Wireless telegraphy is, in theory, closely allied to heliography, or signaling with flashes light. The light used, however, is produced lectrically and is invisible to the naked eye, wing to the fact that it is made up of very ng waves, called Hertzian waves, which virate too slowly to affect the retina. The eye sa only discern waves which make from 000 billions to 7,000 billions vibrations per Linute. However, the Hertzian ray resemelight in that it can be reflected by a etallic plate and can be refracted by a prism pitch, can be brought to a focus with a tch lens, and may be polarized. Owing to The great length of the Hertzian waves, almost lnbstances are transparent to them. The lertzian waves were discovered by Professor Henrich Hertz, a young German philosopher, aring his experiments with the spark disarge of Leyden jars and of the Ruhmkorff al in 1886 and 1887.

He found that when a spark leaped the gap etween the terminals, electric oscillations ok place in these terminals which set up agnetic waves in the surrounding space, apable in turn of setting up similar oscillaans in any adjacent conductor lying at an gle to them. The waves were detected by ang a "resonator," which was merely a circle fa rectangle of copper wire formed with a rap in one side. When the induction coil was

operation and the resonator coil was held ar the coil, a tiny stream of sparks would 3p across the resonator gap. To better derstand this phenomenon take as a crude ample two vertical rods in a pool of water ad on each a float free to slide vertically on rod. Now, if one of these floats be moved and down upon its rod, it produces waves the water just as the electric oscillation dures waves in the ether. These spread in all directions and on reaching the other at cause it to oscillate up and down, just the magnetic waves produce electric oscillain the resonator.

Without going into a detailed history of development of wireless telegraphy from Bert's experiments, it may be stated that the essential difference between the apparatus ed by Hertz in his experiments and the veral systems now commonly in use lies in The transmitter is practically

the receiver.

the same. A vertical wire called the antenna connected to one terminal of the coil, and the other terminal is connected with the earth, the purpose being to increase the electrical pacity of the terminal rods and produce rger waves. Instead of producing the oscilatons by means of an induction coil, they

are now ordinarily produced by a dynamo and a step-up transformer except for telegraphing over short distances. But even with these changes we would not be able to telegraph over any appreciable distance if dependent upon the Hertz resonator for receiving a message, for, owing to the fact that the waves spread out in all directions from the transmitting antenna, the receiving antenna is acted upon by a very small proportion of the power expended by the transmitter, and this proportion decreases very rapidly as the distance between the transmitter and the receiver increases. In order then to detect the rays at long distances, a very sensitive instrument called the " coherer" has been invented. The coherer in its usual form consists of a glass tube with two metal pistons fitted therein between which a quantity of nickel filings is placed. The latter forms an imperfect electrical contact between the pistons, and takes the place of the spark gap in the receiving antenna. When the oscillations are set up in the antenna by the Hertzian waves, due to their high pressure or voltage, they break through the imperfect contact of the coherer, causing the filings therein to cohere or string together and thus produce a much better electric path through the coherer. The action is microscopic and cannot be detected with the naked eye. However, the coherer, aside from being a part of the antenna circuit, is also made a part of a local battery circuit, which contains a telegraph receiver, and whenever the electric oscillations open a good path through the filings for the local circuit, the telegraph instrument will be energized by the local battery only. In order to break this path after the oscillations have ceased, or, in other words, to cause the filings to decohere, they are constantly jarred apart by means of the "tapper," which is in reality an electric bell with the gong removed and the clapper striking the coherer tube instead. Carbon granules may be substituted for metallic filings, and in this case no tapper is necessary, the coherer being self-restoring.

In transmitting messages a telegraph key in the primary circuit of the induction coil is operated according to the usual Morse code. and this causes sparks to leap the spark gap at corresponding intervals. These signals will then be transmitted by the Hertzian waves to the receiving station, where they will be recorded by the telegraph receiver. The coherer is not by any means the only wave detector in use. Every wireless telegraph company has one or more different types of detectors.

A complete list of wireless telegraph stations of the world, including shore stations, merchant vessels, revenue cutters and vessels of the United States Navy, is published periodically by the Bureau of Steam Engineering of the Department of the Navy. The edition for January 1, 1912, consists of 165 pages. Copies of this publication can be obtained from the Superintendent of Documents, United States Printing Office, Washington, D. C., at a cost of 15 cents. The section devoted to wireless telegraphy in this book as regards the United States is taken from this work and is corrected to June 12, 1912, but many who would like to have the call letters, etc., of foreign wireless shore stations, also the call letters, etc., of vessels of the United States Navy, the United States Army, revenue cutters, and all steamships which are equipped with wireless, should purchase this inexpensive pamphlet. Space forbade the publication of this list in full.

Wireless communication was an established fact for more than ten years before the ships "Republic" and "Florida" collided on January 23, 1909. The wonderful salvage operation which was only rendered possible by the prompt action of the vessel summoned by wireless called instant attention to the importance of wireless as a safeguard from the dangers of the sea. The Republic" might

The

have gone down to the bottom without news of the disaster and with none of the passengers and crew saved, except possibly a few of them who escaped by life boats, had it not been for this most practical invention. It was two days after "La Bourgoyne" sank before the story of the catastrophe became known. next interesting use of the wireless was perhaps the detection and arrest of Dr. Crippen for the crime of murder. There is no more weird story in the annals of crime than how the unseen wireless brought Dr. Crippen to the noose. Stations that were practically unknown became suddenly vitalized, and to-day Cape Sable, Belle Isle, Fame Point and Father Point are household words.

On the 14th of April, at 11.46 P. M., ship's time, the "Titanic" struck an iceberg. Within fifteen or twenty minutes the Captain visited the wireless room and instructed the operator to get assistance. The two calls "C.Q.D." and "S.O.S." began to flash from the aerials, and the message of despair from the sinking vessel was heard by the "Mount Temple," the "Frankfurt" and the "Carpathia." The Captain of the "Carpathia" immediately turned around and succeeded in reaching the "Titanic" after she sank, and rescued a portion of her passengers and crew. Had it not been for the wireless the probabilities are that very few, if any, survivors would have remained to tell the awful tale.