212 MAGNUS ON THE DEVIATION OF PROJECTILES, 



provided only that in both cases the relative velocity is the 

 same. This granted, the phaenomena which take place during 

 the motion of a projectile through the air, can be observed by 

 allowing the projectile to remain stationary, and directing a cur- 

 rent of air against it, with a velocity equal to that which the 

 projectile would have had ; in this manner it is possible to ob- 

 serve the relations of pressure on a rotating as well as on a non- 

 rotating projectile. 



If we consider, first, what takes place when air is made to 

 move against a stationary sphere which does not rotate, it is evi- 

 dent that around the diameter parallel to the direction of the 

 current the motion of the air is everywhere the same. It is 

 otherwise, however, when the sphere rotates. 



For if a body, of as perfect a spherical form as possible, be 

 made to rotate around a fixed axis in calm air, a rotatory motion 

 will be also imparted to the latter, which is particularly strong 

 when the sphere rotates excentrically, but is also very plainly 

 observable when the centre is situated in the axis of rotation. 

 This motion of the air is the same around the axis of rotation ; 

 so that if we consider the surface enveloped by the rotating 

 sphere, whether its centre be in the axis of rotation or not, the 

 force with which the air is moved is the same at all points of 

 the same parallel. 



We pass next to the consideration of the combined actions of 

 rotation and translation, the axis of rotation being perpendicular 

 to the direction of the current of air. On one side the sphere, 

 the motion of air generated by rotation has the same direction 

 as the translatory motion ; on the opposite hemisphere, however, 

 the two currents have opposite -directions. 



Hence on the side where both currents have a like direction, 

 the air moves with a greater velocity than on the opposite one. 

 Now, as by this motion of the air a pressure occurs directed 

 normally against the spherical surface, and as this pressure 

 increases when the force with which the air moves increases, 

 one might imagine that the side of the projectile rotating in 

 the same direction as that of the resistance of the air, wouU 

 suffer a stronger pressure towards the centre than the opposite 

 one, and consequently that a deviation of the projectile towards 

 this opposite side must ensue. Experience however proves 



