Page:Popular Science Monthly Volume 28.djvu/455

 As soon as the excavation was finished, the work of drilling holes in the roof and piers, to be afterward charged with explosives, was begun. At the completion, March 25, 1876, there had been drilled in the roof 5,375 three-inch, in the piers 1,080 three-inch, and 280 two-inch holes; the total length of holes drilled being 50,548 feet of three-inch and 1,897 feet of two-inch holes.

The proximity of the reef to habitations at Astoria, Ward's Island, and Blackwell's Island, made it necessary to devise a system of explosion which, effecting the work of demolition, would at the same time do no damage to life and property. The atmosphere and the rock being the mediums through which the shock would be transmitted, it was essential that the waves propagated through these should be as small as possible.

It was evident, in the first place that, if to each charge its full capacity of useful work in breaking up the rock was assigned, regard being likewise had to the superincumbent weight of water, no external effect of moment would be perceived in the atmosphere. In the second place, it was evident that the magnitude of the rock-wave would depend greatly upon the quantity contained in individual charges, that is, if eighty pounds were required for the individual charge, the vibration of the rock would be much greater than if these charges did not exceed twenty pounds. It was known that eighty-pound charges of nitro-glycerine, fired in numbers of twelve to twenty, did not cause a destructive wave. Again, the reef, after the excavation, being connected with the rock only through the piers and outer edge of the roof, it was inferred that the shock propagated in the rock would be due mainly to the charges necessary to disrupt the piers and roof from their connection with the bed-rock, and not to the charges to break up the roof and piers. The cubic contents of the roof and piers were 63,135 cubic yards, and the amount of explosives—

Being at the rate of 0·79 pound to each cubic yard.

The explosives were placed in tin cartridge-cases. The number used was 13,590, 87 per cent being 22 inches and the remainder 11 inches in length. The number of holes charged was 4,427.

The system consisted of 3,080 mines and 23 batteries. Each battery assigned to 100 mines, which were divided into eight groups of twenty each. The mines of each group were connected in continuous series, and a lead and return wire to the battery closed the circuit.

The mines were fired at two hours fifty minutes, September 24, 1876, and there were no injurious shocks in the atmosphere, in the water or underground.