MOTION OF THE GAS SPHERE 219 



for several explosives (TNT, tetryl, pentolite, etc.) at a variety of 

 depths. The most complete data are for tetryl, and two typical ex- 

 amples for 0.056 pound charges at 5 feet and 330 feet are plotted in Fig. 

 8.3. These curves show the characteristic cusp shape of curve for each 

 cycle, with decreasing period and amplitude of oscillation in successive 

 oscillations. The decreased values of maximum radius and period as a 

 result of increased hydrostatic pressure at the greater depth are also 

 evident. A radius-time curve for a tetryl charge weighing 0.55 pound 

 at 300 feet is shown in Fig. 8.3c, and the effect of charge weight is seen 

 by comparing this curve with Fig. 8.3b for the 0.05 pound charge at the 



DIRECTION 

 OF MOTION 



\ 



H 8" M 



Fig. 8.4 The gas sphere near its maximum and first minimum (one ounce 

 polar ammon geUgnite charge, three feet deep). 



same depth. It should be mentioned that the values of radius near the 

 bubble minimum are somewhat uncertain, owing to "smoke" from the 

 explosion products which obscures the bubble outline. It is also to be 

 noted that the curves in Fig. 8.3 have been plotted without regard to 

 vertical displacements of the bubble center and do not therefore show 

 the upward migration under the influence of gravity. This effect is 

 considered in section 8.5. 



A difference in character of bubble shape during its motion is the 

 distortion from spherical form near its minimum, an effect which is 

 largest for charges fired near the surface. This difference is shown in 

 Fig. 8.4, a sketch of the bubble outline near the minimum radius for a 

 condition of rapid upward migration. (The sketch is based on photo- 

 graphic studies made by the Road Research Laboratory (95).) 



