294 Royal Institution. 



coincides with the tangent to its curved path, momentarily changes 

 its* direction, so that the front extremity or apex falls below its former 

 position. Or, for a single instant it may be regarded as if locally at 

 rest, but turning about its centre of gravity so as to depress the apex. 



If the motion were simply in the direction of the axis, the resist- 

 ance of the air would operate directly against it ; but when the apex 

 is continually tending to turn downwards from that line, the resist- 

 ance acts against it partially upivards, and thus tends to raise the apex. 



Thus, at a given instant, the elongated pi'ojectile may be repre- 

 sented by the rotating pait of the apparatus just described. 



When there is no rotation, the resistance of the air tending to 

 raise the apex is represented by the weight at the lower end, which 

 produces the same effect. 



When a rapid rotation is communicated (suppose from left to right 

 of the gunner), the result will be no elevation of the apex, but a 

 lateral movement, or commencement of a rotation round the vertical 

 — in astronomical language retrograde, if the former rotation be 

 direct ; but which, beginning from the opposite part of the circle, is, 

 relative to the operator, towards the right. 



The form of the projectile used in these experiments diifers from 

 that in the Minie rifle, in that the latter is hollow at its broader end, 

 and thus the centre of gravity is thrown forward towards the apex. 

 Hence, according to the same theory, the effect would probably here 

 be to depress the apex, and therefore to give an opposite deviation : 

 but it does not appear whether any such observations have been 

 made ; and in practice the effect would probably be quite insensible. 



It occurred to the author that a very simple illustration of this 

 deviation of rifle projectiles might be made by merely forming a sort 

 of small arrow, whose head was composed of a cork, like a shuttle- 

 cock, but instead of the feathers, small card vanes inclined in the 

 same direction round it, with a tail to balance it, and which thus in 

 the mere act of throwing acquires a rotatory motion from the reac- 

 tion of the air, to the right or left according as the vanes are in- 

 clined ; and on trying this, there was always observed a deviation 

 in the direction of the axis or point of the missile to the right or left 

 accordingly, relative to the experimenter. It is, in fact, nearly im- 

 possible to throw such a body in a direction perfectly in one plane. 

 The true deviation is, however, peculiarly liable to be disguised by 

 the general resistance of the air on so light a missile, as well as by 

 currents, &c. which it is not easy to guard against. 



The well-known case of the Boomerang exhibits effects closely 

 similar ; for it is found that if so projected that its rotation is from 

 left to right, its deviation will be in the same direction, and vice versd ; 

 that is, supposing (as is the usual case) that its plane is inclined 

 upwards from the operator : — if it be inclined downwards, the devia- 

 tion is in the direction opposite to that of the rotation. 



In the former case the reaction of the air against the flat surface 

 of the missile would tend to increase its inclination upwards, in the 

 latter downwards, with respect to the operator ; and this in eacH 

 case respectively would give the motion stated ; as is easily seen on 

 the principle, and by means of the apparatus, before described. 



