173 
13 III. PRESSURE WAVE 
5. A CALCULATION 
when directed outward), as a function of the distance r from the center of the gas 
globe, are found to be as shown in Figure 8. The long dotted line shows the instan- 
taneous position of the interface be- 
tween gas and water. The distance 24 240 
marked "initial solid" represents elt ae 
the radius of the original sphere of 
solid TNT. a 10 
For times exceeding 0.7 mil- ‘'2 120 
lisecond, Penney uses a rough method Foal MS ao = 
of calculation, primarily for the pur- Fae %6 i 
pose of discovering how the pressure 2 « 
may be expected to change with dis- . os oe 
tance. He concludes that no marked 40 
change should occur in the shape of 
; la ae SID eal aS male Ue Ze 
the pressure wave as it proceeds out- Radius r in Meters 
ward, but its intensity should de- Figure 8 
crease. Beyond a distance of 50 feet 
from the charge, the pressure in the wave should fall off like that of ordinary sound 
waves, nearly in inverse ratio to the distance, but at first the rate of decrease 
should be more rapid. At a distance r feet from the center (i.e., r times the radius 
of the original sphere of explosive) the pressure is x times as great as it would be 
if it varied as 1/r, where z has, for example, these values: 
In conclusion Penney shows a comparison of his curve for the pressure at 50 
feet from 300 pounds of TNT with an experimental curve, which is almost the same as 
that published by Hilliar. The two curves agree in showing an initial pressure of 
1800 pounds per square inch, but Penney's curve drops off more rapidly. The oscilla- 
tory feature in Penney's curve may be due to the peculiar initial condition from 
which he starts his calculation, or it may be that such features are missed in cur- 
rent methods of observation. 
III. PRESSURE WAVE 
6. THEORY OF SECONDARY IMPULSES 
6. THEORY OF THE SECONDARY IMPULSES 
In the absence of an exact theory of the oscillations of the gas globe and 
of the pressure impulses produced by them in the water, some light may be thrown upon 
the phenomena by developing a theory in which compression of the water is ignored. 
As a matter of fact, the actual motion must approximate closely to the non-compressive 
type except during the phase of intense compression of the gas. 
