Popular Science Monflihi
��713
�� ��by it can be made to have any sustained duration desired, so long as the felt-rimmed wheel rubs on the steel strip
��—T
FOUR I RON CROSS- PIECES WHICH BRACE HULL AROUND VI- BRATING STRIP AND PREVENT HULL PLATES FROM SETTING UP INTERFERING VI- BRATIONS OF THEIR rOWN
���It is a queer world, this — down "under the sea." It might be supposed that no sounds at all can be heard under water. The opposite is true. Sounds carry better in a dense medium like water than in a comparativeh' "thin" gas such as air. All sorts of sounds can be heard under water — the throb of some distant ship's propeller, the pounding of engines, the explosion of distant mines, and hundreds of other noises. A micro- phone, placed in a chamber of water at the side of the receiving vessel and con- nected with a telephone receiver, aids in this hearing. Singhalese fishermen, how- e\er, have for centuries carried on com- munication between boats by the simple method of striking an earthen bowl under water, the listening fisherman
��Vibrations from the interior
ship's hull do not interfere with the signals
placing his ear against the bare hull of his boat.
Submarine bells are already in use as fog warnings. Some attempt has been made to adapt them to the send- ing of Morse telegraph signals between one boat and another, as for instance between a warship and a submarine. The sound from submarine bells, however, docs not endure; it is not sustained. In other words, one stroke on a bell sounds to the underwater listener just like any other stroke. All the strokes are short — have no duration. Since all the sounds are "dots," it is obviously impossible to send Morse signals, dependent on both dots and dashes (short and long sounds). Moreover, seamen who have listened to these underwater bells sa>' that in a few feet a bell's sound loses its char- acteristic bell-like tone. The sound simply comes to the ears of the listener as a dull, leaden "click," something like that produced when two ordinan,- table- spoons or a knife and fork are struck
�� �
