in navigating difficult channels in foggy weather. Signal-bells are
located near lighthouses and dangerous reefs. Each calls its own
number, and the vessel's commander may thus avoid obstructions and
guide the ship safely into the harbor. The submarine signal is equally
useful in enabling vessels to avoid collision in fogs. Because water
conducts sound much better than air, submarine signals are far better
than the fog-horn or whistles.

The submarine signal system has also been applied to submarine
war-ships. By this means alone may a submarine communicate with
another, with a vessel on the surface, or with a shore station.

An important and interesting adaptation of the marine signal was made
to meet the submarine warfare of the great European conflict. At first
it seemed that battle-ship and merchantman could find no way to locate
the approach of an enemy submarine. But it was found that by means
of the receiving apparatus of the submarine telephone an approaching
submarine could be heard and located. While the sounds of the
submarine's machinery are not audible above the water, the delicate
microphone located beneath the water can detect them. Hearing a
submarine approaching beneath the surface, the merchantman may avoid
her and the destroyers and patrol-boats may take means to effect her
capture.




III

FORERUNNERS OF THE TELEGRAPH