Page:Popular Science Monthly Volume 28.djvu/457



The now facts obtained by this experience are:

1. That an unlimited amount of explosives distributed in blast-holes in moderate charges, proportioned to the work to be done, thoroughly confined in the rock, and tamped with water, may be fired without damage to surrounding objects.

2. That an unlimited number of mines may be simultaneously fired by passing electric currents through the platinum-wire bridges of detonators.

Substantially the same methods as those which had proved efficient upon the Hallet's Point Reef were applied to the larger and more formidable Flood Hock. Two shafts were sunk from the ridge of the rock (Fig. 4), whence the whole nine acres of the reef—extending 1,200 feet in length and 602 feet in width—was undermined by two sets of parallel galleries, running at right angles to one another.

The piers of rock left between these galleries to support the roof of the mine were about fifteen feet square and twenty-five feet apart from center to center. The roof of the cross-galleries, which ran at right angles to the lines of stratification, was blasted down as thin as it would be safe to leave it. (Figs. 5 and 6). Considerable risk was incurred in this part of the work, from the danger of the rock crumbling, and from the uneven and uncertain thickness of the roof. The average thickness was 18·8 thick, and the minimum thickness ten feet. The exact thickness could not be ascertained beforehand, for no