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 destroyers and patrol-boats may take means to effect her capture.

III

FORERUNNERS OF THE TELEGRAPH

    From Lodestone to Leyden Jar—­The Mysterious “C.M.”—­Spark and
    Frictional Telegraphs—­The Electro-magnet—­Davy and the Relay
    System.

The thought and effort directed toward improving the means of communication brought but small results until man discovered and harnessed for himself a new servant—­electricity.  The story of the growth of modern means of communication is the story of the application of electricity to this particular one of man’s needs.  The stories of the Masters of Space are the stories of the men who so applied electricity that man might communicate with man.