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7 IIf. PRESSURE WAVE 
1. OBSERVATIONS 
the gauge by the pressure gradient in the water, and so would be retarded by friction; 
furthemore, elastic oscillations in the gauge may be set up, with the same result. 
The indications of the gauge would thus be made too small. As a matter of fact, a 
systematic discrepancy of 10 to 15 per cent was noted between the indications of the 
impulse and the maximum-pressure gauges. This was overcome by arbitrarily reducing 
the indications of the maximum-pressure gauges by 10 per cent. Perhaps the correc- 
tion should have been reversed. 
For comparative observations, use was also made of simpler gauges in which 
a mass of plasticine was extruded through a hole by the pressure of the water, this 
mass being subsequently weighed. 
A typical curve thus obtained, representing the average from 3 shots, is 
shown in Figure 4. The curve is drawn by estimation through the rectangles, which 
correspond to successive portions of [pat. The gauges were 50 feet from the charge. 
Time in Milliseconds 
Figure 4 
The following features of the behavior of the pressure wave were inferred 
from the observations: 
(a) Law of Similarity. Charges of various sizes produce equal pressures at 
distances and at times which are in proportion to the linear dimensions of the 
charges. 
(b) Variation with Distance. The magnitude of the pressure wave decreases in 
the inverse ratio of the distance r from the charge, at least if r lies between 30 
and 120 times the radius of the charge. 
Because of these simple features, it can be deduced from the observations 
that the maximum pressure due to W pounds of amatol or TNT at a distance of r feet 
is about 
