This divergency of the strait line A H from the strait line A G is that which, the writers of optics commonly called _refraction_, which, when the passage is easier in the first than in the second medium, is made by diverging from the line of inclination towards the perpendicular; and contrarily, when the passage is not so easy in the first medium, by departing further from the perpendicular. [Sidenote: Motion sometimes opposite to that of the movent.] 8. By the 6th theorem it is manifest, that the force of the movent may be so placed, as that the body moved by it may proceed in a way almost directly contrary to that of the movent, as we see in the motion of ships. For let A B (in fig. 2) represent a ship, whose length from the prow to the poop is A B, and let the wind lie upon it in the strait parallel lines C B, D E and F G; and let D E and F G be cut in E and G by a strait line drawn from B perpendicular to A B; also let B E and E G be equal, and the angle A B C any angle how small soever. Then between B C and B A let the strait line B I be drawn; and let the sail be conceived to be spread in the same line B I, and the wind to fall upon it in the points L, M and B; from which points, perpendicular to B I, let B K, M Q and L P be drawn. Lastly, let E N and G O be drawn perpendicular to B G, and cutting B K in H and K; and let H N and K O be made equal to one another, and severally equal to B A. I say, the ship B A, by the wind falling upon it in C B, D E, F G, and other lines parallel to them, will be carried forwards almost opposite to the wind, that is to say, in a way almost contrary to the way of the movent.