THERE are several types of marine devices for finding the depth of the sea by means of ‘echo-sounding’. One or two of these not only give isolated indications of the depth of the sea, but also provide a more or less continuous record of the sea bed. The British Admiralty uses a low-frequency type of oscillation which is reflected from the bottom of the sea, the time of going and returning being marked on an electro-chemical recorder. A high-frequency system using the vibrations of a quartz piezo-electric oscillator, devised by Langevin and Chilowsky, has been developed commercially in Great Britain by the Marconi Sounding Device Co, An entirely new type of high-frequency echo depth recorder which possesses important advantages was described to the Institution of Electrical Engineers on January 2 by A, B. Wood, F. B. Smith and J. A. McGeachy. This device can give a continuous record of the depth of water beneath a survey motor-boat of about 2 ft. draught travelling at full speed. According to the specification, it had to measure a depth ranging up to 200 feet with a maximum inaccuracy of about one foot. The method employed gives a practical application of the phenomenon of gneto-striction. Two oscillators of this typea transmitter, and a receiverare mounted in water-filled tanks and fitted in a chosen position in the motor-boat. The transmitter is excited into resonant vibration at regular intervals of time depending on the range of depth to be recorded. A short train of high-frequency sound waves is directed vertically downwards to the sea-bed and reflected back to the receiver. The induced currents are amplified, rectified and passed through a recorder. During the time the sound impulse is travelling from the transmitter to the receiver via the sea bed, the recording point has travelled a corresponding distance on the paper. The time for the going and return journey is thus found. The method has been proved satisfactory for depth exceeding 400 fathoms.