The Marconi system of wireless telegraphy was practically employed with useful effect April 28, 1899, on the “Goodwin Sands” light-ship to telegraph for assistance when in collision twelve miles from land and in danger of sinking. It was also used in October, 1899, on board the “Grande Duchesse” to report the international yacht race between the “Columbia” and the “Shamrock” at Sandy Hook, as seen in Fig. 13. Lord Roberts also made good use of it in his South African campaign against the Boers. According to Signor Marconi its present range is limited to eighty-six miles, but it is expected that this will be soon extended to 150 miles. [Illustration: FIG. 13A.--THE COHERER.] Marconi’s receiving apparatus is shown in Fig. 13A, and consists of a small glass tube called the coherer, about 1½ inches in length, into the ends of which are inserted two silver pole pieces, which fit the tube, but whose ends are 1/50 inch apart. The space between the ends is filled with a mixture composed of fine nickel and silver filings and a mere trace of mercury, and the other ends of the pole pieces are attached to the wires of a local circuit. In the normal condition the metallic filings have an enormous resistance, and constitute a practical insulator, preventing the flow of the local current; but if they are influenced by electric waves, coherence takes place and the resistance falls, allowing the local current to pass. The coherence will continue until the filings are mechanically shaken, when they will at once fall apart, as it were, insulation will be established, and the current will be broken. If, then, a coherer be brought within the influence of the electric waves thrown out from a transmitter, coherence will occur whenever the key of the transmitter at the distant station is depressed. Mr. Marconi has devised an ingenious arrangement, which is the subject of his patent referred to, in which a small hammer is made to rap continuously upon the coherer by the action of the local circuit, which is closed when the Hertzian waves pass through the metal filings. As soon as the waves cease, the hammer gives its last rap, and the tube is left in the decohered condition ready for the next transmission of waves. It is evident that by making the local circuit operate a relay, in the circuit of which is a standard recording instrument, the messages may be recorded on a tape in the usual way. [Illustration: FIG. 13B.--DIAGRAM OF THE TRANSMITTER AND RECEIVER.]