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9 ITI. PRESSURE WAVE 
1. OBSERVATIONS 
charge and the surrounding water and then removing the charge and water on one side 
of the diaphragm. 
(f) Various Explosives. Amatol and TNT were found to emit very similar pres- 
sure waves. Guncotton and ammonium perchlorate gave considerably lower pressures, 
but the pressure from ammonium perchlorate was observed to fall off with time much 
less rapidly than that from TNT. 
Gunpowder gave a pressure curve of rounded form, without a steep front, as 
in Figure 2. The pressures were also much less. 
Hilliar's report has been summarized rather extensively here because no 
other comparable-series of observations has been reported, and there is no evidence 
as yet of large errors in any of his conclusions. 
III. PRESSURE WAVE 
2. MULTIPLE WAVES 
2. MULTIPLE PRESSURE WAVES 
Considerable evidence has accumulated showing that an underwater explosion 
produces not one but several pressure waves of comparable magnitude. Besides the 
common observation that several sounds are heard, repeated impulses have been seen 
on oscillograph records. It was observed that the periods between successive im- 
pulses grew shorter; also that the period diminished with increasing depth of the 
point of explosion below the surface of the water. Moving pictures of a model boat, 
below which a charge was detonated, showed the boat to be kicked upward several times, 
at intervals of about 1/20 second. 
The multiple impulses are probably due to oscillations of the gas globe. 
Such oscillations have been observed to occur, but the observations of Ramsauer (5) 
and of Ottenheimer (6) will not be discussed here because their interpretation is 
not wholly clear and the problem is under investigation at the present time. 
Observations of the pressure wave need to be extended to cover these sec- 
ondary parts of the pressure wave. It is important to find out whether the second 
impulse is larger than the first, and whether there is a difference in wave forn. 
III. PRESSURE WAVE 
3. QUALITATIVE THEORY 
3. QUALITATIVE THEORY OF THE PRESSURE WAVE 
An exact theory of the motion of the water produced by an explosion can be 
constructed only by laborious methods of numerical integration. The main features to 
be expected in the phenomenon can be predicted, however, by means of reasoning based 
on the elementary principles of compressive waves and of hydrodynamics. The quali- 
tative theory thus obtained will first be described, for purposes of orientation. 
Then the important phases of the process will be discussed in more exact terms. 
