THE SEARCH-LIGHT
All liners and war vessels now carry apparatus which will enable them to detect danger at night time, whether rocks or an enemy's fleet, icebergs or a water-logged derelict. On the bridge, or on some other commanding part of the vessel's structure, is a circular, glass-fronted case, backed with a mirror of peculiar shape. Inside are two carbon points almost touching, across which, at the turn of a handle, leaps a shower of sparks so continuous as to form a dazzling light. The rays from the electric arc, as it is called, either pass directly through the glass lens, or are caught by the parabolic reflector and shot back through it in an almost parallel pencil of wonderful intensity, which illumines the darkness like a ray of sunshine slanting through a crack in the shutter of a room. The search-light draws its current from special dynamos, which absorb many horse-power in the case of the powerful apparatus used on warships. At a distance of several miles a page of print may be easily read by the beams of these scrutinisers of the night.
The finest search-lights are to be found ashore at naval ports, where, in case of war, a sharp look-out must be kept for hostile vessels. Portsmouth boasts a light of over a million candle-power, but even this is quite eclipsed by a monster light built by the Schuckert Company, of Nuremberg, Germany, which gives the effect of 816,000,000 candles. An instrument of such power would be useless on board ship, owing to the great amount of current it devours, but in a port, connected with the lighting plant of a large town, it would serve to illumine the country round for many miles.
In addition to its value as an "eye," the search-light can be utilised as an "ear." Ernst Ruhmer, a German scientist, has discovered a method of telephoning along a beam of light from a naval projector. The amount of current passing into the arc is regulated by the pulsations of a telephone battery and transmitter. If the beam be caught by a parabolic reflector, in the focus of which is a selenium cell connected with a battery and a pair of sensitive telephone receivers, the effect of these pulsations of light is heard. Selenium being a metal which varies its resistance to an electric circuit in proportion to the intensity of light shining upon it, any fluctuations of the search-light's beams cause electric fluctuations of equal rapidity in the telephone circuit; and since these waves arise from the vibrations of speech, the electric vibrations they cause in the selenium circuit are retransformed at the receiver into the sounds of speech. This German apparatus makes it possible to send messages nine or ten miles over a powerful projector beam.
In the United States Navy, and in other navies as well, night signals are flashed by the electric light. The pattern of lamp used in the United States Navy is divided transversely into two compartments, the upper having a white, the lower a red, lens. Four of these lamps are hung one above the other from a mast. A switch-board connected with the eight incandescent lamps in the series enables the operator to send any required signal, one letter or figure being flashed at a time. During the Spanish-American War the United States fleet made great use of this simple system, which on a clear night is very effective up to distances of four miles.
Large arc-lamps slung on yards over the deck give great help for coaling and unloading vessels at night time. The touch of a switch lights up the deck with the brilliancy of a well-equipped railway station. The day of the "lantern, dimly burning," has long passed away from the big liner, cargo boat, and warship.