Tom acknowledged the truth of this statement; and Mr. Seymour assured him, that, when they came to play at ball and marbles, he should be able to give him a practical demonstration of the fact; for he would show him, that whenever a body strikes a surface obliquely, or in an inclined direction, such a _resolution of force_ will actually take place: “and now, Tom,” said his father, “give me a marble; for I wish to explain the reason why it turns round, or revolves on its axis, as it proceeds forward.” “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.” [Illustration: Fig. 7. Diagram for marble’s motion.] “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.”