A later device, called the Fessenden oscillator, will transmit or receive sounds a distance of twenty miles. The principle of its operation is that of setting up wave vibrations by very large transmitters; these vibrations are carried by the water and taken up by receivers on other submarines. It has been found that the human voice will set up vibrations in the Fessenden transmitter so clearly that wireless conversation may be carried on under water for several hundred yards. I discovered in my earlier experiments that when a submarine was lying submerged, with all machinery shut down, the noise of the machinery in an approaching ship could be detected quite a distance off without the use of any special kind of receivers. In this way the commander of a submarine can always note the approach of an enemy simply by shutting down his own machinery. The warning thus given him comes long before he could sight the enemy ship were he on the surface. After a little experience one can tell the type of ship approaching from the sound, as every type of ship has sounds peculiar to her class. The smash of paddle wheels, the deep, slow pound of the heavy merchant ships or battleships, the clack and the whir of the higher speed machinery on destroyers or torpedo boats, are all easily recognizable when one becomes familiar with them. At the present time all the larger submarines are fitted with wireless outfits on their decks which they may use when on the surface to communicate with other submarines or with their base. =Torpedo Tubes.=--These are used to start the automobile torpedo on its course toward the enemy. In simple form they are tubes about eighteen inches in diameter and seventeen feet long, placed in line with the axis of the vessel. They are fitted with doors both internal and external to the submarine. The inboard door of the tube opens into the interior of the vessel and permits the loading of the torpedo. When the torpedo is to be discharged the inboard door is closed and securely fastened. The outer door is then opened, and through the operation of quick-opening valves compressed air is admitted back of the torpedo and the torpedo is driven out of the tube in the same manner that the bullet is driven out of an air rifle or the cork out of a pop-gun. Some of the larger modern submarines carry several torpedo tubes firing in line with the axis of the vessel both forward and aft. Some carry torpedo tubes on their decks which may be made to train to fire broadside on either side of the vessel. [Illustration: A WHITEHEAD TORPEDO Courtesy of the Scientific American