A Treatise On The Principles And Practice Of Harbour Engineering

2/8 HARBOUR ENGINEERING. another lever, which, in conjunction with a strong spring, impels the hammer against the rim of the bell. Whistling ’ buoys are actuated by the rise and fall of the buoy in a swell. In the Courtenay type, air is drawn into a long central tube during the period of rising. The entrance is controlled by a valve, and when the buoy descends the imprisoned air is expelled through the whistle, emitting a penetrating sound of no particular musical value. Whistling buoys are best adapted to fairly deep water where there is an almost constant swell. Sound, however, in air is but an imperfect medium for the notification of danger. It gives no reliable indication, and indeed often conveys a very misleading impression as to locality. Zones of silence are found to lie within the sonorous area.1 Yet, until fogs are dissipable by human agency, it is difficult to see what other means could be universally substituted ; and, certainly, it is reliable in so far as it signifies the imminence of danger, though in many cases the exact location of the warning is a matter of conjecture. Subagueous Signals.—In water, the sense of direction is more determin- able, though, even then, with approximation merely. Ä system of submarine signalling through the agency of a bell struck by a clapper at depths varying from 10 feet to 30 feet below the surface of the water, has recently been promoted by the Boston (U.S.A.) Submarine Signalling Company. The sound is transmitted through the water to a receiver fixed in the ship’s bottom, and thence to a megaphone, with results which have been considered very satis- factory so long as the instrument is immersed to depths of not less than 10 feet and preferably of about 25 feet. The distance traversed has reached 8, 10, and even 15 miles. By turning the ship in various directions, the quarter from which the sound emanates can be easily determined as the sound waves only affect the receiver when it faces the direction from which they corne. There is manifestly much scope for the development of this priuciple of sound transmission. The chief difficulty hitherto has been that of ensuring a constant and regular striking of the warning bell without the uecessity for human attendance. The discovery of some convenient automatic action which is at once simple and reliable should lead to the general adoption of a system which is much more effective than that of the trans- mission of sound in air.2 1 In some interesting experiments carried out off the Isle of Wight, fog sound-signals could not he heard at all at 2 miles from the coast, although they were distinetly audible 10 miles out and were heard again at half a mile from land. 2 At present there are three systems in vogue: viz., (1) Bells suspended from light- ships and struck by the agency of compressed air controlled by a code-ringing device in the engine room ; (2) Bells supported by buoys and worked by the aid of discs, acting on the principle of a sea anchor, so that the difference in movement between disc and buoy operates delicate mechanism ; and (3) Bells supported on tripods resting on the sea floor, and having electrica! communication with a shore station.