HELL-GATE, IMPROVEMENTS AT. 



473 



after the excavation, being con- 

 nected with the rock only 

 through the piers and outer 

 edge of the roof, it was in- 

 ferred that the shock propa- 

 gated in the rock would be 

 due mainly to the charges ne- 

 cessary to disrupt the piers 

 and roof from their connec- 

 tion 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 



Pounds. 



Rend-rock 9,127 



Vulcan powder 11,853 



Dynamite 28,935 



Total 49,915 



Being at the rate of 0'79 pound 

 to each cubic yard. The ex- 

 plosives were placed in tin 

 cartridge - cases. The num- 

 ber used was 13,597, 87 per 

 cent, being 22 inches and the 

 remainder 11 inches in length. 

 The number of holes charged 

 was 4,427. The system con- 

 sisted of 3,680 mines and 23 

 batteries ; each battery as- 

 signed to 160 mines, which 

 were divided into 8 groups of 

 20 each. The mines of each 

 group were connected in con- 

 tinuous series, and a lead and 

 return wire to the battery 

 closed the circuit. The mines 

 were fired at 2" hours 50 min- 

 utes p. M., Sept. 24, 

 1876, and there were 

 no injurious shocks 

 in the atmosphere, in 

 the water, or under- 

 ground. The new 

 facts obtained by 

 this experience are : 



1. That an unlimited 

 amount of explosives 

 distributed in blast - 

 holes in moderate 

 charges, proportioned 

 to the work to be done, 

 thoroughly confined in 

 the rock, and tamped 

 with water, may be fired 

 without damage to sur- 

 rounding objects. 



2. That an unlimited 

 number of mines may 

 be fired simultaneously 

 by passing electric cur- 

 rents through the plat- 

 inum-wire bridges of 

 detonators. 



Flood Rock. Substantially the same methods 

 as those that had proved efficient upon the 

 Ballet's Point Reef were applied to the larger 

 and more formidable Flood Rock. Two shafts 



EXCAVATIONS OP PI.OOD HOCK. 



were sunk from the ridge of the rock, whence 

 the whole reef 1,200 feet in length and 602 feet 

 in width, about nine acres was undermined 

 by two sets of parallel galleries, running at right 



