Example: Range 7000; atmosphere 20; wind 70; velocity 2260; travel 400; tide +10. Find the correction to be applied to the gun-arm.

Solution:

I. Set ruler XX at 7000 on scale KK.
II. Set pointer a at 2000 on scale MM.
III. Set pointer e at 20; f at 70; g at +10; h at
2260, string cc at 400 on 20-second interval
line, and scale dd such that the number
400 cuts the intersection of the string with ruler XX.
IV. Set pointer b at 16 (atmosphere normal);
move slide z till b cuts the 20 curve of atmosphere.
V. Set b at 50 (zero wind) and slide z till b cuts
the 70 curve.
VI. Set b at 0 tide and slide z till b cuts +10.
VII. Set b at 2200 I. V. and slide z till b cuts 2260.
VIII. Set b at 400 on scale dd and slide z till b cuts
300 (normal) on scale dd.

The range correction is now found on scale mm opposite pointer a. This number is now set on the gun-arm of the plotting-board and each next plotted position will read on the range-scale of the gun-arm just that many yards more or less than the true range, i.e., the corrected range.
See Fig. 65.

The Deflection-board.

NOMENCLATURE.

Platen.—The rectangular sliding frame.

Wind-arm.—The arm pivoted to the board on the left of the platen.

Wind-component Scale.—The scale above the movable end of the wind-arm.

Drift-curve.—The curved edge of the metal plate attached to the left end of the platen.