559 



so tha'W close to the minimunv meaaxarements wers not at all reliable . The 

 overall precision of the bubble radius raeasurenent is estimated to be 

 about 2- , Some improvement in these measurements coUld have been made by 

 providing standards of length of nearly the same size as the bubble diameter, 

 and by very sharp focussing, 



III. RESULTS 



5. Firat Bubble Periods in Free Wa,ter 



These results constitute what are believed to be reliable measurements 

 of the first bubble periods for TNT, tetryl, and torpex-2 ^5;^ in the absence 

 of any interfering siu:faces, and at such depths that migration effects can 

 be considered negligible. All charges were in the form of cylinders of 

 height slightly greater than the diameter. A few values are given for 

 pentollte and blasting gelatin, but these cannot be considered as giving a 

 d«finitive value of the period. Some of the results included are for shots 

 made in water of such depth that the surface corrections would be small, but 

 not negligible. 



Table I shows the results at several depths for cast TNT charges of 

 density 1.5. Depths were measured by a Boxordon depth gauge^), but in a few 

 cases the meter wheel reading of the length of cable let out was used. Periods 

 were read from piezoelectric records.* The period constant K^ was defined bv 

 the equation3) 



wl/3 



Kn -^ (5.r, 



where T^ ■ the period (sec) 



Zq - H+33> where H is the initial charge depth (ft) 



W - charge weight (lb) 



n " refers to the first, second, or third bubble period. 



The TNT in each chargs weighed slightly raor'. than a half-pound; to that 

 weight t;as added the TNT eqiilvalent of the 44 gm tetryl booster and 1 gm 

 for the detonator. As far as bubble energiss go, pressed tetryl appears to 

 be about 3jJ more energetic than TNT, ,o the weight of tetryl used was con- 

 •.-^rtsd to the eq\:iva1^nt freight of ''. by r.u'.+ lplylng by 1.03. 



The average value for the pori-,' constant for cast TNT obtained in this 

 way is 4,36. Thi« standeurd ddTiatiou of the mean is shown together with the 

 average in the taole. 



* In a few cases, indicated in Table I, when piezoelectric records were not 

 available, the periods were read from radius- time curves plotted from the 

 Fastax pictures of the bubble cycle. These values are probably accurate to 

 2% or better, as indicated by the correlation when both measurements were 

 avail lable. 



