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 gun 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 sights due td a mistake in the supposed distance of the object 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 case when more than one ship is engaged on each side.
It is necessary, therefore, to depend upon some form of range-finder (q.v.). No perfectly satis factory range-finder has yet been produced, but acceptable ones are beginning to appear. 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 cated, and A B C be a prolongation of the axis of the bore. Let B be the point at which the 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 assumed that the projedtile strikes the water at the end of three seconds. Since we have from physics the general formula S = 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 AEFG 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, • = BE — = 16.1 feet; = CF = 2 32.2 32.2 2 = 64.4; and = DG = 2 144.9. The formula S = is only true in vacuo; but since projectiles are very dense, it is approximately true in air when t 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
then be represented by the line A M G, which is very much nearer the horizontal plane than the curve A E F G. So that if the object fired at be at 0 instead of at G, as supposed, and if its height is 0 P, the low-velocity shot will miss it, while the high-velocity one will hit it, notwith standing the error in the assumed range. The difficulty of obtaining the correct range is en hanced by the movements of the ship firing and the one fired at, the combined movement con stantly changing the distance, sometimes very rapidly. It is necessary, therefore, to have the range transmitted to the gun as quickly as pos sible after being determined by the range-finder. This renders it desirable to have many range finders—one for every group of guns, if possible. The reason why so few hits are made in a naval battle in comparison to the shots fired is largely due to the uncertainty of range. At the naval action of Santiago the ships were all moving quickly, and the ranges were changing with great rapidity; the result was that the United States ships made less than 10 per cent. of hits, while the Spaniards probably made not much over 1 per cent. In both fleets the smoke interfered very much with the shooting; but the greatest source of error was the uncertain range.
The personal error of the gun-pointer, which tends to make him habitually shoot high or low, or too soon, or too late, can only be corrected by practice at known ranges. The state of the atmosphere affects the range, but this need not be considered. The horizontal movement of ships has already been referred to. If from a ship which is not moving, the centre of a vessel 200 feet long moving at the rate of 20 feet per sec ond (12 knots per hour) is fired at, the shot will fall astern of her if she is so far away that it takes more than 5 seconds for the projectile to reach her. If the firing ship is moving in the opposite direction at 12 knots, the shot will miss her if the projectile only takes 2.5 Seconds. The allowance is the same sort as that made when shooting at birds on the wing or at moving ani mals; but it must be very much more carefully estimated. The rolling and pitching of a ship are fruitful sources of error in firing, particularly with poorly trained gun-pointers. Good training and plenty of target practice reduce the errors in a great measure, and nearly eliminate them when the angle of roll or pitch is small. The greatest effect of rolling through small angles is seen in the decreased speed of tire, for the gun pointer of a heavy gun must 'wait for the roll' before he can fire, and if the gun is not trained correctly it will not be on the target when the sights roll past it. For this reason, and because the motion is slower then, it is desirable to fire near the end of the roll; this must always be an approximate rule, as one roll may be through an angle of 5 degrees and the next only through 2 or 3. Guns of 7-inch calibre and less are now fitted with elevating gear which permits them to be kept on the target throughout moderate rolls.