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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 AE, if there were no current. If they row towards the point E, the boat will arrive at the point B, moving in the line AB.

In this case the boat is impelled by two forces, that of the oars in the direction AE, and that of the current in the direction AC. The result will be, according to the parallelogram of forces, a motion in the diagonal AB.

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 CE, DF. The resistance of the water produces forces on the side of the boat, in the directions GL and HL, which, by the composition of force, are equivalent to die diagonal force KL, in the direction of the keel.