projectile, the change in the range during the time of flight of the projectile and for several other variables, in order to obtain the " gun range " with which fire is opened.
Change-of-Range Calculators. When two ships are moving at high speed on different courses, the range between them is constantly changing at a varying rate. This rate of change of range can amount to upwards of 1,500 yd. per min. under action conditions, and it therefore is of great importance that it should be correctly known. The elements upon which the rate of change depend are the course and speed of the firing ship and the bearing, course and speed of the target. The first two of these are known, the third can be easily observed, but the last two are not known and can only be judged approximately. To obtain the rate at any given moment involves the solution of two triangles, the functions of which have just been mentioned. Several different types of calculators are in use for this purpose and that universally used in the British service is the Dumaresq, invented by Rear-Adml. L. S. Dumaresq. In this instrument the known and unknown elements are set graphically and the resulting rate of change of range, corresponding to the combination of the settings, is read off in " yards per minute," which is what is required. The speed of the enemy must always be guessed in the first place, but instruments known as inclinometers are being experimented with whereby the angle between the course of the target and either the line of fire or the course of .the firing ship can be observed with fair accuracy at any moment.
Range Clocks. Some type of clock which can be made to run at the rate of change of range is in use in all navies. In the British serv- ice the Vickers clock is used. This consists of a powerful clockwork escapement which drives a horizontal circular plate at a constant speed. Above this is a similar plate, the speed and direction of whose motion are controlled by the position of a friction roller mounted between the two plates. This roller is arranged to travel along a common diameter of the two plates. The motion of the upper plate can be varied from zero, when the roller is at the centre, to the maximum in either direction when the roller is at the ends of the radii. The pointer of the clock is geared to the upper plate and the face of the clock is graduated in yards. By altering the position of the roller the speed and direction of the pointer, in yd. per min., can be adjusted at will. Arrangements are made so that large corrections in range can be put on the perimeter of the clock without interfering with the motion of the pointer, so that the clock can always be run at the " gun range " that it is desired to transmit to the guns. This is the simple method of using the clock, but there are many other uses to which it can be applied.
Deflection Calculators. It is a difficult matter to calculate the cor- rect deflection for hitting a target at long range. The deflection depends, first, on the lateral motion given to the projectile by the speed of the firing ship. This is at its maximum when firing on the beam and vanishes when firing right ahead, but it is known and can be allowed for. Next comes the adjustment necessary for the move- ment of the target across the line of fire, between the moment of firing and the arrival of the projectile at the target. The amount of this correction depends on the distance and speed of the target and the angle between its course and the line of fire. This adjustment can be obtained with a fair degree of accuracy by calculation from the data that are used for obtaining the change of range, and a variety of deflection calculators are in use for this purpose. There is, how- ever, a variable component caused by the effect of wind on the pro- jectile. Allowance can be made for the wind at the firing ship, but at long range the wind effect at the target may be entirely different. Also the direction of the wind in the upper air, through which the trajectory of the projectile passes, is an unknown factor. All deflec- tion calculators, therefore, have their limitations, and the usual practice is to calculate the proper setting as far as is possible and then to correct it by observation of the fall of shot.
Bearing Indicators. -These instruments are mounted in the con- trol positions and are sometimes, as in the Evershed type used in the British nayy, arranged to transmit the bearing of the target elec- trically to the guns and to the transmitting station. The instrument consists of a bearing plate, mounted with the zero adjusted to the fore-and-aft line of the ship, and graduated in degrees from o to 180 on each side. On the plate is mounted a telescope or binoculars. The movement of the telescope in azimuth, as it is kept trained on the target, works an electrical transmitter. The bearing of the target with reference to the fore-and-aft line of the firing ship is thus transmitted to the different stations. This forms a ready method of indicating the correct target to the guns, and from the observations the rate at which the bearing is changing can be obtained. This rate of change of bearing is used for calculating the course and speed of the enemy, and is also a measure of the deflection due to the relative movements of the firing ship and the target.
