the centre being always fixed by the two theodolites reading angles from fixed stations. The whole area for about 40 ft. or 50 ft. round the two caissons was thus sounded, and a very fair idea of the contour of the bottom obtained. About 3000 soundings were thus taken, representing a large amount of work and trouble with the sounding gear, the mooring chains, the winches, and last, though not least, with rough weather, and distortions thereby produced in the raft itself.
THE PNEUMATIC CAISSONS; QUEENSFERRY.
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The raft was also used for fixing the guide piles or columns against which the caissons would come to lie, and, as a precautionary measure, the centre was set 1 ft. up the slope of the rock, as it was tolerably certain that during the sinking operations the caisson would slide away from the newly cut face.
As soon as the soundings had been taken, and the guide columns fixed for the reception of the caissons, heavy mooring blocks were laid down to the south-east, south, and south-west, with heavy cable chains attached, the ends of which were temporarily secured to the staging. Two stout wire hawsers were also prepared to pass round the caissons as soon as they would be placed in position.
To hold the caissons with some amount of security when once they would take the ground, and give them a good bearing all round the edge, it had been decided to place a large number of sandbags upon the rock, and bring these up to the same level and somewhat above even as the highest point of rock the cutting edge of the caisson would be likely to touch. In order to still more securely fix this bed of sandbags, two rectangular piers were built up first of bags filled with concrete, and these were placed (see Figs. 29 & 30) opposite the highest point of the rock on the circumference of the 70-ft. circle. The concrete was of good strength, there being 27 cubic feet of stone to 41⁄2 cubic feet of cement and 41⁄2 cubic feet of sand. It was mixed dry, slightly wetted, and at once filled into bags. About twenty-six of these went to a yard, and they were put into boxes with hinged bottom and lowered down by a steam crane and emptied. They were put together by a diver, and, when a couple of feet of pier had been built, a number of sandbags of much larger size were put all round it to keep it in place and prevent the tide from washing out the cement. When the concrete piers had been completed, the remaining space under the cutting edge was laid with sandbags, except in two or three places where channels were left through which the air could escape, and through which also the sandbags and other debris could be pushed down the rock during the sinking of the caissons. In two places well within the air-chamber, and near where the centre of the caisson would come to be, two piers of sandbags were built up upon which the ceiling of the air-chamber would be supported. It remained now to lay down the pipes for the
