he 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