. Fig. 10. Diagrammatic representations of sounding across a narrow valley, (a) Transmission and echo assumed to be narrow parallel beams, (b) Transmission spreading, and two echoes being recorded as in actual practice. as such with the ' listening' gear, and it follows that echoes from the deepest part of the depression would be missed, and the profile of the bottom could only be assumed to take the form represented by the dotted line. The recorder however is more selective and sensitive than the human ear and it can distinguish echoes from points at diff'erent distances. It seems very probab
. Fig. 10. Diagrammatic representations of sounding across a narrow valley, (a) Transmission and echo assumed to be narrow parallel beams, (b) Transmission spreading, and two echoes being recorded as in actual practice. as such with the ' listening' gear, and it follows that echoes from the deepest part of the depression would be missed, and the profile of the bottom could only be assumed to take the form represented by the dotted line. The recorder however is more selective and sensitive than the human ear and it can distinguish echoes from points at diff'erent distances. It seems very probable that the ' simple cross- over ' of the kind shown in PL XXX, fig. 4, is in fact the result of crossing a depression of the type shown in Text-figs. 10a and b. That is to say the 'crossover' is not a precise representation of the actual profile of the bottom but almost certainly represents echoes received simultaneously from both sides of a depression. In Text-fig. 10b positions A, B, C, D and E are presumed to be equally spaced points on the ship's track. It must be remembered that in practice continuous soundings are being taken between these points, but the points shown are sufficient to illustrate our theory. At position A echoes are being recorded from A^ and Ao, the latter being weak. At C the echo strength from C^ and C, is nearly equal and at E the signal being received from E^ is now the secondary trace and will probably be of just sufficient strength to give a recording under good conditions. From this it will be seen that a rounded, or U-shaped, depression may be recorded as a V-shaped 'crossover' as represented by the pecked lines in Text-fig. 10/;, and that analysis of such a 'crossover' may give a more accurate representation of the shape of a depression on the bottom than can be obtained by the ' listening' gear.
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