[Illustration: THE VOICE AND EAR OF THE SUBMARINE A Fessenden oscillator, before being installed. The flange of the oscillator is riveted to the shell of the ship and its diaphragm is caused to vibrate by the sound waves, which pass through water more distinctly than they do through the air. To send out signals it is caused to vibrate mechanically by electrical apparatus.] =Invisible Conning Tower.=--For night observation it has been proposed to use transparent conning towers built of clear glass, in which the commander takes his station and just sticks his head above the crest of the waves in order to direct his vessel against the enemy. This has not as yet come into general use because of the difficulty of securing sufficiently clear glass in the desired form. Experiments have been made, however, which show that quite a large transparent conning tower cannot be seen on a submarine at rest even when within a couple of hundred yards; the application of these conning towers will greatly increase the submarine's efficiency for night work. =Submarine Sound Receivers.=--All modern submarines are fitted with devices which enable the commanders of submarines to communicate with each other when running under water even when considerable distances apart. One of these outfits consists of a signal bell and a powerful receiver with which sounds may be transmitted and heard. Conversations may be carried on by the Morse and other codes for distances of ten or twelve miles. [Illustration: TORPEDO TUBES ASSEMBLED READY FOR INSTALLATION IN A SUBMARINE BOAT Left view, the breech end of the tube. Right view, the outboard doors, which must first be opened before the torpedo is expelled from the tube by compressed air. When the torpedo is expelled it starts a compressed-air engine supplied with air stored at high pressure within the torpedo, and will run several thousand yards under its own power.]