For the wind that blows in the line C B will (as hath been shown in art. 6) give to the point B an endeavour to proceed in a strait line perpendicular to the strait line B I, that is, in the strait line B K; and to the points M and L an endeavour to proceed in the strait lines M Q and L P, which are parallel to B K. Let now the measure of the time be B G, which is divided in the middle in E; and let the point B be carried to H in the time B E. In the same time, therefore, by the wind blowing in D M and F L, and as many other lines as may be drawn parallel to them, the whole ship will be applied to the strait line H N. Also at the end of the second time E G, it will be applied to the strait line K O. Wherefore the ship will always go forward; and the angle it makes with the wind will be equal to the angle A B C, how small soever that angle be; and the way it makes will in every time be equal to the strait line E H. I say, thus it would be, if the ship might be moved with as great celerity sideways from B A towards K O, as it may be moved forwards in the line B A. But this is impossible, by reason of the resistance made by the great quantity of water which presseth the side, much exceeding the resistance made by the much smaller quantity which presseth the prow of the ship; so that the way the ship makes sideways is scarce sensible; and, therefore, the point B will proceed almost in the very line B A, making with the wind the angle A B C, how acute soever; that is to say, it will proceed almost in the strait line B C, that is, in a way almost contrary to the way of the movent; which was to be demonstrated. But the sail in B I must be so stretched as that there be left in it no bosom at all; for otherwise the strait lines L P, M Q and B K will not be perpendicular to the plane of the sail, but falling below P, Q and K, will drive the ship backwards. But by making use of a small board for a sail, a little waggon with wheels for the ship, and of a smooth pavement for the sea, I have by experience found this to be so true, that I could scarce oppose the board to the wind in any obliquity, though never so small, but the waggon was carried forwards by it. By the same 6th theorem it may be found, how much a stroke, which falls obliquely, is weaker than a stroke falling perpendicularly, they being like and equal in all other respects.