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THE POPULAR SCIENCE MONTHLY. 



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 PLiN. 



"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 an- 

 chors 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. A\^hen 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, w'hile 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 thtf 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 withdrawn, tlie leading wires are connected with the battery, and the 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 diame- 

 ter, and should reach to about four feet below the level to w^hich it 

 is desired to break the rock. The broken rock is removed by means 



