GUNNERY, NAVAL. It is no easy mat ter to hit a moving target at sea, with a gun mounted upon an unstable platform which is also moving in relation to the tar get. Two elements must be accurately known to obtain a hit. Firstly, the correct range, so that the gun, when fired, shall be at the proper elevation to throw its shell to that distance and, secondly, the deflection, or lateral correction, to allow for the wind and the movement of the target whilst the shell is in flight. A gunlayer firing a single gun at short range can follow the path of his shot and see it strike the water or target, and can correct his sights so that the next shot will hit. Under battle conditions, with a number of guns firing, this be comes a sheer impossibility for anyone stationed at the guns. Hence the necessity for controlling the gunfire of a ship from a position remote from the guns themselves. The control officer is stationed high up in the ship, where he is in the best position to see the fall of the shot and where he has all the available informa tion for estimating the hitting range and arrangements for com municating it to the guns. The gunlayers keep their guns laid upon the target and fire on the command of the control officer.
Let us consider the simple case of a ship firing at a target at a range of about i o,000 yards. The range is obtained by the range finder, and certain corrections, which will be explained later, are applied to the rangefinder range to obtain the "gun range" which, with the estimated deflection, is passed to the guns. As the target and the firing ship are both moving, the range must be changing at a certain rate : this "rate of change of range" is estimated and applied to the gunsights at set intervals. The control officer fires a salvo and watches the fall of shot. He makes a spotting correc tion in range and deflection, calculated to get the next salvo to fall upon the other side of the target to the first one. If this happens he halves his original spotting correction and applies it in the opposite direction and fires again. This third salvo should fall close around the target, and if the rate of change of range has been correctly estimated, subsequent salvoes will continue to hit. This is a very simple case of a bracket system which is used, in one form or another, by all control officers.
Corrections.—The corrections, which must be applied to the rangefinder range in order to obtain the hitting or gun range, are due to (I) the differences in muzzle velocity of individual guns, (2) the effect upon muzzle velocity of the temperature of the charges, (3) the direction and force of the wind, and (4) the height of the barometer and thermometer. The first two are usually applied direct on to the sights of each individual gun and remain more or less constant for any one day. The last two fac tors vary from hour to hour and are applied as a correction to the rangefinder readings. The rate at which the range is changing is another variable factor, since it depends upon the bearing be tween the ship and the target, which is constantly changing. The same applies, in a lesser degree, to the deflection, and con stant corrections in both rate and deflection are necessary to maintain hitting after it is established.
Thus there are many problems to be solved, even in the simple case of firing at a target at io,000 yards. Battle ranges have now been extended up to 20,000 yards and more, and as the range in creases the problems with which the control officer is faced be come much more complicated. At long ranges the observation of the fall of shot becomes extremely difficult, and as the range in creases errors caused by the roll, yaw and pitch of the ship be come accentuated. The shells in their flight reach a very high altitude and the wind and atmospheric conditions in the upper air cannot be known with sufficient accuracy to enable correc tions to be made. The effects of the variations in the muzzle velocity, due to the wear of the guns, to changes in temperature of charges and to more obscure causes, become more pronounced. Range observation, even with the most perfect instruments and with the best trained observers, becomes less accurate as the range increases. It is therefore a matter of great difficulty to make an accurate forecast of the initial gun range and deflection. It has become the practice to obtain the hitting range by a process of "trial and error," using the gun as its own rangefinder with a large bracket system, after obtaining what assistance is possible from the instruments of observation. This can be done as long as the fall of shot can be observed, but becomes impos sible as soon as this condition ceases to exist. Aircraft are now used to assist in the observation of fire at extreme ranges, and it is in this direction that future developments may be expected.