RECOLLECTIONS AND REFLECTIONS
sponds to the bowler at cricket, throws, not bowls, the ball full pitch at the batsman, and by putting spin on the ball can make it swerve in the air either to the right or to the left, or up or down. This interests the physicist as well as the sportsman, for it is a striking illustration of the hydro-dynamical principle that a ball moving through air always tends to follow its nose, the nose being the point on the ball which is in front. Thus if a ball is moving horizontally in the direction of the arrow, the foremost point on the ball will be N, the point right in front of the centre C. If the ball is not spinning, this point, like every other part
of the ball, is moving horizontally, and the ball, following its nose, will do so too. But suppose the ball is spinning, as in the figure, about an axis at right angles to the plane of the paper, N, in consequence of the spin, will be moving downwards (this spin is .what is called top spin in lawn tennis) and the ball will tend to follow it and will therefore dip ; if the spin were in the opposite direction, the nose would be moving upwards so that the ball would tend to soar. If the axis of spin were vertical instead of horizontal, the nose N would have a sideways motion and the ball would swerve either to right or left. If the axis of the spin were not truly vertical or horizontal, the ball would both swerve and dip or rise.
We saw Baltimore under exceptionally favourable circumstances, for our host was a professor in Johns Hopkins University, and both he and his wife belonged to old 166