ANGLE OF ELEVATION. in air, feet. yards. yards.
04' 1,531 1,000 1,115 15' 1,383 2,000 2,350 35' 1,258 3,000 3,732 04' 1,161 4,000 5,277 50' 1,087 5,000 7,007 42' 1,028 6.000 8,975 In addition to checking the velocity of the projectile by direct pressure and friction, the air acts upon it in other ways. It tends to move it in the direction in which the air currents are moving and, in conjunction with the rotation of the projectile, causes a certain amount of deflec tion due to gyroscopic action which is called drift. As this gyroscopic action is almost exactly constant for any particular gun and velocity of projectile it results in an angular error which is allowed for in primary ad justment of the gun-sights. This is effected by inclining the rear bar-sight to the left or inclining the axis of the telescope sight. Other irregular lateral deflections or errors are corrected by a sliding leaf on the rear sight bar, or on the telescope bar or set ting device. Vertical or range errors are cor rected by increasing or decreasing the angle of elevation. Gun-fire is commonly divided into two classes, direct and high-angle. In direct fire the elevation is small, the trajectory very slightly curved, and the projectile strikes from a lateral direction. This is the ordinary method of shooting. In high-angle fire, advantage is taken of the fact that, with a given muzzle velocity, a projectile will have its greatest range when the gun is elevated at an angle of slightly more than 45 degrees above the horizontal. This enables a short gun to fire a heavy projectile at a low velocity to a comparatively long distance. The range may be varied by changing the ele vation or the weight of the charge. High powered guns (muzzle velocity 2,500 feet per second or greater) are usually fired at angles of elevation of less than 15 degrees, but greater elevations are beginning to be common. The term curved fire is sometimes applied to firing at angles between 15 degrees and 30 degreeS and high-angle fire to elevations of 30 degrees or above. To render a gun effective, it must possess (a) safety, (b) accuracy, (c) destruc tive power, (d) adequate speed of fire. Abso lute safety is impossible, but accidents should be rare and only of the kind that it is impossible to wholly provide against. A reputation of
unsafeness tends to demoralize the men who handle the gun. The greatest sources of danger in a modern gun are (a) premature explosion of the charge, (b) explosion of the projectile in the bore, (c) jamming of the projectile in the bote, (d) hang-fire, (e) weakness of breech mechanism. Accuracy is of course imperative, as an inaccurate gun is useless except at very short range. In a field gun the principal causes of inaccuracy are: (1) variations in the angle of sight, (2) drift (due to gyroscopic action of the projectile), (3) wind, (4) jump, (5) axis of trunnions not horizontal, (6) variations in the velocity of the projectile due to inequalities of powder, (7) error in supposed range, (8) personal errors of the gun-pointer, (9) varying conditions of the atmosphere as regards height of barometer, temperature, heat-waves, hygro metric conditions, (10) effect of strong or de fective light on the sights or on the objective. Guns mounted on board ship are affected by these causes and by (11) horizontal movement of the ship and of the enemy—changes of course and speed, and (12) rolling and pitching of the ship. Field guns are frequently directed at targets beyond intervening objects and in that case they have difficulty in (13) pointing in the correct direction, and (14) a jarring of the sights by the shock of discharge which may throw them temporarily out of adjustment. The combined effect of these errors may be con siderable. Some of them permit of partial in dependent correction. The total remaining error is overcome by observing the fall of the projectiles. On board ship, this is effected by spotters at the masthead; on shore, by range observers in kite-balloons or airplanes. The sights are readjusted by the information so derived and this readjustment is made as often as necessary. In order to make spotting easier and more certain the guns on large ships are fired in salvos (usually half or all of the guns which will bear). Observation of the fall of projectiles on land is usually more difficult than at sea except by the forces which have com mand of the air, or are strongest in the air. Whichever side has a marked superiority in air power possesses a great advantage in fire control and gun efficiency.