HEAD - ON 
867 
in a shock. Further details on these gauges are found in 
references (2), (4), and (5), 
3S. The theory of the Hilliar gauge and the crusher gauge ig 
given in references (2) and (3), under the assumption that the 
fyuce of the niston is nerpendicular to the face of the shock, 
(Fig. 2). This orientation of the gauge ie here called "side- 
on", It 18 possible thet the approximations made in references 
(2) and (3) are fair, but it 18 difficult to estimate the cor- 
rections which should be made for them. In particular, the 
diffracting effect of the gauge was not calculated, end the possi- 
bility of "irregular reflection" from the gauge face wes neglected. 
These points will now be considered. 
4. 4A side-on shock will cause a weve to be diffracted from the 
Eip: 2 
D leading corner, C, of the block 
(Fig. 2). According to references 
YY.) (2) end (3) the pressure on the 
face CD ie the same ae in the in- 
Cc 
—{—______+— cident shock (p). According to 
SIDE-ON 
a well-known acoustical paradox, if CD is rigid and the wave 
aiffrected from C is neglected, the nreseure on CD ie 2p, in- 
dependent of the angle of incidence. Friedlsnder (6) has cel- 
culated the actual preasure on CD in the acoustic enrroximation 
without neglecting the dieturbance from C and has shown that the 
wave diffracted from C is a mechanism for reducing the pressure 
from 2p to tlie neighborhood of p, the value which should be 
a ae 
