The Benson signal., 1918-04-27

above the level of the gun there is
ten-mile wind blowing straight across
the line gun target, the thermometer
Is up to 85 and the barometer has
dropped to 20. Will the projectile hit
the target now? Not unless you cor
rect the elevation Of the gun for some
of these conditions and it's laying for
the others. How are you going to
do it?
First, you must find the force with
which your projectfle cleaves the air.
This is called the ballistic cO-ef1cient,
and is reduced by simple formula to
figure. Knowing the weight Of the
projectile, it's form and its diameter
the ballistic co-eff1cient is easily cal
culated.
The Air as a Check.
You have now taken the first step.
You have the force of projectile to
penetrate the atmosphere under stand
ard conditions. But this force varies
with the atmosphere. On a heavy,
muggy day the atmosphere is dense,
and you will be surprised the check
ing effect it will have on your projec-
tile, BY means of elaborate tables one
can find just what this effect is, and
we apply it to the original formula.
Also in the atmospheric factor is in
cluded the temperature, for in hot
weather our powder charge when ig-
nited exerts more strength in decom-
posing into gases than it does on a
cold day.
So far so good. What about this
troublesome wind that is blowing? IF
1t blows against the projectile it
checks 1t; If with it, 1t helps it along.
SO we have two varying factors,
which the ballistician juggles by utiliz-
ing trigonometry until he gets them in
terms where he can once more change
his ballistic eo-efficient, making 1t
larger or smaller as the wind i'm
pedes or accelerates the projectile.
Not quite so easy as you thought, ,sis
it? But we are not through yet.
We know that under range table
conditions our propelling charge has
force that gives the projectile an
initial velocity of so many foot Sec
onds, say 2,25O. But our powder has
been stored in a dugout whose tenn
perature is not normal, so by more fig
uring we find out just what this real
velocity for the day is. Now we can
begin to figure what elevation the
gun must have to travel that 10000
yard path to the target.
By dividing our old friend G, as the
ballistic co-efficient is termed by the
distance in feet that the projectile
must travel, we get reference num-
Der. This number we take with us
and bury our noses in more tables un-
til we find opposite this number in
column headed by the muzzle velocity
another number. This number, multi
piled by G again, gives us the sine Of
twice the angle Of departure. A
quick glance into a table Of lOgerith-
mic sines gives this to us, and dividing
by two we have figure in degrees and
minutes that represents the angle
which the gun must be elevated from
the horizontal to send it's projectile
10000 yards today. Wait a minute
Our target is 200 feet above the level
Of the gun. Also because Of the curv-
ature Of the earth this 200 feet height
has been reduced somewhat. So once
more we delve into mathematics to cor
rect the curvature of the earth and
height Of site Of the target. The re-
suit we subtract from the angle found,
and this time our gun is ready for
business. Is it? No; 1t Is not so easy.
The Influence Of the RlfIlng.
Every rif1ed piece Of ordnance i'm
parts twist to its projectile, and
this twist causes the projectile to devi-
ate from it's course. In our service this
deviation, called drift, Is to the right,
and is in ratio with the range. It Is
constant, therefore, for each range, and
we find it very easily, either by math-
ematic computation or from drift ta-
bles. Also we go back to our wind
problem and find what effect the wind