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Now let us suppose that the ball, instead of moving from C to D, moves from E to D. The force with which it strikes D being expressed by DE′, equal to ED, may be resolved into two, DF and DC′. The resistance of the cushion destroys DC′, and the elasticity produces a contrary force in the direction DC, but less than DC or DC′, because that elasticity is imperfect. The line DC expressing the force in the direction CD, let DG (less than DC) express the reflective force in the direction DC. The other element DF, into which the force DE′ is resolved by the impact, is not destroyed or modified by the cushion, and therefore, on leaving the cushion at D, the ball is influenced by two forces, DF (which is equal to CE) and DG. Consequently it will move in the diagonal DH.

(91.) The angle EDC is in this case called the “angle of incidence,” and CDH is called “the angle of reflection.” It is evident, from what has been just inferred, that the ball, being imperfectly elastic, the angle of incidence must always be less than the angle of reflection, and with the same obliquity of incidence, the more imperfect the elasticity is, the less will be the angle of reflection.