1454 
equal pressure, and using the known relation between critical 
angle and pressure (Fig. 3), it is possible to determine the 
theoretical boundary of the disturbance. This boundary is plotted 
in Fig. 8 as a broken line; the agreement between this theoretical 
curve and the observed facts is surprising. 
There is another test recorded with the spark camera which 
permits a comparison with the calculated boundary of the distur- 
bance. The test arrangement is shown in Fig. 9 which is an en- 
largement of a single picture of the taken series of 24 pictures. 
A charge is exploded in water below the center of a horizontally 
suspended bar out of the same hard sponge ebonite mentioned above. 
The sudden attenuation of the initial shockfront at the points 
where the shockwave hits the sides of the bar must propagate along 
the initial wavefront during the further propagation of the shock- 
wave. The range of the disturbance should be in agreement with 
the calculations which were carried out in the above section. 
Enlargements of the single pictures of the taken series 
were prepared and the spherical shockwave drawn on each of them. 
The radius was chosen to fit the outer parts of the wavefront 
(Fig. 9). The range of disturbance must show up in these pictures 
as deviation from the spherical shockfront because the lower 
pressures in the range cf disturbance correspond to a smaller 
velocity of propagation. These points where the observed shook- 
fronts begin to deviate from the circles were marked on each en- 
largement and all of them transferred to Fig. 10. They determine 
26 
