/ 



256 -10- 



The following limitations and qualifications should 

 be noted. First, the effect of the free surface has been 

 ignored, so that these results are valid if the mine is 

 sufficiently far from the sea level, say, more than 3.5 L ft. 

 Second, it is assvuned that the bubble does not meet the 

 sea bed In the course of its motion. This is approximately 

 equivalent to B > •8L. Third, the sea bed is assumed 

 to act like a rigid wall. 



The validity of this last assumption, should be tested 

 by experiment. But there are plausible reasons for supposing 

 that as far as the balancing phenomenon is concerned, the 

 sea bed does act very much like a rigid wall. During the 

 largest part of the time of pulsation of the bubble, the 

 bubble is large and the pressure in the water Is low; there- 

 fore, a sand bottom could be considered as rigid. The 

 balancing effect is determined in the main by this portion 

 of the period of pulsation. 



4. An exanple . 



Consider a mine with 1500 lbs. of T.N. T, in water 

 of depth 150 ft. Figure 3 is a graph showing the relative 

 magnitudes of the peak pressure in the secondary pulse 

 when the mine is placed at varying distances from the 

 bottom of the sea. The best location of the mine is 21.0 ft. 

 from the bottom; the resulting peak pressure at any point 

 in the water, as conrputed in part II, is 2015/R atmospheres, 

 where R is the distance in feet from the point to the center 

 of the contracted bubble. If the mine is initially placed 

 at any other distance from the bottom, this value for the 

 peak pressure is to be multiplied by the factor whose graph 

 is drawn in figure 3. j^c 



This graph demonstrates a remarkable sharpness in the 

 peak presnure curve as a function of the distance of the 



■«• The correction dve to the free surface is treated in ' 

 part III, section S' 



t In reality the graph will not be quite as sharp because 

 of deviations from the assumptions listed on page 12. 



