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Similar observations will apply to almost every body impelled by instruments projecting from its sides, and acting against a fluid. The motions of fishes, the act of swimming, the flight of birds, are all instances of the same kind.

(85.) The action of wind upon the sails of a vessel, and the force thereby transmitted to the keel, modified by the rudder, is a problem which is solved by the principles of the composition and resolution of force; but it is of too complicated and difficult a nature to be introduced with all its necessary conditions and limitations in this place. The question may, however, be simplified, if we consider the canvass of the sails to be stretched so completely as to form a plane surface. Let AB, fig.14., be the position of the sail, and let the wind blow in the direction CD. If the line CD be taken to express the force of the wind, let DECF be a parallelogram, of which it is the diagonal. The force CD is equivalent to two forces, one in the direction FD of the plane of the canvass, and the other ED perpendicular to the sail. The effect, therefore, is the same as if there were two winds, one blowing in the direction of FD or BA, that is against the edge of the sail, and the other, ED, blowing full against its face. It is evident that the former will produce no effect whatever upon the sail, and that the latter will urge the vessel in the direction DG.