under battle conditions, is in the approximate proportions of
1: 1.5: 4, which would thus produce a shell effect (supposing
the hits made by each type of gun to bear a fixed proportion to
the rounds fired), in the proportions of 72: 22: 4, for the 12-in.,
9.2-in. and 6-in. guns respectively. This argument of course
takes no account of the probably greater effect produced by the
dispersion of the larger number of hits of the smaller gun over
the exposed area of the target, nor, on the other hand, does it
take account of the greater armour-piercing power of the 12-in.
shell which would have the result that a larger proportion of the
hits from the smaller gun would be defeated by the enemy's
armour, and so prove innocuous.
The shell effect forms a strong argument for the weight available for the heavy gun armament of a ship being disposed of in the form only of the heaviest gun available. Another strong argument is that deduced from the fact already stated, that, as the calibre of the gun increases, its ballistic powers enable accurate shooting to be made at a longer range.
The accuracy of a gun at any range depends mainly, for practical naval purposes, on what is known as the “dangerous space,” or the limit within which the range must be known in order that a target of a given height may be struck. Again, the dangerous space at any range depends upon the remaining velocity of the projectile at that range, which, as between guns of different calibres but with the same initial muzzle velocity, is greater, the greater the calibre of the gun and weight of projectile, the advantage possessed by the larger gun in this respect being much increased at great ranges. As a practical example, for a target 30 ft. high at a range of 8000 yds., the dangerous spaces of modern 12-in., 9.2-in. and 6-in. guns, which do not differ greatly in muzzle velocity, are 75, 65 and 40 yds. respectively. At whatever range a naval action is to be fought, it is evident that there must be a period during which the enemy is within the practical 12-in. gun range, and outside the practical 6-in. gun range, and that during this period the weight allotted to 6-in. guns will be wasted, and this at the outset of an action, when it is more important than at any time during its progress to inflict damage on the enemy as a means of preventing him from inflicting damage on ourselves. But if all the weight available be allotted to 12-in. guns, the whole of the armament which wfll bear on the enemy will come into action at the same time, and that the earliest, and consequently most advantageous, time possible. This train of argument led to the substitution of 9.2-in. guns in the 8 "King Edward VII." class (the first of which was completed in 1905) for the upper deck 6-in. guns, and eventually in the "Lord Nelson" and "Agamemnon" (completed in 1908) to the abolition of the 6-in. armament, which was replaced by ten 9.2-in. guns.
At the beginning of the present century the subject of " fire control " began to receive considerable attention, and a short statement is necessary of the causes which render essential an accurate and reliable system of controlling the fire of a ship if hits are to be made at long range. In the first place, even with the 12-in. gun, the range must be known with considerable nicety for a ship to be hit. At a target 30 ft. high, at 8000 yds., for example, the range on the sights must be correct within 75 yds. or the shot will faff over or short of the target. No rangefinder has yet proved itself reliable, under service conditions, to such a degree, and even if one were found, it could not be relied upon to do more than place the first shot in fair proximity to the target. The reason for this lies in the distinction which must be drawn between the distance of a target and its " gun range, " or, in other words, the distance to which the sights must be adjusted in order that the target may be hit.
This gun range varies with many conditions, foremost among which are the wear of the gun, the temperature of the cordite, the force and direction of the wind and other atmospheric conditions. It can only be ascertained with certainty by a process of "trial and error," using the gun itself. The error, or distance which a shot falls short of or beyond the target, can be estimated with a greater approach to accuracy the greater the height o-f the observer. It is the process of forming this estimate
which is termed "spotting," a duty the performance of which calls for the exercise of the most accurate judgment on the part of the "spotter," and which requires much practice in order that efficiency may be secured. In practice, the first shot is fired with the sights adjusted for the distance of the target given by the range-finder, corrected as far as is practicable for the various conditions affecting the gun range. The first shot is spotted, and the result of the spotting observations governs the adjustment of the sights for the next shot, which is spotted in its turn, and the sights are readjusted until the target is hit. From this time onwards it is (in theory) only necessary to apply the change in range, due to the movements of our own ship and of the enemy, for the interval between successive shots, in order to continue hitting. This change of range, which may be considerable (e.g. 1000 yds. per minute in the extreme case of ships approaching each other directly, and each steaming at the rate of 15 knots), is in practice extremely difficult to estimate correctly, and the spotting is consequently continued in order to rectify errors in estimating the rate of change in range. For various reasons the "gun range" which has been referred to is not the same for different natures of guns. This is mainly on account of the difference in the height attained by their projectiles in the course of their respective trajectories. While it is possible, by careful calibration (i.e. the firing from the several guns of carefully aimed rounds at a fixed target with known range and under favourable conditions for practice), to make the shots from all guns of the same nature fall in very close proximity to each other when the sights of all are similarly adjusted, it has not been found possible in practice to achieve this result with guns of different natures. Consequently guns of each nature must be spotted for independently, and it is obvious that this adds considerably to the elaboration and complication of the fire control system.
This constitutes one of the reasons for the adoption of the uniform armament in the “Dreadnought” and her successors; another important reason lies in the fact that with the weight available for the heavy gun armament disposed of in a small number of very large guns, a greater proportion of these guns can be mounted on the midship line, and consequently be available for fire on either side of the ship (see fig. 90). Thus in the "Dreadnought," eight of her ten 12-in. guns can bear through a considerable arc on either beam, while in the " Lord Nelson, " although all her four 12-in. guns can bear on either beam, half at least of her 9.2-in. armament (i.e. that half on the opposite side to the enemy) will be at any moment out of bearing, and consequently be for the time a useless weight. The same principle of a uniform armament of 12-in. guns has been adopted in the "Invincible" type, the only large cruisers designed since the inception of the "Dreadnought." Thus the 12-in. gun forms the sole heavy gun armament of all battleships and large cruisers of the " Dreadnought " era. The gun so carried is known as the Mark X., it is 45 calibres in length, and fires a projectile weighing 850 ℔ with a charge of cordite of 260 ℔, resulting in a muzzle velocity of 2700 ft. per second. The Mark XI. gun was designed to be mounted in the later " Dreadnoughts." Following the same line of development as resulted in the Mark X. gun, it is longer, heavier, fires an increased charge of cordite, and has a higher muzzle velocity, viz. of 2960 ft. per second. This gun appears to mark the climax of development along the present lines, since the price to be paid in greater weight, length and diminished durability of rifling is out of all proportion to the small increase in muzzle velocity. Further developments would therefore be looked for in some other direction, such as the adoption either of a new form of propellant or of a gun of larger calibre. A modern gun of lo-in. calibre is found in the battleships "Triumph" and "Swiftsure." The next gun in importance to the 12-in. is the 9.2-in., which forms part of the armament of the "Lord Nelson" and "King Edward VII." classes of battleships, and the principal armament of all armoured cruisers (excepting the “County” class) antecedent to the " Invincibles." The latest gun of this calibre has developed from earlier types in a similar manner to the 12-in., that is to say, it has experienced a gradual increase in length, weight, and weight of charge, with
