No. 6.
No. 7.
These holes were bored with jumpers, 3¹⁄₂ inches in diameter, and were sunk 2 feet deep in the rock. The boring of each hole took upwards of eight hours, in consequence of the hardness of the material, which is gneiss, a stone considerably more difficult to bore than even the granite of Aberdeenshire. The bats or stancheons, although very accurately forged, were occasionally found not to fit truly, owing to unavoidable twists in the holes, which arose from dries or veins in the Rock crossing the line of the hole, and thus disturbing the motion of the jumper. This gave us much trouble, and shewed that, had we determined, as I at first contemplated, to cut a lewis-hole, swelling towards the base, the work would have been almost impracticable. The mode which I had proposed for executing this operation was to bore a number of very small holes, inclined at the proper angle, all round the outside of this lewis chamber, and then to cut out between them; but this, as our after experience in cutting the foundation of the Tower proved, would have occupied an extent of time which we should have been very unwilling to bestow upon a merely temporary erection like the wooden barrack. Even as it was, and with all the retrenchments that could be safely adopted, the preparation of the seats for the six outer or main beams, and those for the six inner braces, employed twelve men for four days.
[17] The accented letters e′, c′, f′, g′, o′, s′, in the figure ([No. 4]), page 85, denote the various parts of the gauging-rule, when applied to the beam, opposite that to which the letters e, c, f, g, o, s, in the text, refer.
No. 8.
After the seats for the timbers had been dressed in this manner, the carpenters were employed cutting the beams to their respective lengths, the piece to be cut off being measured, as already stated, by the length through which the sliding-board f f ([No. 4], p. 85) had been raised above its position on the level platform on which the pyramid had been erected in the workyard at Greenock. At the same time, the stancheons (k k) in the figure ([No. 8], p. 88), and the glands or collars e e (in [figure No. 8]), were let into grooves in the beam, and the holes admitting the screwed bolts a a, to pass through the two stancheons and the beam between them, were bored with an auger, and widened with a red hot iron. The tops of the beams b b (see [fig. No. 9]), having been already fitted in the workyard at Greenock, so as to meet a hexagonal quoin of hardwood e, round which they were assembled as shewn in the figure ([No. 9]), straps of iron d d, were made to pass over the top of the whole, and were secured to the beams with bolts, and a spike at a was driven into the centre to wedge the timbers tightly up, so as to fill a ring which embraced the exterior of the whole. It was obvious, that if the sliding-board (described on [p. 85]) had indicated the true inclination of the seat on the Rock for the end of the beam to rest on, as well as its radial distance from the centre of the pyramid and the corresponding length of the beams, the top of each beam must necessarily meet in its exact place around the central hexagonal quoin. The operation of determining the positions and lengths of such beams on a rugged rock, and placing them with the accuracy requisite, to insure their mitering truly at the top, was attended with a good deal of trouble; and I have judged it advisable to give these details, as they may prove useful to others who may have a similar work in hand.
No. 9.