CHAPTER XI.

Winds, Their Influence and Correction.

F GRAVITY and the resistance of the atmosphere

I were the only factors that influenced the path of

the projectile, that path would lie in a vertical plane. The influence of the winds, however, distort that path considerably.

Winds are named according to the direction from which they blow. It is customary to conceive the horizon to be a large clock dial, with the shooter at the center, and the target at 12 o'clock. A 12 o'clock, therefore, blows from the target toward the shooter, a 6 o'clock wind from the opposite direction; a 3 o'clock wind from the right, and a 9 o'clock wind from the left normal to the plane of fire.

The 12 o'clock wind increases the resistance of the atmosphere, and thereby increases the curvature of the trajectory; the 6 o'clock wind lessens the resistance and thereby lessens the curvature.

All other winds cause the projectile to be deflected in a curve, lying in a horizontal plane normal to the trajectory curve. And inasmuch as the velocity of the projectile decreases the farther it travels, the wind curve increases with the distance. Gravity being a constant, its effect is constant; wind being a variable in velocity and direction, its effect is a variable.

Since a 12 o'clock wind increases and a 6 o'clock wind lessens the resistance of the atmosphere, it

is evident that if a compensating factor correcting the value of C for these winds could be determined, their influence could be readily computed. Let us call this wind factor, fw, then

W

in which v is the velocity of the wind in miles per hour, T the time of flight in seconds, and X the range in feet.

+, if the wind is 6 o'clock.

if the wind is 12 o'clock.

Problem 22.

Given vw say, 20 miles, 12 o'clock,

X 1000 yards, 3000 ft.

T (.30-150-2700-C-.389) 1.865 sec.

To determine fw

Hence, fw=1-0.029-0.971

The variation in range due to 6 and 12 o'clock winds may be very closely approximated by the formula

X' = fw X X + or - 3 TW

For. 36a

in which T is the time of flight and W the velocity of the wind in feet per second.

Hence, X'=0.971X1000-13 X 1.865X30=952 yds.

The variation may be accurately computed by formulae 35, 44 and 45, used in order named. That is, the projectile will strike 48 yards in front of the target.

Hence, the sights should be elevated for 48 yards.

Following problem 22, the reader may construct a table of Range Corrections for the 6 o'clock and 12 o'clock winds, as follows:

Table of Range Corrections for 6 O'clock and 12 O'clock Winds.

Description of cartridge, say .30-150-2700-C-.389.

Problem 23.

To determine the wind deviation, Dw, of any projectile at any range, due to the influence of a 3 or 9 o'clock wind.

Herewith a modification of Didion's formula, for a wind normal to the plane of fire, or a 3 or 6 o'clock. X V

+.002 X R2) For. 37

Dw (in feet) = Vw · (T· in which vw is the velocity of wind per hour, T the time of flight in seconds, X the range in feet, V the initial velocity, and R the hundreds of yards in the range.

Given Vw=1 mile

W

X=1000 yards, 3000 ft.
V=(.30-150-2700-C-.389)
T, for same,=1.865 sec.
R 10 (hundred)

To determine Dw (in feet).

Dw=1X (1.865

3000

+.002 X 102 )

2700

= 1X (1.865-1.111+.2) = .954 feet.

=

For a 20 mile wind, the above problem would be 19.08 feet.

That the influence of a 1 o'clock wind may be determined, it is resolved into its components, AC a 12 o'clock and CB a 3 o'clock wind.

Fig. 10.

The angle ABC is 60 degrees. Solving the triangle (trigonometry) assuming AB (1 o'clock) to be 1 or unity (mile) we have BC, the 3 o'clock wind equals 0.5 or 2, and AC equals 0.866 or approxi

mately 7% of AB, the 1 o'clock wind. That is, the influence of a 1 o'clock wind is equal to the combined influence of a 3 o'clock wind with a velocity of 1⁄2 that of the 1 o'clock wind, and a 12 o'clock wind with a velocity of approximately % that of the 1 o'clock. Similarly, an 11 o'clock wind may be resolved into a 9 o'clock and a 12 o'clock wind, a 5 o'clock wind into a 6 and a 3 o'clock wind; a 7 o'clock into a 9 o'clock and a 6 o'clock wind.

A

Fig. 9.

Fig.10

A 2 o'clock wind is resolved into its components, AC, Fig. 9, a 12 o'clock and BC a 3 o'clock wind.

The angle ABC is 30 degrees. Solving the triangle, assuming AB (2 o'clock) to be 1 or unity (mile), we have BC, the 3 o'clock wind, equals 0.866 or approximately % of AB, and AC a 12 o'clock, equals 1⁄2 of AB, the 2 o'clock wind. That is, the influence of a 2 o'clock wind equals the combined influence of a 3 o'clock with a velocity of approximately % that of the 2 o'clock, and a 12 o'clock wind with a velocity of 1⁄2 that of the 2 o'clock.