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