Page:Popular Science Monthly Volume 28.djvu/460

446 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 circuits, so that if any circuit failed through any fault in the connections, an explosion of its charges would still be insured through the sympathetic 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 ampères were required; to fire a series, 0·615 ampères. 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 0·01 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 terminated 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 eat 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 return wires. To close the circuit through the fuses it was only necessary 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