446 THE POPULAR SCIENCE MONTHLY. 



The number of pounds of rack-a-rock put into drill-holes was 

 240,399 ; of dynamite, 42,331 ; total, 282,730 pounds. There were 

 11,789 drill-holes in the roof and 772 in the pillars, and their total 

 length was 113,102 feet, or more than twenty miles. The whole 

 amount of rock to be broken by the final blast was 270,717 cubic 

 yards, covering an area of about nine acres. 



The primary charges, the office of which was by their detonation 

 to produce the explosion of the charges in the drill-holes, were placed 

 along the galleries at intervals of twenty-five feet, and arranged as shown 

 in Fig. 12. They were placed on timbers extending from wall to wall 

 in each of the galleries, and consisted of two twenty-four-inch dynamite 

 cartridges like those already described lashed to the timber, with one 

 of the "mine-exploders," also already described, bound upon them. 

 The entire mine was divided into twenty-four independent circuits. 

 Within each of twenty-one of these circuits were twenty-five fuses or 

 mine-exploders, while three circuits contained twenty-two fuses each. 

 A wire from the battery on the surface of the rock at the mouth of 

 the shaft led from one fuse to the next, until the twenty-five fuses 

 were in the same electrical circuit, and thence back to the battery. 

 So far as was practicable, adjacent charges were put on different cir- 

 cuits, so that if any circuit failed through any fault in the connections, 

 an explosion of its charges would still be insured through the sympa- 

 thetic action of the adjoining charges. The whole number of these 

 primary charges was 591. Some of the circuits were nearly a mile long. 



The fuses prepared for this work had a resistance of 1 '73 ohms cold, 

 and 2-76 ohms at explosion. To fire a single fuse, 0-205 amperes were 

 required ; to fire a series, 0-G15 amperes. A factor of safety of two was 

 used, and double this current was sent through every fuse at the final 

 blast. The battery consisted of sixty cells, all coupled in one series, 

 each of which had an electro-motive force of 1*95 volts and an internal 

 resistance of O'Ol ohms. The plates were six inches by nine inches — 

 four carbon and three zinc plates in each cell, separated by a quarter of 

 an inch. The ordinary bichromate solution was used. The poles were 

 constituted of two large mercury-cups, into one of which were dipped 

 the twenty-four lead wires, while the twenty-four return wires ter- 

 minated in a third cup. Between this third cup and the remaining 

 pole of the battery stood the apparatus for closing the circuit. It 

 consisted of a stout iron cup containing mercury, in which sat a 

 thin glass tumbler also partly filled with mercury. Two large strips 

 of copper connected the mercury in the iron cup with one pole of 

 the battery, and that in the glass with the cup containing the re- 

 turn wires. To close the circuit through the fuses it was only neces- 

 sary to break the tumbler so as to let the mercury in it mix with that 

 in the iron cup. To do this at the proper moment, a one-quarter-inch 

 iron rod four feet long, terminating at the top in a small round disk, 

 stood with its point in the bottom of the glass. It was long enough 



