apparatus the
strokes of the bell could be heard at a distance of over ten miles.

This system has had a wide practical application for communication
both between ship and ship and between ship and shore. Most
transatlantic ships are now equipped with such a system. The
transmitter consists of a large bell which is actuated either by
compressed air or by an electro-magnetic system. This is so arranged
that it may be suspended over the side of the ship and lowered
well beneath the surface of the water. The receivers consist of
microphones, one on each side of the ship. The telephone receivers
connected to the two microphones are mounted close together on an
instrument board on the bridge of the ship. The two instruments are
used when it is desired to determine the direction from which the
signals come. If the sound is stronger in the 'phone on the right-hand
side of the ship the commander knows that the signals are coming from
that direction. If the signals are from a ship in distress he may
proceed toward it by turning his vessel until the sound of the
signal-bell is equal in the two receivers. The ability to determine
the direction from which the signal comes is especially valuable
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