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PHYSICS: THOMPSON, HICKMAN, RIFFOLT Proc. N. A. S. 
trace a narrow ribbon on the film surface. As the projectile passes through 
the ribbon in the two places successively an interruption of light occurs 
and a dark section appears in the ribbon at each of the corresponding 
instants. This is in general a longitudinally distorted image of the pro- 
jectile and if the speed of the film were adjusted to a certain value it would 
be a miniature reproduction of its cross-section. Some records which 
have been obtained are shown in figure 3. The measurements have been- 
FIG. 3 
made from the base line of the projectile in each case and the sharpness 
of this permits a determination of the length between these lines with 
a precision which is fully equal to that of the time calibration. The only 
observations, then, that are required are those of length. Bach of these 
can be carried out to one part in ten thousand if necessary. The fact that 
a permanent record is provided and that no electrical apparatus is used 
also adds to the dependability of the results. A compact portable form 
has been designed for field use. 
A general apparatus for ballistic investigation is shown in figure 2. 
This includes a pressure gauge^ as well as the velocity equipment. With 
one discharge of the gun a fairly complete ballistic record is obtained on 
the film mounted on the rotating drum D. The velocity at the muzzle, 
the pressure-time curve in the barrel, including the pressure when the 
projectile leaves, that at maximum value, the time calibration, and, by 
