HELL-GATE, IMPROVEMENT OF. 



381 



ami s'mrnlt means discharges, and of enlarging 

 ..:iv:itii>ii a- it :nlv:inc.-il, the progress was 

 Mlmv c(>iii|>:in-(l with hind-tunneling. Up to 

 .Inly, 1872, tho drilling was mostly dono by 

 luiml. Ihf Knrleigh steam percussion-ma- 

 cliiin- wa* then adopted. It proved to he 

 much hetter than the hand-drill, even for the 

 holes required by these operations, drill- 



BURLEIQH nun. i 



ing 80 feet of holes in eight hours, at a cost of 36 

 to 87 cents per foot, including repairs, against 

 a cost of 95 cents per foot by the manual pro- 

 cess. Experiments were made with the Inger- 

 sitll percussion-machine, which proved not in- 

 ferior to the other ; the diamond drill was also 

 used, which was found to be an excellent in- 

 strument, but useful here only for exploring 

 the rock ahead, the varying dip of the rock 

 and the frequent quartz-veins rendering it in- 

 capable for general work. The new Rand 

 drill was employed later, with approved suc- 

 cess. 



The bottom was accurately surveyed in 1871, 

 and the plans and calculations for the work 

 were based on as many as 16,000 sound- 

 ings. The inclination of the tunnel was varied 

 HO as always to leave a roof of 6 to 15 feet 

 thickness with an average of 10 feet between 

 the mine and the water. The tunnels were 

 kept free from water by constant pumping ; in 

 some of these the floor was inclined so as to 

 convey off the water. The total mass of the 

 reef has been computed at 110,461 cubic yards. 

 The utmost care had to be taken not to leave 

 too thin a roof, and not to break through the 

 roof by too heavy explosions ; and the unman- 

 ageable character of the rock, which is a very 

 hard hornblende gneiss of variable inclina- 

 tion, with foliations and intersecting quartz- 

 veins, greatly enhanced the difficulties of the 

 work. Of the expense of tunneling, blasting 

 represented 46 per cent., and the removal of 

 the rock to the shaft 17 per cent. The amount 

 of explosives employed was carefully propor- 

 tioned to the work done, according to the 

 principle that the charges required in blasting 

 are proportionate to the cubes of the lines of 

 bast resistance. The numerical coefficient of 

 nitro - glycerine for this rock expressed in 

 pounds, that is, the weight required for a line 

 of least resistance of one foot, General Newton 

 found by experiment to be .038. This into .75 

 gave the amount required in pounds of dyna- 

 mite (formula, L s x .038 x .75). 



The insufficiency of the appropriations of 

 Congress greatly delayed the work. The 



amount of rock excavated and carried away 

 through the shaft was 47,461 cubio yanln, 

 leaving only about 30,000 yards to be dredged 

 away, provided that the shattered IUHBHCH were 

 not heaped together upon tho bottom by the 

 explosion a result which General Newton 

 sought to avoid by distributed charges. When 

 the works were ready for the explosion, which 

 was to shatter the remaining shell of the reef, 

 the pillars and roof were charged with a mul- 

 titude of small charges of explosive material 

 distributed all through the remaining rock, 

 and every charge was connected with an elec- 

 tric wire, excepting a certain number which 

 were not primed, but must explode from the 

 concussion. In the concluding portion of the 

 work there were 67 men employed, 3 foremen, 

 17 miners and chargers, and 47 helpers. The 

 engineers in charge were : John Newton, lieu- 

 tenant-colonel of engineers, brevet major-gen- 

 eral ; James Mercur, captain of engineers ; Jo- 

 seph H. Willard, first-lieutenant of engineers; 

 Julius II. Striedinger, civil engineer, assistant ; 

 Bernard F. Boyle, mining engineer, overseer ; 

 James Quigley and Robert S. Burnett, assist- 

 ants. Captain William H. Heur was for a long 

 time resident engineer at Hallett's Point. The 

 minimum amount of explosive material would 

 have been employed if every square pier had 

 received one charge in its centre, and every 

 oblong pier two charges ; but General Newton 

 determined to decrease the lines of least resist- 

 ance by using a greater number of blasts, and 

 thus diminish the vibratory and disturbing 

 effect upon the reef. 



The day before the blast the water was let 

 into the mine by means of a siphon over the 

 coffer-dam. The explosion was delayed until 

 Sunday, by reason of a delay in the delivery of 

 explosives, and General Newton received many 

 remonstrances against firing the mine on that 

 day, but refused decidedly to risk the lives of 

 his men by a day's delay. The holes were di- 

 vided into 184 groups of 20 holes each, every 

 eighth group having a separate battery ; and all 

 the wires were conducted over a plate or cross- 

 bar in the bomb-proof near the shaft, which 

 acted as a circuit-closer and effected the simul- 

 taneous ignition of all the primers. This plate 

 was also made the means of establishing the 

 voltaic current; for when the operator, Gen- 

 eral Newton, touched the key of his battery, 

 it caused a torpedo to explode which broke a 

 cord, by which the plate was suspended, and 

 it falling plunged a number of pins, with which 

 its under side was studded, into small cups of 

 mercury, thus generating the electricity which 

 exploded the mine. At 2.51 p. M., the hour 

 of high tide, September 24th, tho blast took 

 place. The explosion lasted three seconds. 

 It was marked by a vast mass of foaming wa- 

 ter, composed of numerous distinct spouts, ris- 

 ing up in the air to the height of 40 or 50 feet, 

 above which issued fragments of rock mingled 

 with mud, which were projected 16 or 20 feet 

 higher, accompanied by a thick clond of black 



