MOTION OF THE GAS SPHERE 



301 



face become a larger part of the total, but the technical problems of 

 photographic or other measurements are much more difficult for large 

 charges. As a result, there are not many experimental data available 

 to test the simple gravity theory, and most comparisons to experiment 

 must be made with more elaborate theories which include surface 

 effects. These theories and their agreement with experiment are con- 

 sidered in later sections. 



As we have seen, the first maximum of the bubble and the period of 

 the first oscillation are not much affected by gravity except as it deter- 



DEPTH (ft) 



Fig. 8.9 Migrations of gas bubbles from 0.66 pound loose tetryl charges. 



mines the hydrostatic pressure, and the discussion of section 8.3 neg- 

 lecting gravity need not be reconsidered here. The vertical position of 

 the bubble center as it first contracts and approaches the first minimum 

 is affected, as are the later phases of the motion. The first systematic 

 investigation of bubble migration for various depths and charge weights 

 was made by H. F. Wilhs and Ackroyd (122). These observers meas- 

 ured the bubble rise at the time of the first and second minima by 

 sound-ranging methods employing two piezoelectric gauges at known 

 positions relative to the charge to determine arrival times of the second- 

 ary pulses at these points. Differences in these times as compared with 

 those for the shock wave then permitted calculation of the rise. A 

 series of charges of various weights (15 grain detonators, 1 ounce tetryl, 

 1 and 5 pounds blasting gelatin) were fired at wide ranges of depths in 

 55 feet of water. 



The results of Willis and Ackroyd showed first a gradual increase in 

 the rise as the charge depth was decreased, in fair agreement with Eq. 

 (8.32) according to which the rise varies with depth d as (d + 33)~^^^ 



