Mr. Seymour then proceeded to explain the laws of impact, by which the movement of each marble was directed. He observed, that the subject embraced two propositions, viz. the direction of the _object_ marble after having been struck, and that of the _striking_ marble after the stroke. He said that, if a straight line were drawn between the centres of the striking and object marbles, it would necessarily pass through their point of contact, and, if continued, would represent the path of the latter after the blow. In order to find the direction of the _striking_ marble after the shock, he told him that he must imagine a tangent to the path of the _object_ ball drawn from its centre, and then a line parallel to it, from the centre of the striking marble; the latter of which would be the required path. Mr. Seymour now inquired whether there was any other game of marbles at which they could amuse themselves. “The game which we call ‘_lagging out_,’” replied the boy, “is amusing enough. It consists in striking your marble against the wall, and making it rebound, so as to hit any other marble that is placed at a certain distance from it, or to come within a span of it.” “I understand,” said his father, “and, like _ring-taw_, it may be made subservient to our purpose of illustrating the doctrine of forces; although I think that the principle of _reflected motion_ may be more readily explained by the rebounding ball.” Mr. Seymour here took the elastic ball, and threw it obliquely against the wall, from which it rebounded in an opposite and equally oblique direction. He then sketched the annexed figure, and proceeded as follows:--“When I threw the ball against the wall B, in the direction A B, having struck it, it glanced off, making an angle, in its passage back again, equal to that which it made in its approach to the wall. If I draw the perpendicular B D, this fact will be rendered more apparent, and you will perceive that the angle A B D is equal to the angle C B D: the former is termed the _angle of incidence_, the latter the _angle of reflection_; and these angles, remember, are always equal, provided the ball under experiment be perfectly elastic.” [Illustration: Diagram showing the physics of rebounding.] “Do you mean to say,” asked Tom, “that the more obliquely I throw the ball against the wall, the more obliquely it will rebound?” “Exactly; that is my meaning: and see whether you cannot explain the fact, for it depends on the composition and resolution of the forces, a subject which I should hope you thoroughly understand.” Tom pondered for some time over the drawing, and at length observed, that there was one difficulty which he could not immediately surmount. “State your difficulty,” said Mr. Seymour.