380

HISTORY OF ENGINEERING.

Book I.

upright headers are cut into dovetails, for their insertion into the mass of rough masonrybelow.

“ From these beginnings, I was readily led to think, that if the blocks themselves were,both inside and out, all formed into large dovetails, they might be managed so as mutuallyto lock one another together, being primarily engrafted into the rock; and in the round andentire courses above the top of the rock they might all proceed from, and be locked to, onelarge centre stone.

“ It is obvious that this method of dovetailing, while the slope of the rock was makinggood by cutting the steps formed by Mr. Rudyerd also into dovetails, it might be said thatthe foundation stones of every course were engrafted into, or rather rooted into the rock ;which would not only keep all the stones in one course together, but prevent the coursesthemselves, as one stone, from moving or sliding upon each other. But after losing hold ofthe rock by getting above it, then, though every stone in the same course would be bondedin the strongest manner with every other, and might be considered as consisting of a singlestone, which would weigh aconsiderable number of tons,and would be further retainedto the floor below by the ce-ment, so that, when completed,the sea would have no actionupon it but edgeways; yet asa force, if sufficiently great,might move it, notwithstand-ing its weight and the smallhold of the sea upon it, amibreak the cement, before timehad given it that hardnesswhich it might be expectedto acquire afterwards, I hadformed more expedients thanone for fixing the courses "toone another, so as absolutelyto prevent their shifting.”

The six lower courses ofstone, already described, werethus engrafted in dovetail re-cesses, cut out of the solid rock,and were therefore perfectlyimmovable from any forceacting horizontally againstthem; as the work wouldnot now have this natural ad-vantage, it was necessary inthe seventh course to adoptsome means which should pro-duce a similar effect; in thecourse number six, at thecentre, a hole was cut, 1 footsquare, and eight others, 1 footsquare and 6 inches deep, weredisposed at regular distancesround the centre; into thesewere introduced cubes of marble, which were to act as joggles. A plug of strong hardmarble, from the Plymouth rocks, 1 foot square and 22 inches in length, were set inmortar in the central cavity, and there fixed by wedges; this plug stood up 9 inches, andthe centre stone of the seventh course, which had a square hole made in it, covered thisplug, and, after being properly grouted, was held firmly together.

After this centre stone was fixed, the four that surrounded it were placed, united by asmany dovetails, projecting from the four sides of the centre stone ; these were secured bydovetailed wedges and grouted as before, and each held down by a couple of trenails. Thewhole formed a circular stone 10 feet in diameter, and weighing 7 tons, which was held bya centre plug and 12 trenails, the circumference of which admitted eight dovetailed recessesto be made in it, and to receive eight other stones of about 12 cwt. each, and in thismanner did the work proceed until the whole of the solid part was complete. Oneplug in the middle, of a foot square, and each, joggle of a foot cube, with the trenails, addedto the strength of each course.

On the fourteenth course was set out the winding staircase and entrance doorway.

E

Fig. 388.

FORTY-THIRD COURSE.