260 PHYSICS: WEBSTER AND THOMPSON 
figure 1, c. The piston p is that used in the ordinary gauge gun and bears 
upon the middle of the girder. From the mass of the piston and the dimen- 
sions of the spring we may calculate the natural frequency as about 16,000 per 
second. It is evident, however, that the damping is so great that no vibra- 
tions of the instrument appear. The upper side of the spring is held by two 
knife edges, on the end of a strong screw held in a massive frame attached to 
the gun. The arrangement is easy to see from figures 1 and 2. The registra- 
tion is by means of the image of a spot of light illuminated by an arc lamp 
and focused by a good photographic lens upon a film carried by a rotating 
drum, after being reflected upon a small concave mirror attached to the end 
of the bar. 
FIG. 1 
Instead of rotating the drum it is possible to rotate a totally reflecting 
prism, so that the film does not need to move. When used to give the p, t 
curve (from which the p, s curve, or ordinary indicator diagram may be ob- 
tained by integration — figure 4) the drum is rotated at high speed, say 30 to 
50 meters per second peripheral velocity. For maximum pressures it is neces- 
sary to move the drum only about 1/100 of a turn as the individual exposures 
are made. The film when developed shows the series of straight lines the 
height of which represents the maximum pressure in the barrel. One set of 
these is shown in figure 3. The information which is available from the p, t 
curve is of great importance, and the succ^s of the instrument in giving these 
curves accurately and with certainty is very gratifying. For example, in 
