The Saint Paul globe., 1897-10-25
VELOCITY OF CANNON BALLS
Can Now BE Told to Seconds IA
finitesiEaaI Fraction

Can Now BE Told to a Seconds Ia
fnitest1nal Fraction.
An invention of the greatest Importance
to military science has reached Sandy Hook
from Fortress Monroe, where it had been
submitted TO tests which demonstrated its
instrument made by Prof. John A. Brashear,
of Allegheny, and called a photo -chrono-
graph. Its purpose Is the measuring Of the
speed of canno- balls. The photo-chrono-
graph at Sandy Hook is the second construct
ed for the United States government, but is
= more complete than its predecessor.
In the present instrument but a single lever
is used to fire the gun, to start the tuning
fork to vibrate, to open the main shutter and
also to release the electric connections which
throw the beam OF light the Photographic
plate. That is, as as the lever is pulled
the cannon is fired to strike a wire at any
point desired. This opens the way to the pho
tographic plate. which is rotating 1,500 revo-
lutions ner minute. A streak is made on the
Photographic plate; the length of which is
determined by the rapidity with which the
ball is moving, as also the point at which the
ball cuts the second wire, where it instantly
strikes eff the beam OF light.
AS this distance be readily measured
the question is to determine how long the ball
is between the two wires. Before teo cannon
ball strikes the first wire any distance in
front of the cannon tuning. fork is set in
vibration, and through delicate opening in
the tuning fork a powerful beam Of light s
sent to the Photographic plate. The vibra-
ticns of the tuning fork are known exactly,
and as this makes a snuous streak of light
alongside CT the streak produced by the NOV
ing cannon ball it is only a question Of meas
uring the vibration of the tuning fork COW
cred by this streak, as also the fractions
vibration. In measuring the fractions of the
vibration OF tuning fork is where the most
delicate work comes in.
After the Photographic plate is developed
it is placed under a divided circle and the re-
la:ions of the two Photographic streaks meas
ured with micrometer. So exact has this
method been found that the movement of a
cannon ball two or three inches can readily
be timed
Heretofore measurements of such short in
tcrvals have been utterly impossible. because
no Photographic shooter that had weight
could be moved in such brief space Of time,
and it is in this very point that the new ma
chine is such perfect one of It's kind.
Instead of moving something that has mass
the light from an electric arc impinges on
two Nicols prisms which lie at right angles
to another. In this position it is lri9ossi-
BILE for light to pass through at all, and it was
DR.. Crehore who discovered that if a power
full current of electricity be passed through a
coil placed between these Nicols prisms the
light itself be rotated without rotating
the prisms.
1t can be readily seen that the electricity
acts as the shutter, and as an electric current
has no weight it can be moved in an incred-
IDLY short space of time; and even with the
leg that accompanies all motion, any view
lasting over the one hundred thousandth part
of a second can be photographed.