“Let me beg you,” said Mr. Seymour, “to treasure this fact in your memory; you perceive by it how greatly the progressive direction of a body may be influenced by a rotatory motion around its axis; and, indeed, the theory of the rifle gun[(20)] is easily deduced from it. It will also explain the effect which a rotatory motion produces in steadying or disturbing the direction of a projectile. It is for such a reason that the balancer constantly whirls round his balls or oranges, as he throws them into the air, with the intention of catching them again; and that in playing at Bilboquet, or cup and ball, you find it necessary to give a spinning motion to the ball, in order to catch it on the spike--but we will consider this subject presently. I am now desirous of laying down a few propositions upon the subject of rotation, the knowledge of which is essential for the explanation of the motions of revolving bodies.”
Mr. Seymour proceeded to state that every body had three principal axes upon which it might revolve, but that the shortest was the only one upon which it could permanently and steadily rotate--that should it, in consequence of the impulse given to it, begin to spin upon any other than the shortest axis, it would, during its revolutions, be constantly showing a tendency to approach it; whence it followed that, under such circumstances, it would be unsteady and wabbling in its motions.
In order, however, to make this proposition intelligible to the children, Mr. Seymour performed the following simple experiment.
Having tied some string to a common curtain ring, as represented by figure 1, he twisted it round by means of his thumb and finger, until it acquired considerable velocity, when the ring was seen to rise gradually into the position represented by fig. 2. Thus, in the simplest manner, was a revolving body shown to exchange its longer for its shorter axis.
The children declared that they perfectly comprehended the subject, and Tom observed that, in future, whenever he wished to make a ball spin steadily, he should take care to make it turn on its shortest axis.
“You are quite right, Tom,” said Mr. Seymour; “and the skilful bowler at cricket, in order to give his ball a steady axis of rotation, always holds it with the seam across, so that the tips of his fingers may touch, and he takes care to hold it only with such a grasp as may be sufficient to steady it, for by a turn even of the wrist it may be made to proceed unsteadily; and this leads me to consider another equally important proposition--viz. that the axis of rotation should coincide with the direction in which it is moving forward, or, in other words, with its line of motion. Now, where this is not the case, it is evident that the unequal action of the air will cause the body to deviate from its straight course, since its two sides, having different velocities (the rotatory and progressive motions conspiring on one side, while they are in opposition on the other), will be differently affected by such resistance; the resistance, of course, increasing with the velocity. It is upon this principle,” continued Mr. Seymour, “that Sir Isaac Newton has explained the irregular motion of the tennis-ball.”
“But do explain to us, papa,” said Louisa, “why it is so necessary to spin the ball in order to catch it on the spike?”
“Rotatory motion, my dear, when directed according to the principles I have endeavoured to enforce, will always steady the course of a body. In playing at bilboquet, your object is so to throw up the ball that its hole may descend perpendicularly upon the spike which is held for its reception; and in order to accomplish this, you make the ball spin upon an axis, at the extremity of which is the hole; the consequence is obvious.”
Louisa observed, that she well remembered an allusion to this game in Miss Edgeworth’s Essays on Education; and that, unless she was much deceived, the advantage to be gained by spinning the ball was referred to centrifugal force, and its effect in preserving the “parallelism of motion.”