The tendency of the axis of rotation to remain parallel to its original ;lirection is the cause of a peculiar deviation, termed by the French clerivation. It was pointed out by Robins, that when the elevation of the piece is high, and the trajectory greatly curved, the axis of the bullet, or of rotation, does not remain tangential to the trajectory, but very soon begins to make an angle with it, which angle continues to increase to the end of the range (fig. 3); the consequence is, that there is an in equality in the resistance of the air on different sides of the axis ; the bullet or picket presents an increased rotating surface to the condensed air, that is, there is a greater resistance from friction on one side (below) the axis of rotation than on the other ; the effect of this resistance will, for the reason before given, cause the bullet to deflect or derivate. But this resistance is almost wholly below, and of the same nature as that shown, fig. 1, as acting on the anterior hemisphere of the spherical bullet, whilst little or none of the opposing resistance of condensation of air from the meeting of two currents can be called into play (or rather, it will be in a position where it can have little effect) consequently, the bullet or picket, supposing it to have a right-handed rotation from above downwards, viewing it from behind the piece, will have a derivation to the right. The path of the centre of gravity of the shot will be a line of double curvature, but the axis of rotation will remain constantly parallel to its original direction. Robins proposed that the bullets should be formed like eggs, the longer axis being placed in the direction of the axis of the piece, and the larger end to the front, in order that, the centre of gravity being forward, the greater resistance of the air acting behind might throw the point down, and constantly keep the axis tangential to the trajectory. More recently, easelo•cs, grooves cut round the base of a eylindro-conical bullet (see fig. 6), have been proposed by M. Tamiasier, and employed with the same object. But a alight oonsideration will show that the action of this increased resistance behind the centre of gravity is not simply a force tending to produce angular motion in the axis of the bullet in the plane of the trajectory round au axis per pendicular to it, but is of the nature of the action arising from the friction of the rotating bullet or picket, when its axis is in clined to the trajectory, and which we have just been discussing. But it must be remembered, that we were then assuming either that the resistance of the atmosphere was equal on both aides of the centre of gravity, or that any alight inequality there might be was not suffi cient to overcome the effect .of rotation in keeping the axis parallel to its original direction ; whereas now, from the object of the canelures, in order that they may be effective, we must assume that the increased resistance behind the centre of gravity is sufficiently great to give angular motion to the axis of rotation. For the same reason, then, that the bullet or picket would in the former case deflect to the right, supposing it to have a right-handed twist, in this case the portion behind the centre of Gravity would deflect more, proportionately to the greater resistance on it, than the portion in front ; that is, the axis or head of the bullet must turn to the left, making an angle with the plane of the trajectory. In this position the combined forces will tend to depress the head, and so on, if we consider it in its successive positions, with the forces acting on it, we shall see that the effect of the increased resistance behind the centre of gravity will be to make the bullet assume a second motion of rotation—one motion of rotation, the original one, being round the longer axis, while the second will be round an axis making an angle with this, and directed to the 'left of the tangent of the trajectory at every moment ;---though the deflection from this oblique surface would be to the left, and counteracting the deflection it was intended to counteract, yet the wobbling motion the bullet must assume would be very destructive to accuracy and penetration. When from the form of the bullet the resistance is greater in front of the centre of gravity, and sufficient to give angular motion to the axis of rotation, the second axis of rotation will bo directed to the rigid of the tangent of the trajectory, and there will be increased deflection to the right.
These effects may be proved experimentally with the gyroscope. It is, therefore, better, as the derivation when the axis of rotation remains parallel to its original direction is pretty constant for the same dis tances, to allow for it when laying the gun, than to try and correct it by the form of the shot.
Another curious effect arising from the action of rotation, tending to keep the axis of rotation parallel to its original direction, is, that st certain low elevations the range of the elongated projectile is absolutely greater in the atmosphere than it would be in vacuo. This is evidently a point of great importance practically. It is thus explained by Sir W. Armstrong : " In a vacuum, the trajectory would be the same, whether the projectile were elongated or spherical, so long as the angle of elevation and the initial velocity were constant ; but the presence of a resisting atmosphere makes this remarkable difference, that while it greatly shortens the range of the round shot, it actually prolongs that of the elongated projectile, provided the angle of elevation do not exceed a certain limit, which in my experiments I have found to be about tr. This appears at first very paradoxical, but it may be easily explained. The elongated shot, if properly formed, and having a sufficient rotation, retains the same inclination to the horizontal plane throughout its flight, and consequently acquires a continually increasing obliquity to the curve of its flight. Now the effect of this obliquity is, that the projectile is in a measure sustained upon the air, just as a kite is supported by the current of air meeting the inclined surface, and the result is, that its descent is retarded so that it has time to reach to a greater distance." The fullowing example, to prove the truth of this statement, is taken from ' Elementary Lectures on Artillery, It. M. Academy,' by Major Owen, ILA., and Captain Dames, ILA., from which also some of the figures are taken : " The initial velocity of the projectile from Sir W. Armstrong's 12-pr. gun is said to be 1080 feet per second ; what would be its range in vacuo when fired at 2' of elevation If 1100 be taken as the velocity, 680 yards would be the range. From the table of ranges given in the 'Manual of Artillery Exercises,' 050 yards is the range obtained with this gun at an angle of 2' of eleva tion. The range of the projectile in the air exceeds therefore that in vacuo by about 140 yards, if the initial velocity is 1080 feet a second, and by 100 yards should the velocity be 1100 feet a second." A 32-pr. shot fired at an angle of 2' with a velocity of 1600 feet per second, ranges less than I of the distance it would range in vacuo.
The Interior of a rifle barrel is a cylinder, with a certain number of grooves cut in it ; these grooves are parallel to one another, but make an angle with the axis of the cylinder or barrel—that is, they are dis posed spirally round it. The bidlet fitting these, when forced through the barrel by the action of the powder, acquires a motion of rotation round the axis of the piece in addition to the motion of translation, anal this motion it of course retains after leaving the piece. The rifle barrel is in fact a female screw, and the action is the same as that of a nut through which a screw is driven by pressure.
The elements of a rifle may be considered to be a cylinder, grooves, bullet, and method of making the bullet fit the grooves or take the rifling. No rubs; for these various points have at present, at all events, been established ; though perhaps Mr. Whitworth in his experiments has done more than anyone else to establish some of them on a scientific basis. But at present the diameter of the cylinder, that is, the bore ; the number, and depth, and inclination of tho grooves; the form and weight of the bullet, and the method of making it take the rifling—vary with every nation, and indeed with almost every gunmaker. We can therefore merely attempt to give some description of one or two of the most general forms of rifle. Before doing so, however, it may be as well to consider what are the points to be borne in mind, or what are the conditions of the problem of constructing a perfect rifle. This will &saint us in coming to a conclusion as to what rifle combines the before-mentioned element," in the best proportions.