“It is owing to the centrifugal force, which gives it a motion in the direction of a tangent to the circle, and, consequently, overcomes the force of gravity.”

“Your answer is pat,” replied his father: “as long as you give your hoop a certain degree of velocity, the tangential, or centrifugal force, overcomes gravity, in the manner you have already witnessed;[[20]] but, when that is slackened, the hoop will fall on its side; not, however, until it has made several complete revolutions. Now, answer me another question. Why is it so difficult to make the hoop proceed straight forward, without turning to the right or left?”

“I suppose it arises from the same cause as that which altered the direction of my marble as it ran along--the inequality of the ground.”

“That,” replied his father, “would undoubtedly have its influence; but it is principally to be referred to the impossibility of your constantly giving a straight blow by the stick. When it is moving forward, a slight inclination towards either side will cause the parts to acquire a motion towards that side, those which are uppermost being most affected by it; and this lateral, or sideway motion, assisted sometimes by the irregular curvature of the hoop, causes its path to deviate from a rectilinear direction; so that, instead of moving straight forward, it turns to that side towards which it began to incline; and, in this position, its tendency to fall is still farther counteracted by the centrifugal force. It is from a similar cause that the bullet, unless rifled, will have a tendency to go to the right or left, from any unequal impulse which it may have received at the moment of its exit from the barrel. I have yet one other question, and, as its answer will lead us into the consideration of a mechanical subject of some importance, I must beg you to bestow all your attention. In trundling your hoop, have you not often observed that, although the blow inflicted upon it by your stick might have been violent, yet the effect produced by it was comparatively small, in consequence of the hoop having been struck by a disadvantageous part of the stick?”

“Certainly! I have frequently observed that, if the hoop is struck by the stick either too near the hand, or too near the end, much of its force is lost; and I have also noticed the same thing in striking the ball with my cricket-bat.”

“The fact is,” said Mr. Seymour, “that every striking body has what is termed its centre of percussion, in which all the percutient force of a body is, as it were, collected; thus, a stick of a cylindrical figure, supposing the centre of motion at the hand, will strike the greatest blow at a point about two-thirds of its length from the wrist. I may, perhaps, at some future time, return to this subject, and explain several mechanical effects which are dependent upon it.[(21)] Now away with you, and trundle your hoop, or spin your top; as soon as the vicar arrives I will rejoin you.”

In the course of an hour Mr. Seymour and his reverend friend proceeded to the play-ground, where they found the children busily engaged in their several diversions.

“I rejoice to find you at so classical a pastime,” said the vicar, as he approached Tom, who was busily engaged in spinning his top. “The top, my boy, is a subject which the great Mantuan bard did not consider beneath the patronage of his muse: but, hey-day! this is not the ‘volitans sub verbere turbo’ of the immortal Virgil; the top of antiquity was the whip-top, the peg-top is a barbarous innovation of modern times: a practical proof of the degeneracy of the race. Even boys, forsooth, must now-a-days have their activity cramped by inventions to supersede labour: well may we regard the weapons, which our sturdy ancestors wielded as instruments rather calculated for giants than men, if such pains be taken to instil into the minds of youth the mischievous spirit of idleness.”

“My dear sir,” said Tom, who was always grieved at displeasing the vicar, “if it will gratify you, I will spin my whip-top, for I have an excellent one which my papa has lately given me.”

“Well said! my dear boy. ‘Puer bonæ spei.’--What a pity would it be to damp so noble a spirit; get your whip-top.”