PHYSICS: THOMPSON, HICKMAN, RIFFOLT Proc. N. A. S. 
have been used. For example, a very successful method was recently 
developed at the Bureau of Standards by Duncan and Curtis in which 
straight lines were produced across the photographic film by the interrup- 
tion of light passing through slits attached to the tuning fork. 
In the ballistic investigations which have been conducted at Clark 
University during the past two years, a great many of the records ob- 
tained have required such a time scale; for example the pressure-time 
curves for a gun, the curves for the vibration of the gun barrel, and later 
the velocity measurements which are described in this paper. The Bureau 
method was first adopted. We have been able, however, to develop a 
new apparatus which permits the placing of equally sharp lines across the 
film at very high frequencies. This is of considerable importance in the 
small arms ballistic work, inasmuch as it is necessary to have the film 
moving at high speed. 
The apparatus for the time records is shown in figures 1 and 2. The 
tuning fork has a small needle attached to one prong (a weight being at- 
tached to the other) against which there bears a light stiff filament of steel 
ribbon held in place by a massive frame attached to the base mounting 
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0 / z 3 a 
1 I I I I I I I I 
FIG. 1 
of the tuning fork. (The filament is practically a duplicate of the one used 
in Professor Webster's Phonometer to record the motion of the vibrating 
disk.)^ Upon this filament is mounted a small mirror. Light is reflected 
from it and focused on a stationary slit. The fork is struck with an elec- 
