deck.  A red lamp is flashed to indicate a dot in the Morse code, while a white lamp indicates a dash.  The Ardois system is also used by the Army.  The perfection of wireless telegraphy has caused the Ardois and other signal systems depending upon sight or sound to be discarded in all but exceptional cases.  The wig-wag and similar systems will probably never be entirely displaced by even such superior systems as wireless telegraphy.  The advantage of the wig-wag lies in the fact that no apparatus is necessary and communication may thus be established for short distances almost instantly.  Its disadvantages are lack of speed, impenetrability to dust, smoke, and fog, and the short ranges over which it may be operated.

There is another form of sound-signaling which, though it has been developed in recent years, may properly be mentioned in connection with earlier signal systems of similar nature.  This is the submarine signal.  We have noted that much attention was paid to communication by sound-waves through the medium of the air from the earliest times.  It was not until the closing years of the past century, however, that the superior possibilities of water as a conveyer of sound were recognized.

Arthur J. Mundy, of Boston, happened to be on an American steamer on the Mississippi River in the vicinity of New Orleans.  It was rumored that a Spanish torpedo-boat had evaded the United States war vessels and made its way up the great river.  The general alarm and the impossibility of detecting the approach of another vessel set Mundy thinking.  It seemed to him that there should be some way of communicating through the water and of listening for sounds underwater.  He recalled his boyhood experiments in the old swimming-hole.  He remembered how distinctly the sound of stones cracked together carried to one whose ears were beneath the surface.  Thus the idea of underwater signaling was born.

Mundy communicated this idea to Elisha Gray, and the two, working together, evolved a successful submarine signal system.  It was on the last day of the nineteenth century that they were able to put their experiments into practical working form.  Through a well in the center of the ship they suspended an eight-hundred-pound bell twenty feet beneath the surface of the sea.  A receiving apparatus was located three miles distant, which consisted simply of an ear-trumpet connected to a gas-pipe lowered into the sea.  The lower end of the pipe was sealed with a diaphragm of tin.  When submerged six feet beneath the surface the strokes of the bell could be heard.  Then a special electrical receiver of extreme sensitiveness, known as a microphone, was substituted and connected at the receiving station with an ordinary telephone receiver.  With this receiving apparatus the strokes of the bell could be heard at a distance of over ten miles.