The introduction of coast fog-signals is of comparatively recent date. They were, until the middle of the 19th century, practically unknown except so far as a few isolated bells and guns were con cerned. In times of fog the mariner can expect no certain assis tance from even the most efficient system of coast lighting, since beams of light of high power are frequently entirely dispersed and absorbed by the particles of moisture, forming a sea fog of even moderate density at relatively short distances from the shore. The careful experiments and scientific research which have been de voted to the subject of aerial-acoustic fog-signalling have produced much that is useful and valuable to the mariner, but unfortunately the practical results thus far obtained have not been so satisfactory as might be desired owing to (I) the very short range of the most powerful signals yet produced under certain unfavourable acoustic conditions of the atmosphere, (2) the difficulty experienced by the mariner in judging at any time how far the atmospheric condi tions are against him in listening for the expected signal, and (3) the difficulty in locating the position of a sound signal by phonic observations. The future of marine fog signalling may lie in the direction of wireless signals, but it is unlikely that acoustic signals will be dispensed with even if reduced in number.
noticed in the illustration of the, Bishop rock lighthouse, the jib being attached to the lantern (fig. 4). Sometimes the explosive is combined with aluminium in the charge to give a brilliant flash in addition to the detonation.
The acetylene fog-gun is an automatic or semi-automatic signal in which a mixture of air and acetylene is exploded at short inter vals in a gun chamber. It is economical in working cost and occu pies little space ; but its power cannot be compared with that of a compressed-air siren. The admission of air and acetylene is controlled by an automatic gas-valve and the charge is fired by a spark. Signals can be sounded as frequently as every ten seconds. An acetylene gun was established at the Dhu Heartach rock lighthouse (west coast of Scotland) in 1912. The gun continues in action without attention from the time when it is started until it is stopped by the keeper. The gas is generated automatically on the carbide-to-water principle. On the Clyde there are two isolated beacon structures which are equipped with automatic guns started and stopped by wireless control from Gourock pier. Several other stations in Scotland and elsewhere are provided with these guns. Whistles and Reed Horns.—Whistles, whether sounded by air or steam, are not used in Great Britain although they are still employed as fog signals in the United States, Canada and Sweden. It has been objected that their sound bears too great a resem blance to steamers' whistles; and they are wasteful of power.
Reed horns, in their original form, were the invention of C. L. Daboll, an experimental one, of his manufacture, being tried in 1851 by the United States Lighthouse Board. The reed horn was adopted by Trinity House in 1862 after being improved by W. Holmes and many examples from his designs are now in use at secondary stations in Britain and America. Normally they are sounded at a pressure of 15 lb. per in. with air furnished by power-driven compressors, semi-diesel engines being commonly used in modern installations. When operated by hand the work ing pressure is 5 lb. per square inch.