AND ON A PH/EXOMENON OF ROTATING BODIES. 21 7 



moves against that in the approaching current, and thus a lateral 

 motion of the same ensues, repelling the vane placed on this 

 side. When the velocities of both masses of air have a certain 

 ratio to each other, this repulsion attains a maximum, and 

 then its direction is perpendicular to that of the currents. 

 Hence, as above mentioned, the vanes were pressed most in 

 a lateral direction when the velocity of the air generated by 

 rotation had a certain definite ratio to that of the approaching 

 current. 



The application of these experiments to the deviation of sphe- 

 rical projectiles is self-evident. 



If we suppose that a sphere, during its progressive motion, 

 rotates so that the axis of rotation is situated always in the 

 principal normal of the trajectory, and if we call the motion of 

 such a sphere a right rotatory one when the front part of it 

 turns from left to right, in reference to an observer behind the 

 gun, and, on the contrary, a left rotatory motion when the front 

 part of the sphere turns from right to left, then it follows im- 

 mediately, from the described experiments, that a decrease of 

 atmospheric pressure takes place against the right side of a right 

 rotating sphere, and an increase on its left side ; hence such 

 a projectile deviates towards the right. Similarly, a left rota- 

 ting sphere deviates to the left. 



When the axis of rotation is perpendicular to the plane of 

 the trajectory, or if this be a curve of double curvature, per- 

 pendicular to its osculating plane, then, although no lateral de- 

 viation occurs, the pressures on the upper and lower hemispheres 

 are different. If the upper one move in the direction of the 

 progressive motion, the pressure from above on the sphere is 

 greater than that from below ; hence the projectile descends, and 

 its range is less than it would be if upper and under pressures 

 were equal. If the lower hemisphere rotates in the direction 

 of the progressive motion, the pressure from below on the pro- 

 jectile is greater, and it ascends, making its range a longer one. 

 In one position only, that is, when the axis of rotation is tan- 

 gential to the trajectory, no deviation is caused by the sphere's 

 rotation. In every other position of the axis, either a lateral or 

 longitudinal, or some combination of both deviations must ensue. 



This explanation of the deviation of spherical projectiles, suf^ 

 SCI EN. MEM.— Nat. Phil. Vol. I. Part III- R 



