“I suppose,” said Tom, “it depends upon the action which I give to it by my thumb and finger when I shoot it out of my hand.”
“You are, undoubtedly, capable of thus giving to your marble a certain spinning motion, the effect of which we shall have to consider hereafter; but I fancy you would be greatly puzzled to make it proceed without revolving, give it what impulse you might by your hand.”
“I have sometimes tried,” said Tom, “to make it do so by pushing it along with a flat ruler, but it always rolled in spite of me.”
“Then it is clear, from your own experiment, that its rotation cannot arise from the cause you would assign to it. If you will attend to this diagram,” continued his father, “I will endeavour to explain the operation. It is evident that, as the marble moves along the ground B D, the motion of the point B will be retarded by the resistance occasioned by its rubbing on the ground; while the point C, which does not meet with any such resistance, is carried forward without opposition, and it consequently must move faster than the point B; but since all the parts of the marble cohere or stick together, the point C cannot move faster than B, unless the marble revolves from C to E; and as the several points of the marble which are successively applied to the floor are retarded in their motion, while the opposite points move freely, the marble during its progressive motion must continue to revolve.”
“But you said, papa, that whenever a body moved in any direction, except that of a straight line, it must have been acted upon by more than one force; and yet the marble not only runs along the ground, but turns round; at the same time, by the simple force of my hand.”
“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.
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.