In submarine vessels explosions may occur either through a collection of gases from the batteries or by reason of leaks in the pipes or tanks of the fuel supply system, or through the bursting of the air flasks belonging to the boat, or the air reservoirs in the automobile torpedoes. The greatest danger is from explosive gases and have been the cause of all explosions in modern submarine craft, and the greatest danger in this connection is the liability of a leak in the gasolene pipes or tanks. This gas is a heavy gas and so goes to the bottom of the vessel, where it is not so easily detected as a gas which rises. There is no certain way of guarding against leaks of gasolene. A leak may occur at any time in a pipe or tank of gasolene through some cause or other no matter how carefully inspected, and the gas from this is so active that it will go through the tiniest hole imaginable--even through a hole which water will not penetrate. The crew of a submarine is always subject to this danger unless the tanks are built outside the hull of the ship. How the Air May Become Poisoned. There is a constant danger of asphyxiation to the men in the submarine. A very small leakage of gas or the exhaust from an internal combustion engine may make the air so impure that those aboard will be overcome. A great deal of care must be taken to keep the air pure and to warn the crew at the first sign of danger from this. When submarines first came into practical use, it was found a good idea to take a number of little white mice down with the vessel to warn all if the air began to become impure. As soon as this occurred, the mice became distressed and squealed as loudly as they could, thus warning those aboard the ship of danger. The mice felt the impurity of the air quicker than the men, not because they had any special gift to discover when the air was bad, but because they breathe much more quickly than man--take shorter and many more breaths. Now, however, a chemical device has been invented which is affected in such a way as to ring a loud bell, if the air in the vessel becomes impure to such an extent that there is any danger. Breathing the same air over and over may fill the vessel with carbonic acid gas. There should be no great danger from this, however, as submarines are now built sufficiently large to provide enough actually pure air for each man aboard for forty-eight hours, and it is hardly conceivable that a submarine need be submerged more than half that length of time under any conditions. Of course, then, too, there is the danger of accident due to carelessness or ignorance. In other words, it is just as difficult to make a fool-proof submarine as a fool-proof anything else. Wherever anything is constantly dependent upon the continuous careful attention of human beings, there is constant danger of accident, whether it be on board a submarine, a railroad train, steamship or in connection with anything else.