of powder if the gun were made long enough to contain the whole of the powder gases, so that the forward pressure on the base of the projectile would cease just as the shell reached the muzzle. Such a gun would, however, be unwieldy, and in practice we cut the gun short and allow a good deal of the gas pressure to go to waste out of the muzzle. It is readily seen with how much greater force the missile would be thrown if the waste pressure could be brought to bear upon it in addition to that which is actually employed.

Length of Gun.-The length of a gun is expressed by the number of calibers in its total length. Modern rapid-fire field guns are from 27 to 35 calibers long. A caliber is the diameter of the bore measured between opposite ribs of the rifling. Generally speaking, the bore is measured from the face of the breech-block to the muzzle.

Chamber of Gun.-The bore of a gun is divided, for the purpose of internal ballistics only, into two parts, the chamber and the bore proper. The powder does not completely fill the chamber, nor is it a solid mass, but it is in granulated form. If the powder charge in a 3-inch shell were compressed into a solid block, it would be found to fill a small portion of the chamber, possibly a third. On ignition the powder gases first fill the chamber. The higher the velocity of ignition, that is, the more rapidly the gas is evolved from the powder, the sooner will the pressure of the chamber overcome the resistance of the projectile, causing it to move up the bore. This, in a field gun, occurs when the pressure rises to about 11⁄2 tons to the square inch. Henceforward the pressure of the gas acts as an accelerating force upon the projectile until the latter leaves the muzzle.

Effects of Powder on Design of Gun.-In the design of a gun, the caliber, weight of projectile, and muzzle velocity being fixed, consideration must be given to the powder in order that the size of chamber, length of gun, and thickness of walls throughout the length may be determined. To

produce a given velocity in a gun a larger charge of powder that is slow for the gun is required than is of a powder that is quicker. The larger charge requires a larger chamber space, and thus increases the diameter of the gun over the chamber. The maximum pressure being less than with the quicker powder, the walls of the chamber may contain less metal. The slow powder will give higher pressure along the chase, therefore the walls of the gun must here be thicker, the weight of the gun being increased throughout its length.

If we do not wish to increase the diameter of the chamber we must, for slow powder, lengthen the gun in order to get the desired velocity. On the other hand, with a powder that is too quick for the gun very high and dangerous pressures are encountered, requiring excessive thickness of the walls over the powder chamber. The gun in this case may be thinner walled along the chase.

It is evident from the foregoing considerations that each gun must be designed with a particular powder in view, and that a gun so designed and constructed will not be as efficient with any other powder.

Now let us follow the evolution of gunpowder and the consequent changes in the design of the guns.

Forty years ago the only explosive used in guns was coarse black powder. The whole of the charge was converted into gas almost immediately upon ignition, thus developing a very high pressure in the powder-chamber upon ignition, which rapidly fell as the shell moved up the bore. Guns of this period were, therefore, made of a very pronounced bot le shape, enormously thick at the breech. The old-time fieldpiece was, of course, lighter in metal, because the powde charge, and therefore the pressure, was comparatively small; but the general shape was much the same as that of the guns used in permanent works and water-batteries. As an improvement on the old form of black powder, pebble powder, in the form of cubical grains of from one-half inch to one and one

half inches, was introduced. It was found that the cubes burned more slowly than the grains, and, since the gas was evolved more slowly, a smaller initial pressure resulted, and a better maintained pressure as the projectile moved up the bore. Guns were then made thinner at the breech and thicker at the muzzle.

Prismatic powder, pressed into large six-sided prisms, was the next step; this was followed by slow-burning brown powder, known as cocoa powder. Now we have smokeless powder, in thick cords, tubes or tapes, for long guns, and

fine strings for short ones. This has enabled us to adjust the pressures in the bore so as to get the maximum of work out of the gun with the minimum of metal. It is to be observed that to-day there is slight difference in the metal at the breech and at the muzzle.

A simple and convenient means of showing graphically the pressure in any gun is by the use of pressure curves.

Figure 1 gives the curve for a certain 12-inch gun of old type when black powder was used.

Here the height of the curve represents the pressure in tons at that particular point in the bore. We note how the pressure rises from zero to 24 tons per square inch before the shell begins to move, and runs up to 25 tons before the shell has traveled half of its length. The pressure then rapidly falls, as the stopper, so to speak, is drawn out, till at the muzzle it is only 3 tons per square inch.

As a contrast to that of the old 12-inch gun, the curve

for a modern 6-inch gun is shown. The pressure nowhere exceeds 15 tons per square inch and diminishes gradually toward the muzzle. We may also note how in both cases the pressure curve corresponds in a general way to the profile of the guns. This is, as has been shown, because the gun was cast or constructed to withstand the pressure to which it would be subjected, its shape not being merely accidental.

CHAPTER III

EXTERIOR BALLISTICS

Exterior ballistics treats of the motion of a projectile after it has left the piece.

We have already seen that the trajectory is the course of the projectile during its flight. To be mathematically exact, it is the curve GMT, described by the center of gravity of the projectile during its passage through the air.

Every trajectory is theoretically an analytical curve. That is, it is such a curve that it may be analyzed with respect to its ordinates. In other words, the horizontal distance of any point on the curve bears a definite and fixed relation to the vertical distance of that point, from another given point.

M
y

FIG. 1.

Thus in Figure 1, the horizontal ordinate x, of the point M, bears a definite relation to the vertical ordinate y, and given certain other factors, such as the resistance of the air, the velocity of the projectile, etc., etc., knowing the value of either ordinate, the other may be determined.