oOo Ont Aor W Pr 
521 
Sy GS 
Weight Length of path Maximum Pressure amplitude Ratio of 
g 9 
of hy hy (cms. on record) amp] itudes 
charge Direct Reflected Direct Reflected oF i disochee 
Feet Feet Feet Feet and incident 
pressure 
waves 
lbs. 
none 
recorded 
0.53 
0.15 
0.11 
0.08 
0.15 
0.10 
0.11 
0.14 
The values of efficiency of reflection given in the last column of the above table were obtained 
by referring the reflected pressure te the same distance, 15 feet, as the primary pulse assuming the inverse 
distance law of pressure variation. Thus in shot No. 2 the observea reflected pressure amplitude is 0.53 
cms. the reflected path being 26.6 feet. Referring this value to a distance of 15 feet we obtain 0.904 cms. 
which is 51% of that produced by the direct pressure pulse at this distance. Shots (2) and (3) in the 
above were fired over a2 soft mud bottom in Portsmouth Harbour whilst the others, except No. 1, were fired 
in Spithead at a point where the bottom is hard mud and sand. The mean of al) observations indicates an 
efficiency of reflection of 55% approximately from a bottom of mud and sand. This result has ar important 
bearing on the use of bottom mines of the 'M' type. the maximum pressure, and presumadly the momentum, 
developed by such mines being consequently 50% greater than the corresponding values for ordinary mines 
moored in midwater. Similarly the result is of importance in relation to echo methods of depth sounding 
where a small amplitude pressure pulse is reflected from the sea-bed, 
(b) Surface Reflection. Reflection from the hull of H.M.S. "Gorgon", 
The question of the reflection of a large-amplitude pressure wave from a water-air surface is 
much more difficult than might at first be supposed from a consideration of the work of earlier investi- 
gators, in particuler of Hilliar and Walker. Thus both assume that the pressure pulse is completely 
reflected at the surface, but reversed in phase, the effects at any point in the water being simply 
determined by a superoosition of the positive direct and negative reflected waves, making due allowance for 
the time difference in arrival at the point in question. It will be shown that such is not the case. 
n order to determine the critical depth at which the surface was only just broken (or just not broken, as 
we chose) into spray, charges were fired by these observers at different depths, On the basis of such 
experiments it was found that the maximum pressure applied to the water surface was about 0.3 ton/sq.in. 
at the critical stage when spray is just beginning to form. Consequently it was stated that the wave of 
this pressure amplitude is perfectly reflected, but it was erroneously concluded from this observation that 
water is capable of withstanding a tension of this amount. It will be shown in what follows that such 
is not the casey It has been rightly supposed, however, that the surface is capable of reflecting large 
pressure amplitude (of the order of 0.3 ton per sq. inch) but it must be remembered that at the instant 
of reflection the water below the surface is still under compression from the ‘tail’ of the advancing 
pressure pulse. The effect of the reflected pulse therefore is merely to neutralise this positive 
pressure (either totally or in part) but not necessarily to produce a large tension in the water. 
Typical records showing surface reflections are reproduced in Sheet 7, whilst the experimental 
results for a number of 24, 184 and 100 1b. charges are included in the following table. 
Table coccs. 
