Page:Cassier's Magazine Volume XV.djvu/23

Rh the concrete and masonry were carried up to their present height.

One of the most ingenious devices called into being by the necessities of the occasion was the hydraulic spade which Mr. Arrol invented for use in the caissons. This consists of a wrought iron cylinder with a brass casting screwed on to one end, through a stuffing

Cassier's Magazine - General view of the drill roads at the forth bridge.jpg

box in which the shaft of the spade passes. On the other end is fixed a cap into which are screwed short wrought iron pipes to vary the lengths of the spade as required. The spade is forged on one end of the shaft, on the other end of which is a piston with the necessary cup leather. A screw in the lower casting communicates with both ends of the cylinder, and high-pressure water is led to it through a small flexible hose, while another hose is employed to carry off the exhaust water.

Each spade is worked by three men, and the action of it is very similar to the ordinary spade. Thus, with a working face of, say, 15 inches, one man will stand on each side of the spade to lift it by a cross handle and place the point of the spade in the ground, while the third man, stationed behind, opens the cock and allows the water to enter the top, causing an upward movement of the cylinder, which, being arrested, forces the spade into the ground until the piston reaches the end of its stroke. Then, upon reversing the cock, the cylinder falls and sets free the upper end, ready to repeat the operation. By means of this ingenious machine material that had before been taken away only piecemeal by hand in the air chambers was now excavated in large lumps.

As the locks for removing the material were of a novel form, an illustration of them is given on page 9. While the main piers in the river were in progress the granite piers for the approaches to the main structure were being built. After these had attained to a level of about 10 feet above high