Lastly, it may be broadly stated, that with suitable powders, a charge of one-third the weight of the shot demands for most profitable use a length of bore equal to about twenty-six calibers; a charge equal to half the weight of the shot should be accommodated with a bore of about thirty calibers; while a charge of two-thirds the weight of the shot will be best suited by a bore thirty-five calibers long. Of course, in each case, greater length of bore will give increased velocity, but it will be gained at the expense of additional weight, which can be better utilized elsewhere in the gun.
The amount of work performed by gunpowder, when exploded in a gun, is a subject which has engaged a vast quantity of attention, and some highly ingenious methods of calculating it have been put forward. Owing, however, to the impossibility of ascertaining how fast the combustion of large grains and prisms proceeds, a very considerable amount of experience is required to enable the gunmaker to apply the necessary corrections to these calculations; but, on the whole, it may be said that, with a given charge and weight of shot, the muzzle velocity may now be predicted with some accuracy.
You now have the chief data on which the designer bases his proposals, and lays down the dimensions of the gun to suit such conditions as it may be required to fulfill. In actual practice, the conditions are almost always complicated, either by necessities of mounting in particular places, such as turrets and casemates; or by the advantages attending the interchangeability of stores, or other circumstances; and it requires great watchfulness to keep abreast of the ever-growing improvements of the day.
I will now conclude with a few words on the power of heavy guns, when employed in various ways. The first consideration is accuracy of fire. No matter how deadly the projectile may be, it is useless if it does but waste itself on air. Accuracy is of two kinds--true direction and precision of range. All modern guns are capable of being made to shoot straight; but their precision of range depends partly on the successful designing of the gun and ammunition, so as to give uniform velocities, and partly on the flatness of the trajectory. The greater the velocity, the lower the trajectory, and the greater the chance of striking the target. Supposing a heavy gun to be mounted as in the fortresses round our coasts, and aimed with due care, the distance of the object being approximately known, we may fairly expect to strike a target of the size of an ordinary door about every other shot, at a range of a mile and a half. Here we have carriages mounted on accurately leveled platforms; we have men working electric position finders, and the gunners live on the spot, and know the look of the sea and land round about.
Now, consider the case of guns mounted in ships. You at once perceive the difficulties of the shooter. Even supposing the ship to be one of our magnificent ironclads, solid, steady, yielding little to the motion of the water, yet she is under steam, the aim of her guns is altered every moment, some oscillation is unavoidable, and she can only estimate the range of her adversary. Great skill is required, and not only required, I am glad to say, but ready to hand, on the part of the seamen gunners; and low trajectory guns must be provided to aid their skill.
If we go to unarmored ships of great tonnage and speed, we shall find these difficulties intensified; and if we pass on to the little gunboats, advocated in some quarters for attacking ironclads in a swarm, we shall find that unsteadiness of platform in a sea-way renders them a helpless and harmless mark for the comparatively accurate practice of their solitary but stately foe.
The destructive power of guns is little known to the general public, and many wild statements are sometimes put forward. Guns and plates have fought their battle with varying success for many years. One day the plate resists, another day the gun drives its bolt through. But it is frequently overlooked that the victory of a plate is a complete victory. If the shot does not get through, it does practically nothing. On the other hand, the victory of the gun is but a partial triumph; it is confined to a small arc. I mean that, when the plate is struck at an angle exceeding 30° or so, the shot glances harmlessly off; while, even when perforation is obtained, it is at the expense of the more deadly qualities of the projectile, which must be a nearly solid bolt, unable to carry in with it heavy bursting charges of powder or destructive masses of balls.
About six years ago, an experiment carried out at Shoeburyness taught a lesson which seems to be in danger of being forgotten. We hear sometimes that unarmored vessels are a match for ironclads and forts; and I will conclude this paper with a short extract from the official account of the results of firing shrapnel shell at an unprotected ship's side. I shall say nothing of boilers and magazines, but shall state simply the damage to guns and gunners.
A target was built representing the side of a certain class of unarmored ships of war; behind this target, as on a deck, were placed some unserviceable guns, mounted on old carriages, and surrounded by wooden dummies, to represent the men working the guns. The attacking gun was a twelve-ton nine-inch muzzle-loader, of the old despised type, and the projectiles were shrapnel shell. The charges were reduced to represent the striking force at a range of 500 yards. Two rounds did the following damage inside, besides tearing and ripping the ship's side in all directions.
1st Gun.--Seven men of detachment killed.