The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1891-09-17
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TIMING A CANNON BALL.
HOW This Nice Work Is Conducted at
the Sandy Hook Proving Grounds.
New York Snnl

New York SUM-
Capt. Heath has been burning lot of now-
DER this summer in the big guns at the
United States proving ground at Sandy
HOOK, and to people who don't know a big
gun from mortar the most interesting part
Oh THESE tests has been the method by which
the initial velocity of the projectiles has
been determined. The difficulty 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 second, which Is at the rate of q mile in
less than three seconds. Accurate knowl-
edge OF the velocity of proiectile is of the
greatest Importance to the ordn ance officer,
who has to predict the range of the shot and
its penetration, and to determine the accu-
racy and efficiency of the gun, of the powder
and the projectie itself.
Capt. Heath finds fault with powder that
gives on initial velocity of 865 feet per Sec
| ond to the l,000-pound projectile of the new
| II inch rifle at Sandy Hook, when its velo-
city should be 1.975 feet per second. This a
| difference OF only no feet. and the instru-
| ment that records 1t must be capable of
measuring the time interval to within five
thousandth part Of a second. A difference
| of two or three hundred feet in velocity
means In the big guns an important differ
/ elce In the powder used in the tests. At
Sandy Hook delicate electrical appliance
called the Boulanger chronograph measures
| this velocity accurately and records its own
| measurements. This is the way 1t does it:
In testing the large guns at these proving
grounds two open frames are set up in front
OF the gun distance of 100 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 a COM
plete electric circuit, which includes also an
| electric battery and an electromagnetic. 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 second the projectile has covered the
distance to the second screen and interrupt
Ing its circuit, releases another armature
The interval OF time between the drop of
these two armatures represents the time
| taken by the projectile to travel 100 feet.
This period is so short that 1t Is hardly con-
ceivabe. The cbronograph, however, regis
| tiers IL accurately. Wires run from each of
these screens to the laboratory near them.
where are placed the chronographs, batteries,
switchboard, and other apparatus. The two
| electromagnetic in circuit with the screens
| are embodied in the chronograph. The first
| magnet has an armature, an iron rod about
| three feet long, which is suspended in YER
tical 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
electrc-magnet, which Is placed a short dis
lance below the first, is also released. II
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
another is indicated by the distance of the
cut from the end of the rod. From this the
corresponding interval of time Is easily COM
puted, and this forms the unit for the calcu-
lation of the projectile's velocity in feet per
second. When tits necessary TO obtain very
accurate records several chronographs may
be used for testing the same shot.
Of course, the rate of a projectile's velocity
diminishes rapidly during its flight from the
gun. and the calculations of the Initial veloci-
ty must be confined to short portion of its
path Immediately after it leaves the gun. In
Simultaneous photographs are obtained of
the proJectile as 1t strikes the target. From
these experiments in velocity the ordnance
officer is able to determine the relative
strength of different powders