REMOVING OBSTBUCTTONS. 2Sr> 



thus be removed piecemeal, until the passage shall 

 have been completc'ly cleared, and the possible cause 

 of many serious accidents removed. 



It is seldom that so powerful an instrument as the 

 torpedo is necessary, but submarine blasting is often 

 resorted to ; and, in such cases, a diver comes into 

 requisition. He directs the apparatus, worked either 

 by hand or steam-power, and prepares the ground 

 for the introduction of the destructive agent. When 

 the hole is made he inserts the iron vessel filled with 

 powder or nitro-glycerine. He covers it with cement, 

 and places it in communication with the shore, by 

 means of conducting wires for the electric current, 

 or a fuse which will burn under water. The diver 

 then retires. Either the fuse is lighted, or an 

 electric current is sent through the wires, and the 

 explosion takes place. 



At great depths the action of the powder is pro- 

 digious. Compressed by a column of water, the 

 gases exert an increased force on the rock (if that 

 be the nature of the obstruction), and tear it in a 

 thousand directions. At the surface there is scarcely 

 any indication of the concussion below, except a 

 slight agitation of the water. Dead fish float about 

 the scene of action, and a hollow sound is heard. 

 If, however, the depth of the water be sliglit, the 

 explosion causes the projet-tion of a jet into the air. 



