GUNNERY. 372 GUNNERY. possible, but should be unusual. Turret guns niaj' be cooled by the washing-out hose, which is always handy, and the other guns by the fire hose if necessary. Hang-fires (q.v. ) are chiefiy dangerous in small pieces, and most of these are automatic or semi-automatic, and will not open until the chai'ge is exploded. The jamming of the projectile may occur in spite of all ordinary precautions; but the damage is usually confined to the part of the gun outside of the ship, and therefore will not injure the members of the crew. The use of bad (i.e. deteriorated) powder is very nearly overcome by the employment of a reliable kind of powder and careful inspection and stow- age. As far as weakness of mount is concerned, there is no excuse for a mount to be so weak as to render firing dangerous to the crew. Having secured a gun and mount which is safe, we should next desire accuracy. In a field giui the causes of inaccuracy are : { 1 ) Varia- tions in the angle of sight ; ( 2 ) drift ( due to ro- tation of the projectile) : (3) wind; (4) jump; (.5) axis of trunnions not in the horizontal plane; (6) variations in velocity of projectile due to inequality of the powder; (7) incorrect setting of the sights, due to error in the supposed distance of the object fired at ; (S) personal error of the gun- pointer, which makes him habitually take too fine or too coarse a sight, etc.; (9) state of the atmosphere (height of barometer and thermome- ter and hygrometric conditions) ; (10) efi'ect of strong or defective light upon the sights, etc. In addition to these, guns aboard ship are sub- ject to errors due to (11) horizontal movement of the ship and of the enemy; (12) rolling and pitching of the ship. As ships' gims have all the errors of field guns and some very important ones of their own., we need consider them only. The angle of siglit is the elevation of the object fired at above the horizontal plane through the gun; the error due to this is usually trifiing. This follows from the principle of the 'rigidity of the trajectory.' which may be briefly explained by saying that the range of a gun upon any plane passing through the axis of the trunnions of a gun is practically the same as upon the horizontal plane, if the angle of elevation of the gun above this plane is the same as its elevation above the horizontal : provided always that the angle the inclined plane makes with the horizontal is a small one. The drift of a projectile is very con- siderable; but it is likewise very resrular in amount, and this is definitely known and allowed for by inclining the rear sight-bar to the left through an angle of 2^^ to 3 degrees, or dropping the left end of the axis of a telescope sight until it is inclined the same amount. The correction for the effect of the wind must always be left to the individual judgment of the gun-pointer; tables showing the deviations of projectiles due to winds of varying force are pre- pared, and these are used to train the gun-pointer in making allowances. Jump is the vertical error due to the upward throw of the gun as it is fired. It is very nearly corrected by an allow- ance in marking the sight-bars and drums. If the axis of the trunnions of a gim are not paral- lel to the horizontal plane, the angularity of the sight-bar or telescope is interfered with. The error due to this cause is usually small, except at long ranges, and difficult to correct. It should be avoided as far as possible by firing when the axis of the trunnions is nearly horizontal. The error due to variations in velocity caused by in- equality of powder can be corrected only by test- ing the powder at intervals, and by careful at- tention to its stowage, especially as to the preser- vation of low and regular temperatures in the magazines. The error is usually small. The error caused by incorrect setting of siglits due td a mistake in the supposed distance of the ol)ject is the greatest of all the errors of firing. In the days when guns fired slowly and the fall of the projectiles could be noted, the range could be in- creased or decreased as was seen to be desirable. But in modern ships the guns are so numerous and fire so rapidly that the fall of any particular projectile cannot be observed with certainty; and this is particularly the ease when more than one ship is engaged on each side. It is necessary, therefore, to depend upon some form of range-finder (q.v.). Xo perfectly satis- factory range-finder has yet been produced, but acceptable ones are beginning to ap])ear. The error due to incorrectly assumed distance of the object is much reduced as the velocity of pro- jectiles is increased, provided the increased velo- city is not obtained by lightening the projectile, because the trajectory, or line of flight of the projectile, becomes nearer a straight horizontal line as the velocity increases. In the following figure, let A be the point at which the gun is lo- DIAGRAM SHOWING TRAJECTORY. cated, and A B C be a prolongation of the axis of the bore. Let B be the point at which the ])ro- jectile would be in 1 second, C in 2 seconds, and D in .3 seconds, provided gravity did not act. Then the actual positions will be E. F. and G; and B E, C F, and D G are the distances the projectile would fall, if acted on by gravity alone in 1. 2, and 3 seconds. For convenience we have assvnned that the projectile strikes the water at the end of three seconds. Since we have from physics the general formula S = '{jff/", in which S is the distance in feet a body falls in t seconds and g is the gravity acceleration, or 32.2 feet, we can at once determine the shape of the curve A E F G if we know the time it takes for the projectile to move from A to E, from E to F, and from F to G. In the case assumed, S; = BE =5^ (I)' — 16.1 feet; S, = CF = 32.2 2)= = 64.4; and S3 = DG = 32.2 2 -' - --'--- -- 2 '^ = 144.0. The formula S = '^■2<lt' is only true in roci/o ." but since projectiles are very dense, it is approximately true in air when / is small. If the velocity of the projectile were doubled, the necessary elevation, in order to hit an object at G, would be represented by the line A L. in which L G is the distance the projectile would fall in 1.5 seconds, or 36.2 feet; the trajectory would
