"The several courses, represented in Plate III. are shown as they would appear, when completed with the whole of their wedges and trenails: and besides these, there being also generally two lewis holes upon the upper surface of each stone, those served as temporary fixtures for the work of the succeeding course.
"It was the same evening's tide, of the 11th of August, that the basement was completed and the centre stone of Course VII. was landed. Of the preceding courses, each was begun by the stones that engrafted in the dovetail recesses cut in the rock ; these stones, therefore, being immovable by any assignable force acting horizontally, rendered those so likewise that depended upon them ; but having now brought the whole upon a level, we could not have this advantage any longer; it therefore became necessary to attain a similar advantage by artificial means. For this purpose, the upper surthce of Course VI., (Plate III. Figure 6,) had a hole of one foot square cut through the stone that occupied the centre ; and also eight depressions, of one foot square, sunk into that course six inches deep, which were disposed at regular distances round the centre : these cavities were for the reception of eight cubes of marble, in masonry called joggles. As a preparation for setting the centre stone of Course VII., a parallelopiped (which, for shortness sake, T will call the plug) of strong hard marble from the rocks near Ply mouth, of one foot square and twenty two inches in length, was set with mortar in the central cavity. and therein firmly fixed with thin wedges. course VI. being thirteen inches in height, this marble plug, which reached through, would rise nine inches above it ; upon this, the centre stone (see Plate IV. Course VII.) having a hole through its centre of a foot square, was introduced upon the prominence of the plug, and, being bedded in mortar, was in like manner wedged (with wedges on each side of the plug) and every remaining cavity filled with grout. Hy this means, no force of the sea, acting horizontally upon the centre stone, less than what was capable of cutting the marble ping in two, was able to move it from its place : and to prevent the stone more effectually from being lifted, in case its bed of mortar happened to be destroyed, it was fixed down in the manner above described, by four trenails; which being placed near to the corners of the large square of that stone, they not only effectually prevented the stone from lifting, but aided the centre plug in preventing the stone from moving angularly, or twisting, which it might otherwise have done, notwith standing its weight, which was two tons nearly.
" After setting the first centre stone of Course VII. we immediately proceeded to set the four stones that surround it, and which were united thereto, by four dovetails, project ing from the four sides of the centre stone. These stones being fixed in their dovetails by a pair of wedges on each side at bottom and top, as has already been mentioned, and held down by a couple of trenails to each surrounding stone, and still farther steadied by joint wedges at the head of the dovetails, and also in the mitre, or diagonal joints between each surrounding piece ; the whole formed a circular kind of stone of ten feet diameter, and above seven tons weight : and which being held down by a centre plug and twelve trenails, became in effect one single stone ; whose circumference was sufficient to admit of eight dovetail recesses to be formed therein, so as to be capable of retaining in their places a circle of eight pieces of stone, of about twelve hundred weight each, in the same manner, and upon the same principle, that the radical pieces of stone were engrafted into the dovetail recesses of the rock ; and which being in like manner wedged and trenailed, we proceeded with circular tiers of stone, in the manner shown in Plate IV. Figure 1. It is, however, to be remarked, that the mode of applying the wedges and trenails being sufficiently explained in the seve ral figures of Plate 11]. and also in Plate IV. Figure 1, to avoid a repetition of small work, the several succeeding figures simply show the general shapes and disposition of the rent pieces composing a course, and other incidental larger matters wholly omitting the particular application of the wedges and trenails ; yet it is to be observed, that they were everywhere equally applied, till we got to the top of the solid.