NITROGLYCERINE. 



661 



der, it stands clear of the scow, resting on self- adjusting legs, 

 which adapt themselves to the inequalities of the reef. When 

 the drills are working, the dome is down, out of sight, and the 

 machinery, which at the first glance seems in disorder on the 

 scow, is arranged in order, and is level with the deck. The en- 

 gines which drive the drills are supported on moveable bridges, 

 thrown back when the dome is up ; and the drills work io 

 stout iron tubes passing through the dome, one in the center, 

 and the others arranged round it in a circle about twenty feet 

 in diameter. The dome, when down, serves to protect the div- 

 ers, so that at any time they can go down to regulate the work- 

 ing of the drills, or perform any other service. Without this 

 protection, the divers could not keep their feet, so strong is the 

 current on a rising or falling tide. The divers are protected 

 by a diving suit ; the air is furnished them by a pipe to the 

 back of the helmet they wear, and is forced down by an air 

 pump. When a set of holes are drilled, they are charged with 

 uitro-glycerine, and simultaneously exploded by electricity. 



This simple statement serves to show how much the modern 

 methods of conducting such submarine operations are de 

 pendent npon the advance in chemistry of modern times. In 

 fact, hardly a single appliance used in such operations, from 

 the steam-engine which drives the drills, to the gutta-percha 

 tubes, and the india rubber suits which enable the divers to 

 descend below the water, but what are inventions or discoveries 

 which belong entirely to modern times, and enable men to-day 

 to perform operations which to the ancients would have really 

 been impossible. 



The nitro-glycerine is contained in tin cartridge cases, like 



mammoth candle moulds, ten feet long and from fonr to five 

 inches in diameter. They are connected with the battery by 

 wires. The divers go down and place these in the holes 

 which have been drilled, first pulling out the wooden plugs 

 which hava "^een placed in them after they were drilled, to 



