PHYSICS: C. G. ABBOT 
389 
taining an electromagnet which operated a shutter for the purpose which 
I am about to describe. Opaque cards were placed to cover the left 
hand hole in the first diaphragm through which the shot was fired and to 
cover the hole in the second diaphragm through which the shot passed 
after having accomplished two meters more of its flight. The passage 
of the shot through the tinfoils beyond closed the electric circuit and 
operated the shutter which has been mentioned. 
The beam from the left hand hole of the first diaphragm passed to a 
mirror at 45° which reflected it at right angles upon a photographic plate 
caused to rotate in its own plane by means of an electric motor. The 
beam from the right hand hole of the first diaphragm, after passing 
through the second diaphragm was also reflected by the same mirror 
upon the same photographic plate but diametrically opposite to the first 
beam with respect to the center about which the plate rotated in its 
own plane. Between the mirror and the photographic plate was a long 
slit which restricted the two beams of Hght to mere streaks extending 
respectively from the center to the right hand and to the left hand of 
the photographic plate. Just above the long slit was placed the shutter 
controlled by the electromagnet. 
It will readily be seen that when the gun was fired, first a beam from 
the left hand hole in the first diaphragm passed to the mirror and was 
reflected upon the rotating plate. After the projectile had covered the 
two meters leading to the second diaphragm, the other hole was opened 
and the second beam began to print upon the other half of the photo- 
graphic plate. Immediately thereafter the electric circuit was closed, 
the shutter consequently closed, and so the photographic action stopped 
before the plate had made a half revolution. The delay in the electric 
action had no influence on the work provided the shutter closed before 
the plate made a half rotation. After development the blackened parts 
of the photographic plate were found to indicate a certain angle through 
which it had rotated between the time when the projectile opened the 
first aperture and the time when it opened the second aperture. Having 
determined the speed of the photographic plate by measuring the speed 
of the electric motor, the velocity of the projectile became known. 
It is easy by this means to determine with some accuracy very great 
velocities of translation within a very short path. Even one meter path 
would be quite sufficient. 
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