Dagan and Tulin 



This vanishes never and according to the foregoing criteria the 

 motion is stable. 



However Pies set and Mitchell [ 1956] proved that in fact this 

 spherically symmetric implosion is unstable. This demonstrates 

 that for interfaces which are appreciably curved like in the author's 

 case, the criterion for marginal stability is not always that the 

 normal pressure gradient vanishes. 



REFERENCES 



Plesset, M. S. and Mitchell, T. P., "On the Stability of the Spheri- 

 cal Shape of a Vapor Cavity in a Liquid, " Quart, of App. 

 Math. , 13, 4, 1956. 



DISCUSSION 



Prof. Hajime Maruo 



Yokohama National University 

 Yokohama^ Japan 



I congratulate the success of Dr. Dagan's beautiful analysis 

 of the wave-breaking resistance discovered by Baba. As in the 

 discussion at the 12th ITTC, this resistance component has been 

 regarded so far, as a portion of the wave resistance. However the 

 present analysis indicates that it is not the case. The wave-breaking 

 resistance seems to have a nature akin to that of the spray resistance, 

 Which is the better classification, the author considers, whether it 

 belongs to the wave resistance or to the spray resistance? 



624 



