Vol. 6, 1920 PHYSICS: THOMPSON, HICKMAN, RIFFOLT 
175 
as to the variation of the so-called form factor with the velocity. If the 
results indicate an entirely new law of resistance for each form of projec- 
tile, as has been suggested, this fact should be evident. In this case there 
will not be a linear relation between the datum line for a projectile of 
one form over its range of velocities and that for another projectile with 
a different form. The fact that a projectile in actual flight may gradually 
change its cross-sectional surface presented in the direction of flight owing 
to precession (and thus produce a gradual change in the "law") is an argu- 
ment for making the measurements in a short distance. 
44 cal. 30 cal. 
FIG. 5 
Another ballistic application of the method of determining velocities is 
in connection with penetration. The energy loss, and even the law of 
retardation, that is to say the resistance of a solid with the passage of a 
body through it under given conditions, can be found through spark pho- 
tography and successive determinations of the velocity. Here it is prac- 
tically necessary to be able to measure the velocity in a few feet, this ap- 
paratus thus being particularly adapted. 
While the ballistic application of the experiments is perhaps most 
important, yet the general investigation of the hydrodynamical aspects 
is of considerable interest. It is not necessary to restrict the bodies 
