Let A, _fig. 12._, be the place of the boat at starting; and suppose that the oars are so worked as to impel the boat towards B with a force which would carry it to B in one hour, if there were no current in the river. But, on the other hand, suppose the rapidity of the current is such, that without any exertion of the rowers the boat would float down the stream in one hour to C. From C draw C D parallel to A B, and draw the straight line A D diagonally. The combined effect of the oars and the current will be, that the boat will be carried along A D, and will arrive at the opposite bank in one hour, at the point D. If the object be, therefore, to reach the point B, starting from A, the rowers must calculate, as nearly as possible, the velocity of the current. They must imagine a certain point E at such a distance above B that the boat would be floated by the stream from E to B in the time taken in crossing the river in the direction A E, if there were no current. If they row towards the point E, the boat will arrive at the point B, moving in the line A B. In this case the boat is impelled by two forces, that of the oars in the direction A E, and that of the current in the direction A C. The result will be, according to the parallelogram of forces, a motion in the diagonal A B. The wind and tide acting upon a vessel is a case of a similar kind. Suppose that the wind is made to impel the vessel in the direction of the keel; while the tide may be acting in any direction oblique to that of the keel. The course of the vessel is determined exactly in the same manner as that of the boat in the last example. The action of the oars themselves, in impelling the boat, is an example of the composition of force. Let A, _fig. 13._, be the head, and B the stern of the boat. The boatman presents his face towards B, and places the oars so that their blades press against the water in the directions C E, D F. The resistance of the water produces forces on the side of the boat, in the directions G L and H L, which, by the composition of force, are equivalent to die diagonal force K L, in the direction of the keel. 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.