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 map. Moreover, artillery is generally posted on or near prominent points, which may be found on almost any map. It should be remembered that a map is a horizontal projection, that any range scaled from a map is the base of a right triangle, and that the line of sight of a rifle, when firing uphill, follows the hypotenuse of that triangle.

An accurate knowledge of the range to within a meter is not necessary, as meteorological conditions, especially temperature and illumination (degree of humidity of the atmosphere) affect the range of the projectile. Meteorological conditions may necessitate sight corrections amounting to 100 m. at mid ranges and to 150 m. at long ranges. According to the French Firing Regulations a difference of temperature of ±10 degrees C., changes the range of the Lebel rifle by 18 m. at 1000 m. The altitude of the terrain increases the range on account of reduced barometric pressure; this becomes a factor in the selection of a rear sight elevation at an altitude of approximately 1000 m. According to the French Firing Regulations, the range of their rifle at 1000 m. is increased as follows: At an altitude of 500 m. by 18; at 1000 m. by 42; at 1500 m. by 63; and at 2100 m. by 89 m.

These meteorological influences can be eliminated by obtaining the range from troops already engaged (artillery) or by determining the proper rear sight elevation (not the range) by trial volleys (pars. 190 and 191 German I. F. R.) or by an animated fire at will (Switzerland: fire at will, one round).

To ascertain the range by firing trial volleys is, however, practicable only when the terrain near the target is visible; when the target itself is stationary; when the ground is such that the strike of the bullets can be observed; when the firing party is not under fire; and, finally, when the necessary time is available. Since all of these preliminary conditions will