Mr. Seymour continued--“The various balancing toys will elucidate the nature of the _centre of gravity, point of suspension_, and _line of direction_: the seesaw, rocking-horse, and the operation of walking on stilts, will here come in aid of our explanations. The combined effects of momentum and a change in the centre of gravity of a body may be beautifully exemplified by the action of the Chinese Tumblers. The sling will demonstrate the existence and effect of _centrifugal force_; the top and tetotum will prove the power of vertiginous motion to support the axis of a body in an upright position. The trundling of the hoop will accomplish the same and other objects. The game of _bilboquet_, or cup and ball, will show the influence of rotatory motion in steadying the rectilinear path of a spherical body, whence the theory of the rifle-gun may be deduced. For conveying some elementary ideas of the doctrine of _oscillation_, there is the swing. The flight of the arrow will not only elucidate the principles of _projectiles_, but will explain the force of the air in producing rotatory motion by its impact on oblique surfaces: the revolution of the shuttlecock may be shown to depend upon the same resolution of forces. Then comes the kite, one of the most instructive and amusing of all the pastimes of youth,--the favourite toy of Newton in his boyish days:[4]--its ascent at once developes the theory of the composition and resolution of forces, and explains various subordinate principles, which I shall endeavour to describe when we arrive at the subject. The _see-saw_ will unfold the general principle upon which the Mechanical Powers are founded; and the boy may thus be easily led to the theory of the _lever_, by being shown how readily he can balance the heavier weight of a man by riding on the longer arm of the plank. The theory of colours may be pointed out to him as he blows his soap-bubbles;[5] an amusement which will, at the same time, convince him that the air must exert a pressure equally in all directions. For explaining the theory of sound, there are the whistle, the humming-top, the whiz-gig, the pop-gun, the bull-roarer, and sundry other amusements well known in the play-ground; but it is not my intention, at present, to enumerate _all_ the toys which may be rendered capable of affording philosophical instruction; I merely wish to convince you that my plan is not quite so chimerical as you were at first inclined to believe.” “Upon my word,” said the vicar, “no squirrel ever hopped from branch to branch with more agility,--you are the very counterpart of Cornelius Scriblerus; but I must confess that your scheme is plausible, very plausible, and I shall no longer refuse to attend you in the progress of its execution. Cedo equidem, nec, nate, tibi comes ire recuso,[6] as Virgil has it.”