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68. The main qualitative features of the bubble motion in mid water as 
predicted by theory and observed experimentally, can be summarized as 
Pollows:- 
(1) an oscillation of the gas bubble, 
(2) the rise of the bubble due to tne net hydrostatic force, and 
3) the production of bubble pulses subsequent to the original 
P Pp 
pressure pulse. 
The initial explosion is followed, in effect by subsidiary feebler 
explosions. If the explosion takes place well to the side of the target 
these "explosions" are relatively unimportunt. If a charge is exploded 
vertically below a target, however, the rise of the bubble under gravity 
May cause one of the later explosions to occur very close to the target 
and contribute appreciable damage. This phenomenon of enhanced damage 
when an explosion takes place beneath a target instead of to the side, has 
been observed experimentally and is possibly the most important practical 
consequence of the bubble motion. 
Behaviour of bubble near free and rigid surfaces 
69. Possibly the most remarkable features of bubble behaviour are those 
associated with the presence of nearby surfaces. One extreme aspect is 
the behaviour of the bubble at the free surface of the sea where there is 
virtually no resistance to flow and where the pressure remains constant. 
The second extreme feature is the behaviour of the bubble at a completely 
rigid surface preventing any flow perpendicular to it. The effects of 
such surfaces have been both predicted mathematically and observed 
experimentally on the smallscale.* Briefly they are as follows:- 
(41) A free surface exerts, in effect, a weak attraction for the 
bubble while it is expanding followed by a strong repulsion 
when the bubble is small and contracting. The net effect is 
that the bubble tends to move away from a free surface during 
a complete oscillation. 
(2) A rigid surface exerts a weak repulsion for the bubble while 
it is expanding followed by a strong attraction when the bubble 
is small and contracting. The net effect is that the bubble 
tends to move towards a rigid surface and stick to it when in 
contact. 
In both cases the attractions and repulsions increase with decreasing 
distance from the surface, subject in cause 1 to the bubble not breaking 
surface on its expansion. 
70. ‘The repulsion from a free surface and the attraction to a rigid 
surface are in the nature of what is known mathematically as a second- 
order effect. It is not possible, therefore, to give any short account 
of the underlying theory ‘ and attention will be concentrated on the 
qualitative results in connection with the effects of the sea surface and 
the sea=bed on the motion of the bubble. 
x The reader is strongly recommended to see ciné-films of bubble motion. 
