able interest, and determined Mr. Seymour to call at Ivy Cottage, and invite its inmate to the Lodge.

Fig. 33.

THE SUBJECT OF ROTATORY MOTION CONTINUED.-A BALL, BY HAVING A PECULIAR SPINNING MOTION IMPARTED TO IT, MAY BE MADE TO STOP SHORT, OR TO RETROGRADE, THOUGH IT MEETS NOT WITH ANY APPARENT OBSTACLE. THE RECTILINEAR PATH OF A SPHERICAL BODY INFLUENCED BY ITS ROTATORY MOTION. BILBOQUET, OR CUP AND BALL. THE JOINT FORCES WHICH ENABLE THE BALANCER TO THROW UP AND CATCH HIS BALLS ON THE FULL GALLOP. THE HOOP. - THE CENTRE OF PERCUSSION. THE WHIP AND PEG-TOP. HISTORICAL NOTICES. THE POWER BY WHICH THE TOP IS ENABLED TO SUSTAIN ITS VERTICAL POSITION DURING THE ACT OF SPINNING. THE SLEEPING OF THE TOP EXPLAINED. -THE FORCE WHICH ENABLES IT TO RISE FROM AN OBLIQUE INTO A VERTICAL POSITION. -ITS GYRATION.

"Toм, do you remember that I told you a few days ago," said Mr. Seymour, "that, by giving a revolving body a peculiar spinning motion, certain effects were produced, which I should, on some future occasion, take into consideration?" "To be sure I do," replied Tom.

Well, then, attend to me."

Mr. Seymour took a marble, and placing it on the ground, gave it an impulse forward by pressing his fore-finger upon it the marble darted forward a few paces, after which it rolled back again.

"That is most extraordinary!" cried Tom; "the marble came back to your hand, as it were, of its own accord, and without having met with any obstacle."

"And you, no doubt," said Mr. Seymour, "regard it as contrary to the well-known law, that a body once put in motion, in any direction, will continue to move in that direction until some foreign cause oppose it."

"It really would appear so."

"It is, however, far otherwise; the force which I imparted to the marble communicated to it two kinds of motion; the one projecting it forward, the other producing a rotatory motion round its axis, in a direction opposite to that of its rectilinear course; and the consequence was simply this, that when the former motion, on account of the friction of the marble on the ground, was destroyed, the rotatory motion continued, and by thus establishing an action in an opposite direction, caused the marble to retrograde. If, however, you will fetch your hoop, I will demonstrate the fact on a larger

scale."

Tom accordingly produced the hoop; and Mr. Seymour projected it forward, giving to it, at the same instant, a spinning motion in an opposite direction. The hoop proceeded forward to a certain distance, when it stopped, and then ran

back to the hand.

"Let me beg you," said Mr. Seymour, "to treasure this fact in 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 (19) 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 shall also present you with a new missile which has lately found its way into the toyshops, termed the BOMMERENG. It is used by the natives of Australia, and has the curious property, when skilfully directed, after striking

the desired object, to return to the thrower. (20) 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.

Fig. 35.

2

Having tied some string to a common curtain ring, as represented by Fig. 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."

"I do not recollect the passage," answered her father, "but I will admit that the centrifugal force is indirectly instrumental to the effect, although, in my view of the subject, it is more philosophical to refer it at once to the creation of an appropriate axis of rotation."

"I well remember," observed Tom, "that the rider at Astley's whirled round the oranges as he threw them into the air."

"And I hope that your are now not only acquainted with the principle which rendered such a rotatory motion necessary, but that which must make the shorter the more eligible axis for effecting his purpose; -but can you tell me how it could have happened, that the oranges, which were