“The revolution of the marble, my dear boy, is brought about by no less than three forces: look attentively at the diagram, and you will easily comprehend my explanation. There is, in the first place, the rectilinear motion given to it by your hand; then there is the friction of the ground: since, however, this latter acts in a contrary direction, it merely tends to lessen or counteract the velocity with which the under-surface proceeds, and consequently to give a relatively-increased progressive motion to its upper part; then comes that force by which its several parts cohere, and which may be represented by C H; so that the two forces producing the revolution of the point C, are justly expressed by the lines C G, C H: but these are in the direction of the two sides of a parallelogram, the point will therefore move along the diagonal C E. I have here a toy for you, which will serve to explain still farther the causes of rotation to which I have alluded.” Mr. Seymour produced a watch-glass, in the hollow of which stood a dancing-figure of thin card, as here represented. [Illustration: Toy man standing on the flat part of a convex piece of glass.] He placed it upon a black japanned waiter,[16] which he held in an inclined position, when it immediately slided down the inclined plane, as might have been expected. He next let fall a drop of water upon the waiter, and placed the watch-glass in it. Under this new arrangement, instead of sliding, the watch-glass began to revolve as soon as an inclination was given to the surface; and it continued to revolve with an accelerated velocity, obeying the inclination and position of the plane, as directed by the hand of the operator. “What a very pretty effect is produced by the rapid revolution of the figure!” observed Louisa. “Its use in the arrangement,” said her father, “is to render the accelerated motion more obvious.” “I perceive it revolves faster and faster, or, I suppose I ought to say, with an accelerated velocity,” said Tom.