954 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



shoe. At that instant the disk is stopped, its graduation giving the depth 

 directly. The accuracy of this device is claimed to be 1/10,000 of a 

 second. 



In the "echometer" designed by Hecht, a steel disk connected to a 

 pointer is situated between two electromagnets. One of these is stationary 

 and the other is revolved continuously by a sjoichronous motor. Before 

 the sound impulse is sent, both electromagnets are energized and the steel 

 disk is held by the stationary electromagnet in such a manner that the 

 pointer is on the zero point of the dial. The sending of the impulse dis- 

 connects the stationary electromagnet so that the moving electromagnet 

 is free to take the disk along with it. When the echo arrives, the rotating 

 magnet is disconnected and the stationary magnet is energized so that the 

 disk and the pointer stop at an angular position corresponding to the echo 

 time and, therefore, the depth. The pointer is held in this position for a 

 few seconds, permitting a reading to be taken. It is then returned auto- 

 matically to its starting position. 



Another dial-indicating instrument is the "fathometer" developed by 

 the Submarine Signal Corporation. A modification for shallow depths by 

 Dorsey^^^ is illustrated in Fig. 12-23. In this instrument the pointer is 

 represented by a slot in a disk attached to the rotor of a synchronous motor 

 driven by a 1025-cycle tuning fork. In front of the revolving disk is a 

 frosted glass dial and behind it is a circular neon tube which lights up 

 instantaneously when the echo strikes the receiver, thus illuminating the 

 depth reading on the stationary dial. The sound impulse is dispatched 

 by the action of a photoelectric cell which receives a flash of light from a 

 mirror attached to the rotor when the latter passes through its zero position. 



Recently, the recording type of echo-sounding device has come into in- 

 creased use. It is a simple matter to record oscillographically the reflected 

 impulse, together with what motion may be produced by the direct wave 

 which travels from the source to the receiver through or around the hull 

 of the ship. Records of this kind resemble those taken in reflection seis- 

 mic exploration.^" However, such oscillograph records are required only 

 in connection with experimentation and research. Because of the rela- 

 tive strength of the reflected impulse, its well-defined character, and the 

 absence of interference from other refractions or reflections, a complete 

 record can be dispensed with. The echo-depth recorders now used work 

 automatically and record continuously both impulse transmission and 

 echo depth; in other words, they trace the water surface and the bottom 

 contour. 



An automatic recorder developed by the British Admiralty^ * is illus- 



»« H. G. Dorsey, J. Wash. Acad. Sci., 25(11), 469-476 (Nov., 1935). 

 1" See records in B. Gutenberg, Lehrb. Geophys., 3, 585 (1926). 

 138 J. S. Slee, J. Inst. El. Eng. (London) 70, 269-280 (1932). 



