St. Paul daily globe. [volume] (Saint Paul, Minn.) 1884-1896, 1891-10-23
TIMING A CANNON BALL. :

How This Work Is Done at the.
Sandy Hook Proving Grounds.
Capt. Heath has been burning lot Of
powder this summer in the big guns at
the United States proving ground at
Sandy Hook, says the New York Sun.
and to people who don't know big gun
from mortar the most interesting part
of these tests has been the method by
which the initial velocity of the projec-
tiles has been determined. The diff
Guilty of such a calculation is indicated
by the fact that with our modern high,
power guns the shot is often expelled at
an initial velocity of 1000 feet per Sec
ond, which is at the rate of mile i'm
less than three seconds. Accurate
knowledge of the velocity of projectile
is of the greatest Importance to the ord
nance officer. who has to predict the
range OF the shot and its penetration,
and to determine the accuracy and eff
cheney of the gun. of the powder and OF
the projectile itself.
Capt. Heath finds fault with powder
that gives an initial velocity OF .S65
feet per second to the I.00O-pound pro.
iectile of the new 12-inch rifle at Sandy
Hook, when its velocity should be 1875
feet per second. This is difference of
only 10 feet, and the instrument that
records it must be capable of measuring
the time interval to within the five
thOuSandth part of second. A differ
ence of 200 Or 800 feet in velocity means
in the big guns an important difference
in the powder used in the tests. At
Sandy Hook a delicate electrical ap-
pliance called the Boulanger chrome
graph measures this velocity accurately
and records its own measurements. This
is the way it does it:
In testing the large guns at these
proving grounds two open frames are set
up i'll front of the gun at distance
$50 feet from each other. Wires are
stretched back and forth across each of
these frames, making screen through
which the projectile must pass. The
wires in each of these screens form
complete electric circuit, which includes
also all electric battery and an electric
magnet. When the gun is fired the
prOjectile breaks the wire in the first
screen, interrupts the circuit, and at the
same time the armature of the magnet
is released. In very small fraction OF
q second the proiectile has covered the
distance to the second screen and inter-
rupted It's circuit. releasing another ar
mature. The interval of time between
the drop of these two armatureS repre-
SClllS tllG luC ldhCll ny GllC lIOJcClllcS
to travel $50 feet.
This period is so short that it is hardly
conceivable. The chronograph, how
ever, registers it accurately. Wires Iun
from each of these screens to the labora-
tory near them, where are placed the
chrOnographs, batteries, switchboard,
and other apparatus. The two electro
magnets in circuit with the screens are
embodied In the chronograph. The first
magnet has an arnature, all iron rod
about three feet long, which is suspend
ed in a vertical position from its core.
When the wires of the first screen are
broken this iron rod falls. When the
shot ruptures the wire of the second
screen the armature of the second cleo
tro-magnet, which is placed at short
distance below the first. is also released.
1t operates as knife, and, striking
the side of the falling rod, makes a
slight mark. The distance through
which the rod drops while the shot is
passing from one screen to the other is
indicated by the distance of the cut
from the end of the rod. From this the
corresponding interval of time is easily
computed, and this forms the unit for
the calculation of the projectile's veloc-
ity in feet per second. When it is nee
essary to obtain very accurate records
several chronographs may be used for
testing the same shot.
Of course, the rate of projectile's
velocity diminishes rapidly during its
flight from the gun. and the calcula-
tions of its initial velocity must be COLI
fined to short portion of its path i'm
mediately after it leaves the gun. In
Simultaneous photographs are obtained
of the projectiie as it strikes the target.
From these experiments in velocity the
ri L, Il.
relative strength OF different powders.