He proceeded to observe, that the force acting in the direction A B would certainly be resolved into two others, viz. one in the direction F B, and another in that of D B; “because,” continued he, “these lines are the adjacent sides of the parallelogram, of which A B is the diagonal; and I well know that whenever a force strikes obliquely, it is thus resolved.” “That is all very well explained,” replied his father; “pray proceed.” “Now comes the difficulty,” continued Tom; “for the force D B will of course be destroyed by the wall, and that represented by F B, which is the only one that can remain, would carry the ball to E.” “It certainly would do so,” answered his father, “if the ball were perfectly devoid of elasticity; but remember that, in consequence of this property, the force D B will be exchanged for one in an opposite direction, B D.” “I had entirely overlooked the elasticity,” said Tom; “I now see my way clearly, for in that case there must be two forces acting in the directions B D, B E, which will, of course, drive the ball down the diagonal B C.” “Your demonstration is perfectly correct, my boy; and I think you will now admit that I could not have adduced a more beautiful instance of the composition and resolution of forces; for, in the first place, you resolve the diagonal force into two others, and then you recompound these again to produce another diagonal one.” “But I think you told us that the angles of incidence and reflection were only equal when the rebounding body was perfectly elastic.” [Illustration: Diagram showing more physics of rebounding.]