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.
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.
Some manifestations of electricity had been known since long before the Christian era. A Greek legend relates how a shepherd named Magnes found that his crook was attracted by a strange rock. Thus was the lodestone, the natural magnetic iron ore, discovered, and the legend would lead us to believe that the words magnet and magnetism were derived from the name of the shepherd who chanced upon this natural magnet and the strange property of magnetism.
The ability of amber, when rubbed, to attract straws, was also known to the early peoples. How early this property was found, or how, we do not know. The name electricity is derived from _elektron_, the Greek name for amber.
The early Chinese and Persians knew of the lodestone, and of the magnetic properties of amber after it has been rubbed briskly. The Romans were familiar with these and other electrical effects. The Romans had discovered that the lodestone would attract iron, though a stone wall intervened. They were fond of mounting a bit of iron on a cork floating in a basin of water and watch it follow the lodestone held in the hand. It is related that the early magicians used it as a means of transmitting intelligence. If a needle were placed upon a bit of cork and the whole floated in a circular vessel with the alphabet inscribed about the circle, one outside the room could cause the needle to point toward any desired letters in turn by stepping to the proper position with the lodestone. Thus a message could be sent to the magician inside and various feats of magic performed. Our own modern magicians are reported as availing themselves of the more modern applications of electricity in somewhat similar fashion and using small, easily concealed wireless telegraph or telephone sets for communication with their confederates off the stage.