EAST RIVER AND HELL GATE IMPROVEMENT. 443 



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The new facts obtained by this experience are : 



1. That an unlimited amount of explosives dis- 

 tributed in blast-boles in moderate charges, pro- 

 portioned to the work to be done, thoroughly 

 confined in the rock, and tamped with water, 

 may be fired without damage to surrounding 

 objects. 



2. That an unlimited number of mines may be 

 simultaneously fired by passing electric currents 

 through the platinum-wire bridges of detonators. 



Substantially the same 

 methods as those which had 

 proved efficient upon the 

 Hallet's Point Reef were 

 applied to the larger and 

 more formidable Flood 

 Rock. Two shafts were sunk 

 from the ridge of the rock 

 (Fig. 4), whence the whole 

 nine acres of the reef — ex- 

 tending 1,200 feet in length 

 and 602 feet in width — was 

 undermined by two sets of 

 parallel galleries, running at 

 right angles to one another. 

 The piers of rock left be- 

 V '^ e o tween these galleries to sup- 



port the roof of the mine 

 were about fifteen feet 

 square and twenty-five feet 

 7 apart from center to center. 

 The roof of the cross-gal- 

 leries, which ran at right an- 

 gles to the lines of stratifica- 

 tion, was blasted down as 

 thin as it would be safe to 

 leave it (Figs. 5 and G). Con- 

 siderable risk was incurred 

 in this part of the work, 

 from the danger of the rock 

 crumbling, and from the un- 

 fv /Mi'^^^^ 1 even and uncertain thickness 



of the roof. The average 

 thickness was 18*8 thick, and 

 the minimum thickness ten 

 feet. The exact thickness could not be ascertained beforehand, for no 



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