170 



The pressure P can only be zero when '"(t^) is zero, i.e. at fixed values 



u- 



Hence 

 the duration of the positive pulse must be a constant fraction of the total period T. Using the 

 proportionality factor obtained from the full step by step calculation for 1 oz. of T.N.T. at 6 feet 

 depth yields equation (lO) for the duration D 



0.218T 



(10) 



The shape of the pressure wave . 



Equation (2l) giving the pressure as a function of bubble radius and the constant m cannot 

 be used directly to plot a pressure-time curve. A rough idea of the shape of any given pressure- 

 time curve may, however, be obtained for any case by calculating from these equations the "shape 

 factor" -p-Q. This is equal to the ratio of the area under the pressurt-time curve to the area of 

 a triangular wave form having the same peak pressure and duration. The factor thus gives a measure 

 of the "hollnwness" of the pressure.time curve. 



Values of P . I, and this "shape factor" for a few charge weights and depths have been 

 given in Table 1 to demonstrate the effect of thesi- variables on the "shape factor". In order to 

 visualize the meaning of a particular value of the "shape factor" a few i nag i na ry pressure w.we forms 

 have Seen sketr.ed in Figure 9 and their "shape factors" computed. The curve labelled "0.2U" has 

 been drawn to r semole closely the actual pressure-time curve calculated for 1 oz. of T.|il,T, at 

 6 feet depth. 



TABLE 1 . 



PEAK PRESSURE. DURATION, IMPULSE AND SHAPE FACTOR . 



(Pressures and impulses multiplied by the nurrerical value of the 

 distance in feet from the point considered to the bubble centre) 



I 



