and top, and is provided with self-adjustable legs so arranged as to be all let go together after it is lowered. This caisson or dome is simply a framework affording a fixed support to the drill-tubes, twenty-one in number, through which the drills operate. It is connected with the scow by four chains, communicating with four hoisting-engines, by
Fig. 2.—Deck Plan.
which it is lowered or raised. The scow, having the dome swung by chains, is anchored over the rock to be operated upon, by as many anchors as are required to hold it against all the currents. The diver then goes down to examine the bottom and see that the position is favorable. The position of the scow is changed by lengthening and shortening the moving chains with capstans. When all is fixed, the dome is lowered close to the bottom and established by letting the legs go to adjust themselves on the bottom. The chains which connect it with the scow are unslung, and flexible connections are made between the tops of the drill-rods and the piston-rods of the driving-engines. These connections must be flexible, because, the dome being fixed on the bottom, while the scow holding the drill-engines is certain to swing for short distances back and forth, the connections, if rigid, would be broken. When the drill-holes are completed and ready to be charged, the dome is lifted up, and the scow, carrying the suspended dome, is swung off from the spot to a safe distance (the length of which depends on the amount of the charge), without casting loose the moorings. The charges of explosives, in tin cases of different lengths to suit the varying depths of the drill-holes, are carried to the spot upon a small scow, whence the diver descends to insert them into the holes; the cartridges are handed down to him, already attached to leading wires by the men on the scow; and he is guided from hole to hole by lines connecting the stoppers or plugs inserted in the holes. The scow is withdrawn, the leading wires are connected with the battery, and the explosion is made with invariably certain effects.
To secure satisfactory results in the breaking up of the rock, the drill-holes should be six or eight feet apart, five to six inches in diameter, and should reach to about four feet below the level to which it is desired to break the rock. The broken rock is removed by means
