Reception of Sound

Radio-Acoustic Sound-Ranging.—This method which for ob vious reasons could not be used in war time, was first proposed by Joly, and has recently been developed by A. B. Wood and H. E. Browne for the British Admiralty (Proc. Phys. Soc., April 1923). It consists in the simultaneous emission of a "wireless" signal and an explosion impulse. The former travels with the velocity of light (186,000 miles/sec.) the latter with a velocity about i mile a second in water. If, therefore, one string of the Einthoven gal vanometer on shore records the radio-signal whilst the others record the arrival of the explosion wave at the various hydro phones, we have a very simple and direct means of locating the explosion. The distance of the explosion from the hydrophones is given by ch ct, etc., where c is the velocity of the sound in the sea and t3 are the time-intervals between the arrival of the wireless signal and the explosion-wave at the respective hydrophones. A 9 oz. charge of guncotton can be located in this way at 4o miles, whilst larger charges have been located up to 8o miles or more. At 8o miles the sound-wave through the sea arrives I Z minutes after the wireless signal which denotes the instant of firing the charge. The method permits of very great accuracy and has important applications in navigation and hydrographical survey. For navigational purposes great accuracy is sacrificed to speed, it being possible to give a ship in fog 4o miles away a position within a radius of half a mile in less than 10 minutes of receiving her request for a location. In hydro

graphical survey work the method has been used successfully to fix accurately the positions of buoys and light-vessels out of sight of land. The method is accurate in rough or foggy weather and at all seasons of the year. Various ingenious proposals have been made to safeguard ships in fog, by application of radio acoustic methods (see Joly "Scientific Signalling and Safety at Sea," Phil. Mai. July 1918).

BIBLIOGRAPHY.-LOrd

Rayleigh, Theory of Sound, 2 vols., and Collected Papers; H. Lamb, Dynamical Theory of Sound; H. von Helmholtz, On the Sensations of Tone (1885) ; John Tyndall, On Sound; E. H. Barton, Text-book on Sound; D. C. Millar, Science of Musical Sounds; W. H. Bragg, The World of Sound; W. C. Sabine, Collected Papers on Acoustics; Davis and Kaye, Acoustics of Buildings; I. B. Crandall, Vibrating Systems and Sound; Collective, Mechanical Properties of Fluids; A. B. Wood; Sound—a text-book; H. Bouasse, Acoustique; E. G. Richardson, Sound—a text-book; Geiger and Scheele, Handbuch der Physik, Bd. VIII. Akustik; Glazebrook, Dic tionary of Applied Physics; F. Aigner, Unterwasserschalltecknik; Hydrographical Review, Monaco. Vols. 1, 2, 3. Recent Papers in Proceedings of Royal Society and Physical Society, Physical Review, Royal Society of Canada, Philosophical Magazine, Bell System Techn.

Journal, Journal of the Franklin Institute, etc. (A. B. W.)