PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 225 



scale marks in the first two cases are 7 inches apart, in the last, 13 

 inches. The black horizontal rods were introduced to determine the 

 distortion of the dark gas sphere outline inside the shock front, and 

 supporting wires and detonator fragments are also visible. The first 

 picture shows that the shock front shortly after detonation is very 

 nearly spherical and concentric with the charge. Fine structure of the 

 shock wave just behind the front is evident in the second and, to a lesser 

 extent, third records at later times. These secondary fronts are be- 

 lieved to be real rather than optical illusions, and become invisible at 

 greater distances. Even in this highly symmetrical case, the shock 

 wave close to the charge has a fine structure sufficiently pronounced to 

 be registered by refraction, a result of importance in confirming shock 

 wave pressure-time curves exhibiting similar irregularities. It is per- 

 haps not amiss to point out that these and other photographs give little 

 support to assumptions that the shock wave pressure has a significant 

 time of rise, nor does the gas bubble outline exhibit large irregularities 

 in its expanding stages. 



Shock waves from a cylindrical stick of pentolite (1 Vie inches diam- 

 eter, 5//8 inches long) are shown in Plate 11. The reference marks are 

 7 inches apart in the first two pictures taken 10 and 54 microseconds 

 after initiation, and 13 inches apart in the third picture taken at 101 

 microseconds. The first picture shows detonation about half com- 

 pleted, the shock front and gas products being visible although some- 

 what obscured by general illumination. The second picture is of par- 

 ticular interest, as it clearly shows multiple fronts in regions off both 

 ends of the charge. Two secondary fronts are visible at the detonator 

 end and a single one at the other end. These multiple discontinuities 

 are also observed in piezoelectric gauge records at greater distances, and 

 must have their explanation in the relief of pressure discontinuities at 

 fronts intersecting shortly after their formation, as discussed in section 

 7.7. The multiple shocks are present but closer to the front and less 

 evident at the later time of the last record. The shock wave front at 

 this time is less elongated and pictures at later times show the approach 

 to the spherical shape ultimately realized. 



A clear cut example of the Mach effect in intersecting shock waves 

 from two 0.11 pound loose tetryl charges placed 8.4 inches apart and 

 fired simultaneously is shown in Plate III. The shock fronts have 

 travelled approximately 12 inches from the charges (located below the 

 margin of the picture) and the normals to the fronts at the intersection 

 include an angle of 71°. The characteristic Mach region of intersection 

 discussed in section 7.7 is clearly evident, although the exact details are 

 subject to error from refraction effects. 



B. Photographs of the gas bubble. The pictures in Plates I and II 

 show the initial rate of expansion of the gaseous explosion products after 



