MECHANICS AND USEFUL ARTS. 95 



tended a distance of 200 feet, and are from 8 to 10 feet in diame- 

 ter. The rock is very hard mostly gneiss. The blasting charges 

 are one-quarter, one-half, 1 and 2 pounds respectively, contained 

 in pasteboard tubes well wrapped in glazed gutta-percha cloth, 

 tarred at the ends, with safety-fuse, all water-proof. 30 or 49 of 

 these are discharged at the same time, a gong being previously 

 sounded to draw off the workmen. 



Huge logs, interlaced with iron bands, form " curtains," which 

 are hung at the opening of each tunnelling to prevent the de- 

 tached rock from being hurled into the air without. The force is 

 so terrific, however, that these ponderous curtains are often 

 swung out 8 or 10 feet. Great steam derricks elevate the refuse, 

 and steam pumps lift out the water. Daily observations are 

 taken to guide the work. A miniature levee keeps off the water 

 on the river side. A temporary platform extends out over the 

 reef, from which a fine view of the wild, rushing tides is afforded. 

 Even when but about a depth of 2 feet of water is flowing over 

 the " hog's back," its force is so great as to sweep a strong man 

 instantly off his feet. This gigantic enterprise must proceed very 

 slowly, and require years for its successful completion. 



EXPLOSIVE POWER OF NITRO-GLYCERINE. 



"We condense from the "American Chemist" the following 

 upon the above subject : 



A measure containing 1 cubic foot will hold 796 ounces of 

 blasting-powder, and 997.1 ounces of water; or, in other words, 

 the specific gravity of blasting-powder, as it is used, is about 0.8. 

 This, of course, takes in the interstices, which are filled with air, 

 but as we do not use the powder in a solid lump, this is, for prac- 

 tical purposes, the specific gravity of blasting-powder. Now, the 

 specific gravity of nitro-glycerine is 1.6. Therefore, bulk for bulk, 

 if the explosive power were the same in a given mass, as pre- 

 pared for blasting, the nitro-glycerine would have twice the 

 power. 



In reality the following are the volumes of gas generated by 

 each respectively in explosion : 



One volume of powder, which is considered as most effective, 

 produces : 



Carbonic acid gas 221.4 vols. 



Nitrogen, 74.6 vols. 



Of another kind of powder, which explodes with the gases at a 

 lower temperature, one volume produces : 



Carbonic oxide, 391 vols. 



Nitrogen, G6 vols. 



One volume becomes 457 vols. 



