To memorize distinguishing marks on the enemy as an aid in arriving at a correct estimate of the range, is not considered as simulating actual service conditions, since the power of vision of the men differs. Furthermore, one sees the enemy mostly in a prone position in action, and it is not to be expected that the men will remember the various rules for determining the range by the appearance of the enemy’s skirmishers. The same is true of the regulation which prescribes that the soldier estimating the range is to judge the effect which the target makes upon him. (England and Austria). It will rarely be practicable for infantry to scale the range directly from a map. Infantry ranges are usually so short, in comparison to the scale of most of the maps, especially those available in the field, that it is difficult to take them between the points of a pair of dividers. Moreover, in an infantry action conspicuous objects are not likely to be located often enough near the position of the skirmishers to make it practicable to find them quickly even on the most accurate maps. The artillery is much better off in this direction; its ranges are longer and therefore more easily scaled from a 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,[140] 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. [140] Lieutenant-General ROHNE, _Schieszlehre für die Infanterie_, p. 51, and also par. 70 Austrian I. F. R. The effect of temperature and of the altitude of the battlefield was strikingly illustrated in the Boer war. 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).