FOG-SIGNALS (ante). The importance to navigation along the coast of the United States of these signals, has led to many experiments and improvements. The bells, gongs, guns, etc., used on board ship are little depended upon; but instead, there have been placed at many points on the coast, whistles and horns of great power, which are sounded at frequent intervals when the state of the atmosphere requires it. The sim plest and commonly most powerful signal employed by the light-house board, is the locomotive whistle, operated by a steam boiler with a pressure of 50 to 75 lbs. The sounds from the land are distinguished from those on board ship by time length of the notes and the intervals ,between. The whistles are from 8 to 10 in. in diameter, and are operated automatically. The Daboll trumpet, which is worked by air condensed by a caloric engine, is next in importance. This trumpet itself provides the resounding cavity, and the vibratory motion of the air is produced by a reed. This reed is an iron bar, the larger trumpets being 18 in. long, 2 in. wide, and three quarters of an inch thick, gradually lessening towards the free end. A pressure of 15 lbs. to the sq.in. is the highest power employed. This trumpet is especially valuable in places where water is not procurable, because its motive power is hot-air. The most powerful instrument yet employed as fog signal, is that known as the siren trumpet. The impulse to the air which produces the sound is given by a fiat drum, or a hollow cylinder with a short axis, one end of which is perforated to admit the steam from a pipe connected with a locomotive boiler. On the other side the drum is also perforated with eight holes, in
connection with which is a revolving disk, which is, in its. turn, provided with the same number of holes. As the disk revolves, these eight holes are alternately opened and -shut, allowing egress to as many gusts of steam, which in turn, produce a violent movement of the air, giving rise to a most powerful sound, reinforced by the resonance of a trumpet of suitable length. The requisite velocity is communicated to the disk by an engine attached to a boiler. The sound from this instrument can be heard in still air at a distance of from 20 to 30 tn., even during a dense fog. This trumpet is worked by .a pressure of 75 lbs. of steam in an ordinary locomotive boiler. But, although these sounding horns are very powerful, there is always an amount of uncertainty in the results. The trumpet of which one note may be heard 20 tn. off may send the next note less than two thirds the distance. Sound travels in the quiet, dry open air at mean temperature, at the rate of about 750 m. per hour, and an opposing or crossing wind, traveling 10 m. an hour, disturbs and retards the transmission of sound. The intervention of rain, mist, or fog will also disturb and retard; and temperature has a very distinct influence. Add to these disturbing causes, the fact that no two men hear exactly alike, and the uncertainty of dependence upon sound is apparent.