385 



0.01 0.02 0.03 

 Time in seconds 



Figure 2 - Curves of Size and 



Position of a Gas Globe 



near a Vertical Wall 



The charge was fired 9 inches from a rigid 

 vertical wall. Note that the velocity of 

 the center of the globe is greatest during 

 the compression phases. 



during the compression phase, when the 

 momentum becomes concentrated Into a 

 comparatively small volume of water sur- 

 rounding the globe. Thus the center of 

 the gas globe moves continually toward 

 the wall but advances chiefly In spurts 

 during the compression phases, as shown 

 In Figure 2. 



The effect of a free surface, 

 when the gas globe Is not near enough to 

 the surface to break through. Is approxi- 

 mately opposite to that of a rigid sur- 

 face. The Initial expansion of the gas 

 globe accelerates the water above It up- 

 ward and that below it downward. While 

 the globe Is expanded, the pressure near 

 it is very low, and this deficit of 

 pressure acts so as to check and then 

 reverse the radial motion. Because the 

 pressure remains constant on the free surface, the deficit is less, or the 

 pressure is greater, near the surface than it would be if there were addi- 

 tional water instead of air above the surface; and because of this relative 

 excess of pressure, the water lying either above or below the gas globe is 

 given an excess of momentum downward. During the next compression phase this 

 momentum becomes concentrated in a much smaller volume of water and, provided 

 the effect Is not canceled ty the simultaneous and opposed action of gravity, 

 the globe is carried downward. Arguments from momentum in the water are dan- 

 gerous, however; some further remarks on this subject are Introduced at the 

 end of this report. 



By this action, the gas globe is attracted toward a rigid boundary 

 but repelled from the free surface of the water. Although the action of 

 gravity is always present, to cause the globe to rise through the water by 

 virtue of its buoyant nature, attraction by the bottom in shallow water or 

 repulsion from the free surface of the water may merely decrease the rise due 

 to gravity, or it may actually produce a downward displacement of the globe. 



The effect of a boundary should increase as the gas globe approaches 

 the boundary. Under certain circumstances, however, observation shows that 

 marked departures from sphericity of the globe may occur. Near a free sur- 

 face, for example, part of the gas may blow through the surface while the re- 

 mainder migrates flown Into the water; see Figure 3. Furthermore, a gas globe 



