"The stone to be set being hung in the tackle, and its bed of mortar spread, was then lowered into its place, and beat down with a heavy wooden maul, and levelled with a spirit level : and the stone being brought accurately to its marks, it was then considered as set in its place. The business now was to retain it exactly in that position, notm ithstanding the utmost violence of the sea might come upon it before the mortar was hard enough to resist it. The carpenter now dropped into each groove two of the wedges already described, one upon its head, and the other with its point downward, so that the two wedges in each groove would then lie heads and points. With a bar of iron of about two inches and a half broad, three-quarters of an inch thick, and two feet and a half long, the ends being square, he could easily (as with a rammer) drive down one wedge upon the other, very gently at first, so that the opposite pairs of wedges being equally tightened, they would equally resist each other, and the stone would therefore keep its place ; and in this manner those wedges Delight be driven even more tight than there was occasion for ; as the wood being dry, it would by swelling become tighter ; and it was possible that by too much driving, and the swelling of the wedges, the stones might he broken ; and farther, that a moderate fastening might be effectual, a couple of wedges were also, in like manner, pitched at the top of each groove, the dormant wedge, or that with the point upward, being held in the hand, while the drift wedge, or that with its point downward, was driven with a hammer; the whole of what remained above the upper surface of the stone was then cut off with a saw or chisel; and generally a couple of thin wedges were driven very moderately at the but t.end of the stone ; whose tendency being to force it out of its dove tail, they would, by moderate driving, only tend to preserve the whole mass steady together ; in opposition to the violent agitation that might arise from the sea.
"After a stone was thus fixed, we never, in filet, had an instance of its having been stirred by any action of the sea whatever; but, considering the unmeasured violence thereof, the farther security by trenails will not seem altogether unnecessary, when we reflect, that after a stone was thus fixed in its place by wedges, a great sea coming upon it, (often in less than half an hour) was capable of washing out all the mortar from the bed underneath it, notwithstanding every defence we could give it by plaster or otherwise ; and that when the bed of mortar was destroyed, the sea acting edgewise upon the joint, would exert the same power to lift it up, that the same sea would exert to overset it, in ease its broad base was turned upright to oppose it ; and as the wedges only fixed and secured the several pieces of which each course consisted, to each other, and had no tendency to keep the whole course from lifting together, in ease the w hole should lose its mortar bed ; it seemed theretbre highly neces sary to have some means of preventing the lifting the whole of a course together, till the solidity and continuity of the mortar should totally take away that tendency. Adverting
now to what was said, that a couple of holes, to receive oak trenails of one inch and three quarters in diameter, were bored in the work-yard through the external or projecting end of every piece of stone : we must now suppose these stones set in their places, and fixed by wedges ; then one of the tin ners, with a jumper, began to continue the hole into the stone of the course below, and bored it to about eight or nine inches deep : but this hole was bored of a less size, by one eighth of an inch in diameter, than the hole through the stone above ; in consequence, the trenails, having been pre viously dressed with a plane till they would drive somewhat freely through the upper hole, would drive stillly into the under one, and generally would become so last as to drive no farther before their leading end gotdown to the bottom; and if so, they were sufficiently fast : but as they sometimes happened to drive more freely than at others, the following method was used to render them fist, for a certainty, when they got to the bottom. The leading end of every trenail was split with a saw, for about a couple or inches, and into this split was introduced a wedge, about one-eighth of an inch less in breadth than the diameter of the trenail ; it was a full quarter of an inch in thickness at the head, and sharpened to when therefore the head of the wedge touched the bottom of the hole, the trenail being tbrcibly driven thereupon, would enter upon it, till the whole substance was jambed so fast. that the trenail would drive no further ; and as the wood would afterwards swell in the hole, and till the little irre gularities of boring by the jumper, it became so fast, that, as it seems, they could sooner be pulled in two than the trenails be drawn out again. The trenail (originally made somewhat too long) being then cut off even with the top of the stone, its upper end was wedged cross and cross. There being generally two trenails to each piece of stone, no assignable power, less than what would by main stress pull these tre nails in two, could lift one of these stones from their beds when so fixed, exclusive of their natural weight, as all agita tion was prevented by the lateral wedges. The stone being thus fixed. a proper quantity of the beat mortar was liquefied, and the joints having been carefully pointed up to the upper surface, the grout so prepared was run in with iron ladles, and was brought to such a consistency as to occupy every void space ; and though a considerable part of this was water, yet that being absorbed by the dry stones, and the more con sistent parts settled to the bottom, the vacuity being at the top, this was repeatedly refilled till all remained solid : the top was then pointed, and, when necessary, defended by a coat of plaster.