466 -2- 



Resul ts . 



The 'orms of the pressure-time curves recorded at a point 5 feet above the charge, for various 

 distances of the charge from the bottom are shown in Figures 1 and 2; the shock-wave peaks have not 

 been recorded and portions of the records between the pulses have been omitted. The records exhibit 

 the same general features as those obtained in earlier work where charges were fired near a gravel 

 bottom though there are certain differenves as mentioned below. 



Shock waves :- The recorded parts of the shock waves were similar in form to those obtained 

 previously except that with the charge very near to the bottom the reduction in the tail was not so 

 pronounced. a few measurements were made of the magnitude of the peak shock wave pressures produced 

 with the charge on the bottom. The results indicated that there was an increase in peak pressure 

 above the 'open water' value. Further investigations are required to determine more precisely the 

 action of the bottom in this case (e.g. to determine whether the charge plus bottom acts as a single 

 charge s;f greater weight or as two separate charges). 



waves of unknown origin :- Pulses similar in form and magnitude t(j those observed in the 

 earlier tests were recorded some u to 7 milliseconds after both the shock wave and bubble waves. 

 This is further evidence against these waves being produced by reflections from a denser substrata 

 since the two sets cf tests were made at different sites. 



Bubble waves :- In most cases two bubble waves were observed. In the previous tests, only 

 one bubble wave was observed for charge distances of 1 foot 6 inches or less from the bottom, whereas 

 here two pulses were recorded at 1 foot from the bottom. At distances of 1 foot 6 inches or less, 

 there was a difference in wave form in the two cases, e.g. with the charge fired on the rigid bottom, 

 the buOble wave was more intense and of shorter duration than when fired on the gravel bottom. 



The maximum pressures and impulses in the 1st and 2nd bubble waves at points 2, 3, i and 5 

 feet above the charge are shown in Figures 3, 4, 5 and 6 for various distances of the charge from the 

 bottom. 



The pressures and impulses in the 1st bubble waves with the charge 3 feet from the bottom are 

 about the same as observed before. The present results, however, show an increase in both pressure 

 and impulse with the charge about 6 inches from the bottom. This effect was not previously observed 

 for the pressures, but was observed to some extent for the impulses. 



The variations of pressure and impulse in the 2nd bubble waves with distance of the charge 

 from the rigid bottom were appreciably different from those with the gravel bottom. Thus Figure S 

 shows that the pressure has a maximum value for the charge about 2 feet from the bottom and Figure 6 

 shows that the impulse has a maximum for the charge about i foot from the bottom whereas previously 

 both pressure and impulse decreased with distance of the charge from the gravel bottom. 



NO measurements were made of the rise or fall of the bubble. It is proposed to make these 

 measurements using 'he underwater camera, which will also give information on the shape of the bubble. 



Bubble periods :- The periods of the 1st and 2nd bubble oscillations are shown in Figure 7 

 as functions of the distance of the charge from the bottom. Also shown in Figure 7 is the theoretical 

 curve for the 1st oscillation of 1 oz. of T.N.T. neglecting the effects of the free and rigid surfaces. 

 It is seen that the ist period increases as the charge approaches the bottom, until the charge is 

 6 inches from the bottom where the period is a naximum. This increase is expected theoretically if 

 the bottom behaves as a rigid surface. with the gravel bottom no such increase in period was observed. 

 The bubble periods observed here are greater than those obtained in the gravel pit. This rawtbe 

 due to the proximity cf the sides of the tank to the explosion 



Conclusions . 



The principle effects observed with the rigid and gravel bottoms are summarised below for 

 charge distances of 3 feet or less from the bottom. 



Rigid 



