" In reflecting upon the late structure, it appeared most evidently, that had it not been for the moor-stone courses, inlaid into the frame of the building, and acting therein like the ballast of a ship, it had long ago been overset, notwith standing all the branches and iron-work contrived to retain it : and that, in reality, the violent agitation, rocking, or vibration, which the late building was described to be subject to, must have been owing to the narrowness of the base on which it rested ; and which, the quantity of vibration it had been constantly subject to, had rendered, in regard to its seat, in some degree rounding, like the rockers of a cradle. It seemed therefore a primary point of improvement, to pro cure, if possible, an enlargement of the base, which, from the models before me, appeared to he practicable. It also seemed equally desirable, not to increase the size of the pre sent building in its waist ; by which I mean that part of the building between the top of the rock and the top of the solid. If therefore I still kept strictly to the conical form, a neces sary consequence would be, that the diameter of every part being proportionably increased by an enlargement of the base, the action of the sea upon the building would be greater in the same proportion ; but as the strength increases in pro portion to the increased weight of the materials, the total absolute strength to resist that action of the sea, would be greater by a proportional enlargement of every part, but would require a greater quantity of materials ; on the other hand, if we could enlarge the base, and at the same time rather diminish than increase the size of the waist and upper works ; as great a strength and stiffness would arise from a larger base, accompanied with a less resistance to the acting power, though consisting of a less quantity of materials, as if a similar conical figure had been preserved.
" On this occasion, the natural figure of the waist or bole of a large spreading oak, presented itself to my imagination. Let us for a moment consider this tree: suppose at 12 or 15 feet above its base, it branches out in every direction, and forms a large bushy top, as we often observe. This top, when full of leaves, is subject to a very great impulse from the agi tation of violent winds; yet, partly by its elasticity, and partly by the natural strength arising from its figure, it resists them all, even for ages, till the gradual decay of the material dimi nishes the coherence of the parts, and they suffer piecemeal by the violence ; but it is very rare that we hear of such a tree being torn up by the roots. Let us now consider its par tieular figure.—Conneeted with its roots, which lie hid below ground, it rises from the surface thereof with a large swell ing base, which at the height of one diameter is generally reduced by an elegant curve, concave to the eye, to a dia meter less by at least one-third, and sometimes to half of its original base. From thence its taper diminishing more slowly, its sides, by degrees, come into a perpendicular, and for some height t'orm a cylinder. After that, a preparation of more cir cumference becomes necessary, for the strong insertion and establishment of the. principal boughs, which produces a swelling of its diameter. Now, we can hardly doubt but that every section of the tree is nearly of an equal strength in proportion to what it. has to resist : and were we to lop of its principal boughs, and expose it in that state to a rapid cur rent of water, we should find it as much capable of resisting the action of the heavier fluid, when divested of the greatest part of its clothing, as it was that of the lighter, when all its spreading ornaments were exposed to the fury of the wind : and hence we may derive an idea of what the proper shape of a column of the greatest stability ought to be, to resist the action of external violence, when the quantity of matter is given whereof it is to be composed.
" In Plate V. Figure 1, is a sketch, representing the idea which I formed of this subject. It is farther observable, in the insertions of the boughs of trees into the bole, or of the branches into the boughs, (which is generally at an oblique angle) that those insertions are made by a swelling curve, of the same nature as that wherewith the tree rises out of the ground ; and that the greatest rake or sweep of this curve is that which fills up the obtuse angle ; while the acute angle is filled up with a much quicker curve, or sweep of a less radius : and Figure 2, of the same Plate, represents my con ception of this matter. In this view of the subject, I imme diately rough-turned a piece of wood, with a small degree of tapering above ; and leaving matter enough below, 1 fitted it to the oblique surthee of a block of wood, somewhat resem bling the sloping surface of the Eddystone rock ; and soon found, that by reconciling curves, I could adopt every part of the base upon the rock to the regularly turned tapering body, and so as to make a figure not ungraceful ; and at the same time carrying the idea of great firmness and solidity.
"The next thing was to consider how the blocks of stone could be bonded to the rock, and to one another, in so firm a manner, as that, not only the whole together, but every indi vidual piece, when connected with what preceded, should be proof against the greatest violence of the sea.
" Cramping, as generally performed, amounts to no more than a bond upon the upper surface of a course of stone, without having any direct power to hold a stone down, in ease of its being lifted upward by an action greater than its own weight ; as might be expected frequently to happen at the Eddystone, whenever the mortar of the ground-bed it was set upon was washed out of the joint, when attacked by the sea before it had time to harden ; and though upright cramps, to confine the stones down to the course below, might in some degree answer this end, yet, as this must be done to each individual stone, the quantity of iron, and the great trouble and loss of time that would necessarily attend this method, would in reality render it impracticable ; for it appeared, that Mr.Winstanley had found the fixing twelve great irons, and Mr. Rudyerd thirty-five, attended with such a consumption of time (which arose, in a great measure, from the difficulty of getting and keeping the holes dry, so as to admit of the pouring in of melted lead) that any method which required still much more, in putting the work together upon the rock, would inevitably, and to a very great degree, procrastinate the completion of the building. It therefore seemed of the utmost consequence to avoid this, even by any quantity of time and moderate expense, that might be necessary for its performance on shore ; provided it prevented hinderance of business upon the rock ; because of time upon the rock, there was likely to be a great scarcity, hut on the shore a very sufficient plenty. This made me turn my thoughts to what could be done in the way of dovetailing. In speaking how ever, of this as a term of art, I must observe, that it had been principally applied to works of' carpentry ; its application in the masonry way had been but very slight and sparing ; for in regard to the small pieces of stone that had been let in with a double dovetail, across the joint of larger pieces, and generally to save iron, it was a kind of work even more objectionable than cramping; for though it would not require melted lead, yet being only a superficial bond. and consisting of for more brittle materials than iron, it was not likely to answer our end at all. Somewhat more to my purpose, I had occasionally observed. in many places in the streets of Lon don, that, in fixing the kirbs of the walking-paths, the long pieces, or stretchers, were retained between two headers. or bond pieces ; whose heads being cut dovetail-wise, adapted themselves to, and confined in, the strechers ; which expe dient, though chiefly intended to save iron and lead, never theless appeared to me capable of more firmness than any superficial fastening could be ; as the tye was as good at the bottom as at the top, which was the very thing 1 wanted ; and therefore if the tail of the header was made to have an ade quate bond with the interior parts, the work would in itself be perfect. What I mean will he rendered obvious by the inspection of Figure 3, in Plate V. Something of this kind I also remembered to have seen in Belidor's description of the stone floor of the great sluice at Cherbourg, where the tails of the upright headers are cut into dovetails, for their insertion into the mass of rough masonry below. From these beginnings I was readily led to think, that if the blocks them selves were, both inside and out, all formed into large dove tails, they might be managed so as mutually to lock one an other together ; being primarily engrafted into the rock : and in the round and entire courses, above the top of the rock, they might all proceed from, and be locked to, one large centre stone. After some trials in the rough, I produced a complete design, of which Figure 5, Plate V., is the exact copy ; the dotted lines representing the course next above or below, which in the original was drawn from the same centre, on the other side of the paper ; so that looking on each side separately, each course was seen distinctly ; or, looking through the paper, the relation of the two courses, showing how they mutually broke joint upon one another, was clearly pointed out : and this method of representation was pursued throughout ; but not being practicable in copper plate work, I am under the necessity of introducing the method by dotted lines, though attended with some degree of confusion of the main design.