_A._ If a bullet from a certain point given, be shot against a wall perpendicularly, and again from the same point obliquely, what will be the proportion of the forces wherewith they urge the wall? For example, let the wall be A B, a point given E, a gun C E, that carries the bullet perpendicularly to F, and another gun D E, that carries the like bullet with the same swiftness obliquely to G; in what proportion will their forces be upon the wall? _B._ The force of the stroke perpendicular from E to F will be greater then the oblique force from E to G, in the proportion of the line E G to the line E F. _A._ How can the difference be so much? Can the bullet lose so much of its force in the way from E to G? _B._ No; we will suppose it loseth nothing of its swiftness. But the cause is, that their swiftness being equal, the one is longer in coming to the wall than the other, in proportion of time, as E G to E F. For though their swiftness be the same, considered in themselves, yet the swiftness of their approach to the wall is greater in E F than in E G, in proportion of the lines themselves. _A._ When a bullet enters not, but rebounds from the wall, does it make the same angle going off, which it did falling on, as the sun-beams do? _B._ If you measure the angles close by the wall their difference will not be sensible; otherwise it will be great enough, for the motion of the bullet grows continually weaker. But it is not so with the sun-beams which press continually and equally. _A._ What is the cause of reflection? When a body can go no further on, it has lost its motion. Whence then comes the motion by which it reboundeth? _B._ This motion of rebounding or reflecting proceedeth from the resistance. There is a difference to be considered between the reflection of light, and of a bullet, answerable to their different motions, pressing and striking. For the action which makes reflection of light, is the pressure of the air upon the reflecting body, caused by the sun, or other shining body, and is but a contrary endeavour; as if two men should press with their breasts upon the two ends of a staff, though they did not remove one another, yet they would find in themselves a great disposition to press backward upon whatsoever is behind them, though not a total going out of their places. Such is the way of reflecting light. Now, when the falling on of the sun-beams is oblique, the action of them is nevertheless perpendicular to the superficies it falls on. And therefore the reflecting body, by resisting, turneth back that motion perpendicularly, as from F to E; but taketh nothing from the force that goes on parallel in the line of E H, because the motion never presses. And thus of the two motions from F to E, and from E to H, is a compounded motion in the line F H, which maketh an angle in B G, equal to the angle F G E.