Cavitation (Influence of Free Gas Content) 
rently, came up with the same general conclusion as to the bubble di- 
stribution importance. 
I should like to thank Professor Silberman for pointing out a 
paper of his which was unknown to me. It is unfortunate since the 
Navy Department apparently funded that work and I was remiss in not 
seeing it in the literature. 
I would like to say that all of this work can be considered a 
conservative estimate in that I emphasised free gas bubbles must 
touch the surface. If I gave looser bounds to the calculations and said 
that the bubbles did not have to touch the surface but only had to reach 
within a certain distance from the surface, in fact it would make the 
case I was presenting even stronger. As you may have noticed, the 
actual bubble distri bution was not even used in this work. All we took 
were the total number of bubbles observed. We did not even discrimi- 
nate between bubbles that had an order of magnitude difference in size 
because we felt it was not necessary to make the point. 
On Mr. Bindel's comments, I should like to point out that we 
could only discriminate between bubbles and solid particles larger 
than 25 micrometres and the conclusion that bubbles were not impor- 
tant at all was drawn by inference from the studies that we made in 
the high-speed basin. The analytical calculations of the trajectory by 
Foissey are not familiar to me but I think a comparison would be in- 
teresting between the methods of calculation. I might say that the 
paper pointed out that the calculations that I have used were compar- 
ed with experimental measurements of bubbles in pressure gradients 
in water and in glycerine and there was quite good agreement, sol 
suspect that there may be a close agreement between the work of 
Foissey and the calculation methods that we used. This work does 
agree with the work of Hsieh. The equations used were essentially 
the same. Perhaps it is unfortunate, but in the slide that I showed I 
did not show that some bubbles would be drawn away from the body, 
but given a pressure distribution on the surface - and in my case it 
was different from that of Hsieh and Johnson - and given different 
bubble sizes, some bubbles would by the pressure gradients be. forced 
away from the surface. I should like to reference the work of 
Dr. Brockett at the Naval Ship Research and Development Center. He 
has ina report correlated the noise produced by the collapse of the 
cavity and the visual observation through high-speed photography of 
the collapse of the cavity on the surface. It has been shown that ba- 
sically in the work on this body the cavitation that occurred on the 
surface produced the noise and to the best of our information the 
cavities that were further from the surface and did not touch the sur- 
face, did not produce noise within the significant range of this work. 
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