GUNNERY 325 ion Ar; and the result is the deflection of le path of the shot to the right. Hence the feet of the last ballot in the case supposed is, irst, to throw the shot to the left, while the lequal pressure of the air gradually deflects it 3k again to the right. If the final ballot ,"' i Fio. 1. , r ere on the left side, the deflections would be sversed ; if upon the top, the range would be lightly increased ; and if upon the bottom, the ige would be diminished. These eifects were ivestigated, and the results demonstrated ex- jrimentally, by Magnus. They are much ag- ivated when, by reason of irregular density, centre of gravity of a ball does not coincide rith its centre of figure. They are greater in lall than in large projectiles for three reasons : L The actual amount of windage is very nearly same for all calibres, and hence is relatively for larger calibres than for small ones; lerefore the balloting and consequent rotation nil be less. 2. Large projectiles can be made lore nearly isotropic than small ones, and the Bntres of figure and of gravity are more nearly )incident. 3. The effects of resistance of air very nearly proportional to the surface ex- sed, i. e., to the square of the calibre; while le inertia of the shot and its consequent power > resist these eifects is proportional to its mass, ., to the cube of the calibre. No projectiles ive less lateral deviation than the largest >und shot, whether the range be long or short; it the deviations of small spherical shot are )toriously great. In using elongated projec- les, the purpose is to reduce the total resis- ice encountered in passing through the air id through the target. This is attained by lucing the area of resistance, while the mass not reduced. Less velocity is lost by them consequence of the smaller front they offer atmospheric resistance, as compared with )herical shot of equal weight. After reaching le target they are required, in order to pene- ite it, to make smaller holes than spherical ojectiles of equal weight, and hence, with an mal striking velocity, will penetrate further. Co secure these advantages, the elongated shot FIG. 2. lust always move with its axis as nearly as ossible tangent to its path. But there are 3veral causes which tend to make it rotate ibout its shortest axis, or tumble. To prevent this, and to give stability to the position of the long axis, a rotary motion about this axis is given to the projectile. This motion is totally distinct from the rotation of spherical projec- tiles just described, and the resulting effect of resistance of air is altogether peculiar. By reference to fig. 2 it will be seen that if the axis of the projectile were always parallel to its initial position, the curvature of the path | would cause the resistance of air to act more and more upon the lower side, while the air upon the upper side would be rarefied in the wake of the projectile. The rotation upon the .condensed air beneath causes it to roll to the right or left, according to the original direction of rotation ; to which deflection the name drift is given. But in reality the axis does not con- tinue parallel to its initial position. It describes very slowly a conical surface, the apex of which is the centre of gravity of the shot ; and what is most singular, the direction of this axial motion in pointed projectiles is opposite to that of flat-fronted projectiles. .The conical rota- tion (or precession) of the axis causes an in- creased drift, the amount of which is even greater than the rolling drift already described. With pointed shot this deviation is to the right, but with flat-fronted shot to the left. The point of the former also droops, turning ob- liquely downward and to the right; the flat front turns obliquely upward and to the left. During the flight the former is more nearly tangent to the path than the latter. For uni- form projectiles, the drift at moderate ranges is tolerably constant, and may be allowed for in sighting ; but for round shot it is hopeless- ly irregular, sometimes to the right and some- times to the left. At long ranges the drift of the elongated shot also becomes irregular, and often excessive, amounting sometimes to 200 or 300 yards to the right of the object sighted. There are also vertical deviations, causing over- or under-shooting. In many cases these errors are more serious than lateral drift ; for in- stance, against a battalion of troops, the hull of a vessel, the crest of a parapet, or the body of a deer, where the object is more extended laterally than vertically, and is more liable to be missed by vertical than by lateral error. There is another kind of error which may be called longitudinal deviation, or variation in range. A series of projectiles fired under conditions as nearly alike as practicable will differ in range ; partly because no two charges of powder can be made to give exactly the same initial velo- city, and partly because slight differences in the forms of the projectiles occasion marked differences in the amount of vertical drift. Hence the form of the trajectory is of great importance. To avoid vertical errors as much as practicable, it is desirable to give a high velocity to the shot ; since the swifter its mo- tion, the less curvature will gravitation pro- duce in its path. It is evident that a low or flat trajectory is more dangerous to an enemy than a high one; but the former requires a higher velocity in the projectile than the lat- ter. The trajectories of spherical shot are at
