INDUSTRIAL TROGRESS DURING THE YEAR 1876. ccxv 



extremely hard, foliated, hornblendic gneiss, intersected with nu- 

 merous quartz veins, which rendered the progress of the tunneling 

 work extremely dithcult, and required the exercise of constant vigi- 

 lance on the part of the engineers in charge of the work to avoid 

 getting too near the bed of the river, and the shattering of the roof 

 of rock by too heavy shots. In only one case, it is affirmed, was the 

 roof endangered, and there a wall had to be built to support it. 

 The explosives used in making the excavations were nitro-glycerine, 

 several of its compounds, and common black powder. Since the' 

 year 1872 nearly all the drilling was done with the Burleigh rock- 

 drills and the Rand drill. The diamond drill was used to some 

 extent, principally for exploring the rock ahead of the work ; but 

 the varying hardness of the rocks and their inclination were unfa- 

 vorable to it. Previous to 1872 the drilling was done by hand, and 

 the progress of the work affords a good criterion by which to esti- 

 mate the relative economy of hand and machine drilling. In this 

 case the work of drilling was contracted for at so much per foot. 

 Hand-drilling cost about 95 cents per foot, while the cost of drilling 

 with the Burleigh rock-drills, including repairs, etc., was found to 

 be, on an average, 36 to 37 cents per foot. The work of excavation 

 was finished about the month of July, 1875, and the work of prepar- 

 ing for the final blast, involving the boring of holes in the pillars 

 and roof, the charging of the same with explosives, etc., occupied a 

 number of months. The firing of tlie mine was effected by electric- 

 ity ; and to effect the simultaneous explosion of all the holes, the 

 conducting wires leading from the same were held on a cross-bar, 

 forming a circuit closer ; and the circuit was closed by firing a small 

 torpedo which held the cross-bar up ; when this took place, the bar 

 fell, the brass points to which the wires were attached plunged into 

 small cups filled with mercury, and which were connected with the 

 second set of wires from the primej-s, and the circuit was closed. 

 Every thing being in readiness, the water was let into the vast cav- 

 ern, and on the afternoon of Sunday, the 24th of September, the mine 

 was successfully exploded. The amount of disturbance caused by 

 the explosion of the 52,206^- lbs. of dynamite, rend-rock, and Vulcan 

 powder employed was moderate and circumscribed, and was quite a 

 disappointment to a large body of spectators that had been attracted 

 to the scene in the expectation of witnessing a terrific convulsion. 

 As subsequent soundings demonstrated, however, the reef was very 

 completely shattered, and the depth of the river considerably in- 

 creased even before the removal of any of the debris, which eventu- 

 ally will all be dredged up. Until this is done the complete results 

 of the work of demolition will not be accurately known. The work 

 upon the Flood Rock excavations, it is understood, will now be en- 

 ergetically pushed forward, operations having been held in abeyance 

 pending the consummation of the Hell Gate explosion. 



