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 you 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 thrown perpendicularly upwards while the horseman was on the full gallop, should have fallen again into his hand?” “Ay,” said Louisa, “that puzzled me exceedingly; I should have thought he would have ridden away from them, and that they must have fallen several feet behind him.” “What say you, Tom, to that?” enquired Mr. Seymour. “I suppose,” replied Tom, “that the rider calculated upon the distance he would pass forward before they could fall, and projected them accordingly.” “No, indeed; there is no calculation in the case, nor is any art used to throw the oranges in advance: they are projected perpendicularly from the hand; and if you will only recall to your mind the subject of the ‘Composition of Forces,’ the mystery will vanish.” “I see it all clearly,” cried Tom; “the orange partakes of the progressive motion of the rider; when, therefore, he throws it upwards, it is influenced by two forces which are in the direction of the two sides of a parallelogram, and it consequently describes the diagonal.” “You are quite right; but you doubtless will perceive that, instead of a straight line, the orange will describe a parabolic curve.” “For the same reason, I suppose,” said Tom, “that the stone from the sling described a curve?” “Certainly; but see, I have a diagram which will explain the subject more clearly.” [Illustration: A man on a moving horse demonstrates progressive motion with an orange.]