Methods of Firing. Before the range at which heavy guns are used at sea became so extended, it was the practice to use a single gun for ranging before opening fire with the whole broadside. The differ- ences between the shooting of individual guns, due to the wear of the guns, the temperature of the charges and a variety of other causes, become accentuated at long ranges and no two guns can be built that will always shoot precisely the same. This leads to a pattern or spread always resulting when a number of guns of the same size are fired at the same elevation. This " spread of the salvo," as it is
called, can be reduced by making careful adjustments, but it can never be entirely eliminated. It can be made, however, an approxi- mately constant quantity, known to the control officer of each individual ship. To base the corrections for the whole broadside on the result of the fall of the shot from a single gun, which may differ from the remainder, is obviously liable to lead to large errors. Also, at extreme ranges, the splash made by the fall of a single shot is difficult to see, even when using the best glasses. It is now the general practice to range with a salvo of several guns, usually half the broadside, and to continue firing alternate salvos of an equal number of guns. The object of the spotting officer, who knows the approximate spread that his salvos will give, is to apply such cor- rections as will bring the mean point of impact of his salvos on to the target. He then knows that he is obtaining the maximum hitting effect from the armament that he is controlling.
Directors. Practically all navies have now adopted some form of master sight or director, whereby all guns that are loaded and laid at a prearranged elevation and training can be fired by a single gun- layer. Such a system of firing the guns has many obvious advan- tages, chief amongst which are the elimination of smoke interference of one gun with another, the reduction of the personal errors in lay- ing and the fact that it is much easier to correct for the fall of a salvo that falls " all together " instead of being spread out over an irregular time interval. In the British navy the director installation invented by Adml. Sir Percy Scott is used. In this system a director sight is mounted aloft, or in a director tower well separated from the guns themselves. The sight is similar to the gunsight and all the usual settings can be made on it. The sight is carried on a mount- ing which is capable of being moved in elevation in the same manner as a gun mounting, but the efforts required for moving the sight are naturally much smaller than in the case of a mounting carrying a gun. The motion of the director mounting is communicated elec- trically to training and elevation receivers in the turrets or gun positions, thus causing the electrical pointers on the receivers to indicate the position of the director mounting. Mechanical pointers are fitted in the director elevation and training receivers at the guns, which are geared on to the moving parts of the gun and mounting.
The position of the gun in elevation and training is thus also indicated on the director receivers. By working the elevation and training control gears of the gun so that the mechanical pointers are kept in line with the electrical pointers, the guns are made to follow the motions of the director mounting. When the director is being used the range and deflection are set on the director sight and the director telescope is kept laid on the target by the director layer and trainer. The gunlayers and trainers at the guns keep their pointers in line for elevation and training, and the guns are thus laid to correspond with the position of the director sight. The firing cir- cuits of all guns are brought to a single trigger at the director sight, so that all guns that are ready can be fired simultaneously by the director layer. This brief description indicates the principal func- tions of the director, but in actual practice there are many com- plications. The vertical and lateral distances of the director sight from the guns have to be allowed for and compensation has also to be made for any differences in the levelling of the gun mountings and of the director mountings. Arrangements are also necessary to compensate for the errors of the individual guns due to wear and other causes. On the whole the installation is an intricate one but the results that have been obtained have rendered it invaluable. In capital ships there are 2 director sights, which can be used alter- natively for the main armament, one mounted aloft and the other in a position just above the level of the guns. A director sight is also fitted for use with the secondary armament. Light cruisers are fitted with a director for their main armament, and a modified form of director is in use in destroyers.
Sights. The principles upon which the sights of naval guns are constructed have not been altered in recent years, but a great many alterations in the details have naturally been introduced to meet the requirements of the different types of gun mountings that have come into use. Telescopic sights are invariably used for all modern guns, the telescope being so arranged that, when the sight is set at zero, the axis of the telescope is exactly parallel to that of the bore of the gun. The telescope is carried in a carrier on a sight bracket and has two motions controlled by the adjusting mechanism of the sight. The angle between the axis of the telescope and the bore of the gun can be altered in the vertical plane to the angle of elevation for the required range. The range dial of the sight is graduated in yards to correspond to these angles of elevation; and when the telescope is laid on the target, its axis being horizontal, the axis of the gun is laid at the required angle of elevation. Similarly the telescope can be moved in the lateral plane through the number of degrees required for deflection ; and when the telescope is then pointed at the target the gun is laid at the required angle to the right or left. The gradua- tions of the deflection dials are sometimes in degrees and sometimes in knots, to the right or left, the latter being only exactly correct at a particular range.
Allowance is made for the permanent angle of drift of the pro- jectile by fixing the whole sight bracket at a permanent angle to the vertical. The telescopes used are generally of variable power; that is, their magnification can be adjusted from about 3 to perhaps 20 diam., the high power being used for good visibility and the low power for
