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between the two mediums. The shock wave with a profile consisting of 
practically infinite steepness and a tail of diminishing intensity is 
propagated into the water and produces the initial pressure pulse in the 
environment of the explosive. After a relatively long period following 
emission of the shock wave, a series of secondary pressure pulses of 
considerable breadth and low intensity is emitted. The secondary pulses 
arise from oscillations of the gas sphere, which withdraw energy for 
their production from the slowly diminishing tail of the shock wave. 
Although the secondary pulses may play a certain role, the damage to marine 
structures produced by an underwater explosion must be principally attrib- 
uted to the initial pressure pulse. Because of its probable role in pro- 
ducing damage to structures, the theoretical and experimental investigation 
of the intensity and duration of the initial pressure pulse is of consid- 
erable importance. 
The hydrodynamical analysis of the initial pressure pulse emitted 
by the expanding gas sphere presents certain rather formidable difficulties, 
Although the wave approaches acoustical behavior at large distances from the 
charge, neither the acoustical approximation nor the incompressible approx- 
imation is adequate to describe the motion of the water in the neighborhood 
of the gas sphere in the initial stages, owing to the high pressures and large 
particle velocities involved. For example, in the case of INT, the initial 
pressure in the shock front is of the order of 30,000 atmospheres and the 
initial particle velocity of the order of 1000 meters/second. Furthermore, 
an analysis of the initial stages of the motion is necessary to predict 
the amplitude and duration of. the shock wave, even in its later acoustical 
phase at large distances from the charge. 
