanction of the late Dr Thomas Young, who appears to have connected his preference of this form with its greater efficiency as a source of friction among the parts of a building. In his syllabus of Lectures, under the section “Architecture,” he thus speaks: “For a Lighthouse where a great force of wind and water was to be resisted, Mr Smeaton chose a curve convex to the axis. In such a case, the strength depends more on weight than on cohesion, and also in a considerable degree on the friction which is the effect of that weight. Perhaps a cone would be an eligible form.”
If these views be substantially correct, it may not, perhaps, be altogether inadmissible (without, however, venturing to enunciate any general law) to conclude, that, in the three Lighthouses of the Eddystone, the Bell Rock, and the Skerryvore, this source of union among the outer stones of the lower courses must bear some proportion to the numbers 753, 659, and 927, which are the quotients of the height of the column, divided by the difference of the top and bottom radii of the shaft in each case respectively. This consideration seems too important to be entirely overlooked; and I conceive that, by following out this view, I have been enabled to depart with perfect safety from the intricate and elaborate work required for the connection of the materials by means of dovetailing and joggling, which the adoption of a more concave outline (in which the vertical pressure could not have been so advantageously transmitted to the outer stones of the base), would perhaps have rendered advisable. In the case of the Bell Rock, however, whose construction, in regard to this property, is the least advantageous of the three buildings, it must be borne in mind that the Tower is covered to the depth of 15 feet at spring tides, and that this principle of vertical pressure could not have been safely appealed to during the whole time which intervened between the commencementof the building and the attainment of a height sufficient to render it available, which, in a Tower having so great a part submerged, was of necessity much prolonged. The stones were thus exposed to the full effect of heavy seas, at all levels, during two entire winters, and could not therefore have been safely left, without being kept together by numerous ties and dovetails. It also seemed important, in designing that Tower, with reference to the rise of tide, to give its lower part a sloping form, as the least likely to obstruct the free passage of the waves. The outer stones of the lower courses were also selected of unusual lengthinwards, so as to bring them more under the influence of the vertical pressure of the upper wall.
Before leaving this subject, I may remark, that it is quite possible to construct a Tower of a curved form, in such a manner, that the pressure of the upper part of the pillar shall be distributed to the greatest advantage on every stone, by building the outer walls as inverted arches, so that the section of each stone shall be that of a voussoir, with joints perpendicular to the successive tangents of the curve. This arrangement of the stones is, in fact, practised in sea walls of various kinds, and has even been recommended for circular Towers in an ingenious paper in the Transactions of the Royal Scottish Society of Arts. But in many situations, and at Skerryvore in particular, this mode of transmitting the pressure, so as to throw it perpendicular to the beds of the stones, is inadmissible, as conducing to or involving a greater evil. The evil has already been noticed, and consists in the thrust of the lowest stone (which is of course inclined to the horizon) having a tendency to push out the sides of the Rock on which the Tower is built. This fear, where the Towers are to be placed on small steep rocks or pinnacles, and more especially when these Rocks are traversed by veins nearly vertical, is by no means visionary; and there is good reason to apprehend, that the pressure thus resulting in a line considerably inclined to the plane ofcleavage, might throw outwards a thin portion ofrock, which, under the more conservative influence of a vertical pressure, might continue to retain its connection with the rest of the Rock unimpaired for ages.
Another method of, in some degree, increasing the resistance of a Sea Tower to a horizontal thrust, if such aid be required, is to give the line of courses a continuous spiral form, instead of building them in successive horizontal layers. Were there reason to fear that the entire dislocation of the building might take place in a plane nearly horizontal, this method seems more calculated to counteract the danger than the use of dowels or joggles passing from the course below to the course above; but, as this is one of the accidents least to be apprehended, there does not seem any good ground for resorting to a mode of structure which would lead to considerable intricacy of workmanship, and would, in practice, be attended with difficulty in obtaining a proper vertical bond or union among the several stones.
The only remaining point, in which the example furnished by the Eddystone and Bell Rock Lighthouses has been at all materially departed from, is (as has already been hinted at by an unavoidable anticipation) the mode of uniting the different parts of the masonry together. In both these Towers the stones were dovetailed throughout the buildings, chiefly (at least in the case of the Bell Rock where the foundation was so much below the tide) with the view of preventing the sea from washing away the courses which might be left exposed to the winter storms before the weight of the superstructure had been brought to bear upon them. In the upper part of the Bell Rock my father also introduced a kind of band joggle, which consists of a flat ribband of stone raised upon the upper bed of one course, and fitting into a corresponding groove cut in the under bed of the course above; and this system of tying the adjoining courses together also forms a chief feature in his design for a Lighthouse on the Wolf Rock.[16]When thegreat pressure of the superstructure of these Towers, however, and the effect of the mortar are considered, there seems little probability of one course being dislocated, in defiance of the friction resulting from the weight of the column. An impulse sufficient to produce such an effect would tend to overset the whole superstructure from mere deficiency in weight, and in this case the joggle would have little effect. But if joggles be thought necessary for this purpose, the ribband form certainly produces a better arrangement than that of the cubic joggles employed by Smeaton for connecting the adjoining courses of his building together, as the sectional strength of these scattered square joggles is very small compared to the effect of a shock which could be supposed capable of moving the whole mass of a Tower. In the lower parts of the Skerryvore Tower, I entirely dispensed with dovetailing andjoggles between the courses, and thus avoided much expensive dressing of materials. The stones were retained in their places during the early progress of the work, chiefly by common diamond joggles, and the courses were temporarily united to each other by wooden treenails, like those used in the Eddystone and Bell Rock. These treenails had split ends, with small wedges of hardwood loosely inserted, which being forced against the bottom of the holes in the course below, into which the treenails were driven, expanded their lower ends until they pressed against the sides of the holes; while their tops were made tight by similar wedges driven into them with a mallet. I have, however, adopted the ribband-joggle in the higher part of the Tower, where the walls begin to get thin in the very same manner as at the Bell Rock, where it was used, partly that it might counteract any tendency to aspreadingoutwards of the stones, and partly that it might operate as a kind offalse jointto exclude the water which, when pressed with great violence against the Tower, is apt to be forced through a straight or plain joint. The stones in the higher courses throughout each ring are also connected at the ends by double dovetailed joggles, which unite the two adjoining stones; and the walls are, besides, tied togetherat various points by means of the floor stones, which are all connected by dovetails let into large circular stones which form the centres of the floors. I also ventured to leave out the metallic ties at the cornice, which consisted, at the Eddystone, of chains, and, at the Bell Rock, of copper rings. The reasons which induced me to adopt this change I need not here enlarge upon. It is sufficient to state, that I believe I have nearly balanced the forces which would have tended to throw the cornice outwards, had a greater disproportion existed in the weight of the outer and inner parts of the cavetto, and to point out (Plate VII.) that the Lightroom or highest floor occurs, at such a level, as of itself to answer all the ends which metallic ties could have served.
[16]Account of the Bell Rock Lighthouse, Plate XXI.
[16]Account of the Bell Rock Lighthouse, Plate XXI.