162 
I. GENERAL SURVEY 2 
1. TYPES OF MOTION 
of the water itself, is usually much less than the speed of propagation of the waves. 
In a train of waves traveling in one direction, the water in a region of compression 
is moving momentarily in the direction of propagation of the waves; in a rarefaction, 
the particle velocity is backward (opposite to the direction of propagation). The 
variations of pressure involved in ordinary sound waves are very small. 
It is always possible to regard changes in the pressure of water as propa- 
gated through the water by a succession of small impulses moving with the speed of 
sound. In many types of motion, however, the time required for the propagation of 
such an impulse is so short as to be negligible, and the changes of density themselves 
may also be unimportant. 
The term non-compressive may be applied to motion of such character that it 
is sufficiently accurate, first, to treat the medium as incompressible, and second, 
to assume that pressure applied to the boundary of the medium is propagated instanta- 
neously to all points of the interior. Examples are the motion of water around a 
ship, or the flow of water in pipes that are not too long. 
As a convenient criterion it may be said that the motion of water will be 
essentially non-compressive whenever the motion changes little during the time re- 
quired for a sound wave to traverse the scene of action. At the opposite extreme, 
whenever a mass of water changes its motion considerably during the time required for 
a sound wave to traverse the mass, changes in density must usually be allowed for and 
the laws of compressive motion mist be applied. 
I. GENERAL SURVEY 
2. EXPLOSION PHENOMENA 
2. SEQUENCE OF EVENTS DUE TO AN UNDERWATER EXPLOSION 
We are far from possessing complete experimental or theoretical knowledge 
of whet occurs in an underwater explosion. The general sequence of events appears to 
be as follows. 
When a mass of explosive material detonates, it almost instantly becomes 
gas under a very high pressure (1 or 2 million pounds per square inch), without ap- 
preciable increase in volume. The exploded gas then begins to expand and compresses 
the layer of water next to it; at the same time this layer of water is given a high 
velocity outward, perhaps 3000 feet per second. The layer of water, moving outward, 
then compresses the next layer and also accelerates it outward,and so on. In this 
way a state of high pressure and large particle velocity is propagated outward as the 
front of an impulsive wave. 
The gas globe, pressing:continually on the water, can be imagined to send 
out a succession of impulses of this sort, all of these impulses blending into a con- 
tinuous wave. The pressure of the gas falls as the gas expands, however. Hence the 
meximum pressure and maximum particle velocity occur at or near the front of the wave, 
