EXCAVATION OF SUBAQUEOUS HOCKS BY A LARGE BLAST 45 



it at short range with impunity. This explosive was afterward 

 adopted for the great blast, its strength under water being somewhat 

 greater than that of No. 1 dynamite. It cost a little more than 

 half of No. 1 dynamite and had the great advantage that it could 

 be stored in large quantities without danger to the city, as it was 

 made explosible just before being taken to the mine. 



The rackrock, consisting of 79 parts of finely ground chlorate of 

 potash and 21 parts of di-nitrobenzole, was mixed in small batches 

 in a leaden trough and packed at once into cartridge cases 2J in. 

 in diameter and 24 in. long, made of copper 0.005 of an inch thick. 

 Into each cartridge was inserted a small exploder containing 30 

 grains of fulminate, reinforced by one ounce of dynamite. The 

 cartridge, being loaded its lid was securely soldered, using an alloy 

 melting at 160 F. and a hollow soldering iron heated by blowing 

 through it a jet of wet steam. In all 42,528 cartridges were thus 

 soldered without accident. 



All the loading in the mines w T as done by a gang of 12 men, w T ho 

 first placed the rackrock cartridges, ending with one of dynamite 

 containing the fulminate exploder, besides the usual brass wires, 

 which w^ere secured by means of wooden wedges. The number of 

 pounds of rackrock put in drill holes was 240,399; of dynamite, 

 4331; total, 284,730 pounds of explosives. There were 11,789 drill 

 holes in the roof and 772 in the pillars. To prevent the cartridges 

 from slipping out of the inclined holes the exterior of the cartridges 

 was provided with short brass wires spreading out. 



The primary charges which were to fire those in the drill holes 

 were placed along the galleries at intervals of 25 ft. They consisted 

 of two 24 X li-inch thin copper cartridges filled with No. 1 dynamite, 

 packed solidly and lashed upon a horizontal beam at a height above 

 the floor varying from 3 to 12 ft. according to the height of the 

 galleries. On top of these was lashed a rigid brass shell 8-12 in., 

 containing about one-half pound of dynamite put in loose and a plat- 

 inum wire connected by wires with the battery at the head of the 

 shaft. There were 591 of these primary charges arranged in 21 

 circuits of 25 each, and three circuits of 22 all coming together at 

 the poles of the batteries. Some of these circuits were nearly one 

 mile long. 



The battery was composed of 60 cells all coupled in one series, 

 two large mercury cups constituting the poles. The 24 lead wires 

 dipped into one of these cups and the 24 return wires terminated 



