It is well known to tennis players that a rapidly rotating ball in moving through the air will often deviate considerably from the vertical plane. There is no difficulty in so projecting a ball against a vertical wall that after rebounding obliquely it shall come back in the air and strike the same wall again. It is sometimes supposed that this phenomena is to be explained as a sort of frictional rolling of the rotating ball on the air condensed in front of it, but the actual deviation is in the opposite direction to that which this explanation supposes. A ball projected horizontally and rotating about a vertical axis, deviates from the vertical plane, as if it were rolling on the air behind it. The true explanation was given in general terms many years ago by Prof. Magnus, in a paper “On the Deviation of Projectiles,” published in the Memoirs of the Berlin Academy, 1852, and translated in Taylor's Scientific Memoirs, 1853, p. 210. Instead of supposing the ball to move through air which at a sufficient distance remains undisturbed, it is rather more convenient to transfer the motion to the air, so that a uniform stream impinges on a ball whose centre maintains its position in space—a change not affecting the relative motion on which alone the mutual forces can depend. Under these circumstances, if there be no rotation, the action of the stream, whether there be friction or not, can only give rise to a force in the direction of the stream, having no lateral component.