soundings could distinguish between the solid rock and a concretion of bowlders and shells formed upon it. Had the excavations at any time broken into a large seam, the mine would have been flooded by the inrush of the water, and all the work and probably many lives would have been lost. Occasionally small seams were met and had to be dealt with. One seam was ten inches wide and a hundred feet long; another one, from one to four inches wide and 400 feet long, extending clear across the reef, carried 850 gallons of water a minute. The latter was dealt with after protecting the completed part of the work by building across the gallery a door capable of withstanding the pressure of the water. The seams were all walled, as fast as they were opened, with Portland cement. The total length of the galleries was 21,070 feet.
The galleries were excavated to depths varying with the uneven surface of the reef. The roof was then drilled with holes for the reception of the explosive cartridges, with which the rock was to be finally blown up. The holes were slanted upward at angles varying from 75° to 45°, and were made from eight to ten feet deep—except where the existence of seams open to the river made it impossible to obtain the depth wanted—and of sufficient capacity to receive a rigid two-and-a-half-inch cartridge throughout their entire length.
The holes were charged with rack-a-rock as the principal explosive—a substance formed by mixing 79 parts of finely-ground chlorate of potash and 21 parts of di-nitrobenzole. It is one of the safest explosives to handle, and the ingredients are absolutely inert when kept separate, and they need not be mixed till just before loading the cartridge; it has 109310 per cent the strength of No. 1 dynamite, when fired under water, and costs but a little more than half as much. The mixing was done in small batches on Great Mill Rock, in a lead-lined trough, and the explosive was packed at once into cartridge-cases 214 inches in diameter and 24 inches long, made of copper 0·005 of an inch thick. To prevent the corrosion of the copper by the chemical action of the sulphureted water running through some of the drill-holes, the cartridges were protected by being dipped in melted resin, beeswax, and tallow. The cartridges, after being loaded, were soldered with a steam-heated soldering-iron; were removed as fast as they were filled, and were carried to the mine in boxes containing twenty each; so that the amount of mixed explosive above-ground at any one time was never enough to do more than local damage in case of an accident. These cartridges were inserted in the drill-holes, one after the other, till the holes were filled, the last cartridge in every case being filled with dynamite, with its end left to project about six inches, so that it might receive the full effect of the shock from the initial charges connected with the battery. This cartridge is represented in Fig. 7, and is 15 inches long and 214 inches in diameter. In its forward or projecting end is inserted a small copper shell filled with fulminate of mercury. The other cartridges, charged with rack-a-rock, repre-
