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GENERAL FEATURES OP THE MIGRATION OF A GAS GLOBE 



The migration results from the action of gravity and from the ef- 

 fects of bounding surfaces such as a rigid wall or bottom or the free surface 

 of the water; see Figure 1 . These various actions are not simply superposed 

 upon each other, because the extent of the migration is greatly Increased by 

 the periodic compression of the gas globe and the degree of compression is 

 Itself materially decreased when the rapidity of the migration becomes large. 

 The migratory motion Implies the existence of kinetic energy of translation 

 In the surrounding water; this energy is abstracted from the energy of the 

 radial motion, with the result that the inward motion of contraction ceases 

 sooner than it would in the absence of the migration. 



If the gas globe were fixed in size and far removed from all bound- 

 aries, it would simply rise with an acceleration of 2g, or twice the ordinary 

 acceleration due to gravity; for the water surrounding the gas globe is acted 

 on by a buoyant force equal to the weight of the displaced water, and the ef- 

 fective mass of the water is only half of the mass of the displaced water for 

 the type of motion that results in the upward displacement of the globe (3). 



The effects of a bounding surface or wall in the neighborhood of 

 the globe can be regarded as arising in the following manner: While the gas 

 globe is compressed, the pressure in the water is positive, and this pressure 

 is increased owing to the blocking effect of the wall. The pressure Increase 

 due to the wall is greatest between the gas globe and the wall, and the in- 

 equality of pressure thus produced has somewhat the same effect as if it were 

 due to a gravitational field acting toward the wall. The gas globe then 

 floats away from the wall in accelerated motion. During the expanded phase, 

 on the other hand, the pressure is less than hydrostatic, and the deficit of 

 pressure is greater on the side toward the wall, as it is relieved less on 

 that side by the inflow of water. Thus during the expansion phase motion of 

 the water is developed that acts to carry the gas globe toward the wall. The 

 action on the globe throughout one cycle can be regarded as equivalent to a 

 sort of buoyant force acting alternately away from the wall and toward it. 



The action during the expansion phase predominates, however, both 

 because this phase lasts longer than the compression phase, and because the 

 buoyant action on a large globe is much greater than that on a small one. 

 Since the first phase after detonation is one of positive pressure, the ini- 

 tial effect will be a slight displacement of the globe away from the wall; 

 but thereafter the distribution of momentum in the water will always be such 

 that the gas globe moves toward the wall, the momentum increasing in magni- 

 tude during each expansion phase and losing only a little during each com- 

 pression. The actual velocity of the gas globe, however, will be greatest 



