fixtures, which, as afterwards shewn in figure (No. 8, p. 88), spread outwards from the axis of the beam, and thus formed a kind of dovetail. In order to make the holes capable of receiving the bats and, at the same time, embracing the timbers of the barrack, a quoin of wood (Nos. 6 and 7) _e_, was put down, with bevelled faces or grooves _g_, cut in it for directing the motion of the jumper or boring iron _i_, thus:-- [Illustration: No. 6.] [Illustration: 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. [Illustration: No. 8.]