523 
- 27 - 
A glance at the above table reveals a number of outstanding points. In the first place the 
maximum tension recorded under any circumstances is 170 lbs. per square inch; in the majority of cases 
the tension is of the order of 50 1bs./square inch. It seems improbable that the actual tension attained 
at any point in the water ever exceeds 200 Ibs./square inch. On the other hand, the maximum pressure 
amplitude of the reflected pulse may reach 600 Ibs. per Square inch — see Column 8 of above table, in which 
case only a small fraction (sometimes zero) appears as tension. When the total reflected pressure amplitude 
is smal] the proportion of tension is usually great. 
The results included in the taple cannot possibly be explained on any theory of simple reflection 
from the surface, for the reflected pressure amplitudes recorded are, as a rule, only a few per cent 
(4 or 5 usually) of the expected values if perfect reflection is assumed. In certain cases, however, e.g. 
shots (6) (7) (16) and (18) in table, the reflected pressure is much greater than usual and in one of these 
cases (16) there is no tension recorded. In order to explain these apparently widely varying results the 
following qualitative theory (deleted) is suggested. It is assumed that perfect reflection takes place 
at the water-air surface and that the tension at any point in the water cannot exceed 'p’ 1bs./square Inch 
without rupture and the consequent formation of a new water-air (or vacuum) surface. 
The experimental results are in general conformity with the above theory of critical layers and 
multiple reflecting surfaces. 
It seems not improbable also that this theory will explain certain large discrepancies still out— 
standing between the ‘spray method’ of measuring explosion pressure (see G.W. Walker's reports) and other 
methods suchas Hilliar and the p.e. method now described. The question of the spray phenomena on these 
grounds requires further consideration. 
To summarise the results of this section it appears that, 
(1) The value of the maximum tension which sea-water can support momentarily is probably less than 
200 bs./square inch. Values of 100 Ibs./square inch in ‘water—in bulk" and of 170 1bs./square 
inch between water and steel are the highest tensions recorded. 
(2) The maximum recorded pressure reflected from the sea surface is about 600 1bs./square inch but 
there is no reason to suppose that this is the maximum possible. 
(3) A qualitative theory is proposed to explain these results. On this theory it appears that when 
the depths ef charge and gauge are greater than certain critical values (depending on the size 
of charge and distance from gauge) the reflected pressure amplitude cannot exceed a certain small 
value of the order of 200 1bs./square inch. 
(4) In the circumstances stated in (3) the practical effect of the "negative-momentum' reflected from 
the surface of the sea is negligible. 
Velocity of the Pressure Wave. 
Experiments carried out at St. Margarets Bay S/R Station® in 1920 and 1921 gave an accurate value 
of the velocity of small amplitude explosion waves over a base line of the order of 10 mides. The velocity 
at different temperatures was expressed by the formula 
Ve = 4756 + 13.8t - 0.12 t? feet/second at a salinity of 35% 
Hilliar in 1919 measured the velocity of large-amplitude pressure waves (close to large charges) by 
means of two contacts at a known small distances apart and a chronograph running at a known speed. His 
experiments yielded the result that, within a few per cent, the velocity of the explosion wave (of pressure 
amplitude 0.7 ton/sq.in.) is the same as that of sound waves in water, or about 4900 feet/second, The 
temperature of the sea at the time of the measurements is not specified. 
Im the weoee 
Proc. Roy. Soc. A. 103, p.284, 1923. 
