. 89 
appreciably ; afterwards they begin to rise and to lose their sperical form. What 
happens to them eventually depends on the depth at which the charge was fired. 
If it was fired at a moderate depth, say 30 or 50 feet, the gases rise and vent them- 
selves at the surface before their pressure is completely spent, throwing up tall 
plumes of smoke and spray. But at a greater depth, say 200 feet, the pressure of 
the gases is entirely spent before they can reach the surface ; probably after several 
seconds the pressure falls to less than the normal hydrostatic pressure, owing to 
condensation of steam and other causes ; after this the gases are churned up with the 
surrounding water into an emulsion, which, being of lower specific gravity than sea- 
water, pours up slowly to the surface. 
The supposed history of the explosion products has been traced to a corclusion 
because it is of interest to account for the effects observed at the surface, but it is 
only during the first thousandth of a second or so that the effective part of the 
pressure wave is generated. At the moment when the wave of detonation reaches 
the surface of the exploding sphere the charge may be supposed still to occupy 
exactly its original volume (Section 9), and the water in contact with it, which until 
that moment was under normal pressure, is instantaneously subjected to a pressure 
probably exceeding 200 tons per square inch. The first layer of water is compressed 
and thrown outwards, compressing the next layer, and so on, and in this way there 
is generated the front of a pressure wave, which springs away with a velocity that at 
first exceeds and then rapidly approximates to that of sound in water, The globe of 
gas, expanding much less rapidly than the pressure wave, feeds the rear of the wave 
with a continually falling pressure. At any instant the pressure in the water at 
different points along a radius increases continuously from the boundary of the gases 
to the front of the wave, where the pressuré is a maximum. 
The pressure wave is, of course, nothing but a very strong sound wave; the 
mechanism of its propagation is essentially the same as that of sound, the difference 
is only in the intensity of the pressure transmitted. According to elementary 
acoustic theory the pressure in a spherically diverging sound wave falls off in simple 
proportion to the distance from the source, the time-pressure curve at distance D, being 
a copy of the curve at distance D, with the pressure altered in the ratio = The 
1 
velocity of propagation is independent of the distance. This theory, however, is only 
exact in the case of infinitely weak waves, since it rests on the assumption that the 
compression of the medium is indefinitely small. At a very great distance, from a 
submarine explosion this condition will be approximately fulfilled, and the experiments 
made by Mr. Boulding in connexion with submarine sound ranging have proved that 
at distances of several miles the velocity of the pressure wave is the same as that of 
sound in sea-water. 
On the other hand the simple laws of sound cannot be expected to apply in the 
region near the charge, where the very intense pressure produces considerable com- 
pression of the water. In this region the velocity of propagation of the pressure wave 
must be greater than that of sound (Section 23). Moreover, the front of the pressure 
wave, where the pressure is greatest, must travel faster than the subsequent parts of 
the wave, where the pressure is less, and this involves a tendency for the crest of the 
wave to become flattened, the maximum pressure falling more than in simple pro- 
portion to the distance. At the same time, if the energy of the wave is conserved, 
the time integral of the pressure must fall less than in proportion to the distance. 
The present investigation was confined almost entirely to distances at which the 
pressure did not exceed about 2 tons per square inch (25 feet and upwards from 
a 300-lb. charge and corresponding distances from other charges). Under this 
pressure 100 volumes of sea-water are reduced to 98°7 volumes, that is to say, the 
‘condensation’ is only °013. which is a small quantity though not of an infini- 
tesimal order. It was therefore expected that the pressure wave would behave not 
very differently from an ordinary sound wave, both as regards velocity and in other 
respects, and the experimenfal results proved this view to be correct. 
There must be a great difference in this respect between the propagation of 
explosion pressure waves in water and in air, for the pressures necessary to produce 
the same small condensation in these two fluids are in the ratio 24,000:1. An 
explosion pressure wave in air must certainly deviate widely from the simple acoustic 
laws for a great distance from the charge. 
