EAST RIVER AND HELL GATE IMPROVEMENT. 441 



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 286 two- 

 inch holes ; the total length of holes drilled being 56,548 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 explo- 

 sion 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 sec- 

 ond 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 vibra- 

 tion 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 de- 

 structive 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 con- 

 nection 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 — 



Rend-rock 9,127 pounds 



Vulcan powder 11,853 " 



Dynamite 28,935 " 



Total 49,915 



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



The explosives were placed in tin cartridge-cases. The number 

 used was 13,596, 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 3,680 mines and 23 batteries. Each bat- 

 tery assigned to 160 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 p. m., September 

 24, 1876, and there were no injurious shocks in the atmosphere, in the 

 water or underground. 



