M. J. Tucker and A. R. Stubbs 303 
revolutions of the helix, and if no precautions were taken might well be lost in 
the transmission mark or in the reverberation due to scatterers in midwater. 
To overcome these difficulties and to find out how many complete revolutions 
of the helix elapse before the echo returns, a switching system is used. Figure 
16.1 shows a typical arrangement using twelve sweeps of the helix. Pulses are 
transmitted on only the first six and the receiver is switched off for the first 
four sweeps. Thus, apart from the fifth and sixth sweep, there is negligible local 
reverberation and the number of echoes received allows the number of revolu- 
tions between transmission and echo to be determined. This number is usually 
determined by ear, as successive sweeps are not well separated on the record. 
To overcome errors due to shrinkage of the paper in drying, range marks 
are put on the paper by counting down the tuning-fork frequency to produce suit - 
fe 
Fig. 16.3. Part of a bathymetric chart of the Swallow Bank; area, approximately 30 by 35 miles. The 
depths are marked in fathoms. This figure shows the remarkable consistency which can be obtained 
using the precision echo-sounder, (Chart from A.S. Laughton.) 
