444 THE POPULAR SCIENCE MONTHLY. 



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 in- 

 rush 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 Avere all walled, as fast as they were opened, 

 with Portland cement. The total length of the galleries was 21 ,G70 feet. 



The galleries were excavated to depths varying with the uneven 

 surface of the reef. The roof was then drilled with holes for the re- 

 ception 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 suflScient 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 explo- 

 sive — 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 lOO^V 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 2\ 

 inches in diameter and 24 inches long, made of copper 0005 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 con- 

 nected with the battery. This cartridge is represented in Fig. 7, and 

 is 15 inches long and 2^ inches in diameter. In its forward or pro- 

 jecting end is inserted a small copper shell filled with fulminate of 

 mercury. The other cartridges, charged with rack-a-rock, repre- 



