The reason for this is that though wet guncotton is safe to handle because a very great shock is required to make it explode, dry guncotton is much less so, while a shell or torpedo filled with fulminate of mercury would be more dangerous to its owners than to their enemies, because the slightest jar might set it off prematurely. Every precaution is taken to prevent a torpedo’s exploding too soon and damaging the vessel from which it is fired. When the torpedo is shot out of the tube, by compressed air, like a pea from a pea-shooter, the striker is held fast by the “jammer”: a small propellor-shaped collar, whose blades begin to revolve as soon as they strike the water, till the collar has unscrewed itself and dropped off after the torpedo has traveled about forty feet. A copper pin that runs through the striker-rod is not removed but must be broken short off by a blow of considerable violence, such as would be given by running into a ship’s hull. As a third safeguard, there is a strong safety-catch, that must be released by hand, just before the torpedo is placed in the tube. The explosive charge of two or three hundred pounds of wet guncotton is called the “war-head.” In peace and for target-practice it is replaced by a dummy head of thick steel. The usual target is the space between two buoys moored a ship’s length or less apart. At the end of a practice run, the torpedo rises to the surface, where it can be recovered and used again. This is distinctly worth while, for a modern torpedo costs more than seven thousand dollars. Back of the war-head is the air-chamber, that contains the motive-power of this miniature submarine. The air is either packed into it by powerful pumps, on shore or shipboard, or else drawn from one of the storage flasks of compressed air, a number of which are carried on every submarine. The air-chamber of a modern torpedo is charged at a pressure of from 2000 to 2500 pounds per square inch. As the torpedo leaves the tube, a lever on its back is struck and knocked over by a little projecting piece of metal, and the starting-valve of the air-chamber is opened. But if the compressed air were allowed to reach and start the engines at once, they would begin to revolve the propellors while they were still in the air inside the tube. This would cause the screws to “race,” or spin round too rapidly and perhaps break off. So there is a “delaying-valve,” which keeps the air away from the engines till another valve-lever is swung over by the impact of the water against a little metal flap.