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In this example there are two successive decompositions of force. First, the original force of the wind CD is resolved into two, ED and FD; and next the element ED, or its equal DG, is resolved into DI and DH; so that the original force is resolved into three, viz. FD, DI, DH, which, taken together, are mechanically equivalent to it. The part FD is entirely ineffectual; it glides off on the surface of the canvass without producing any effect upon the vessel. The part DI produces lee-way, and the part DH impels.

H. Adlard, sc.

London, Pubd. by Longman & Co.

(86.) If the wind, however, be directly contrary to the course which it is required that the vessel should take, there is no position which can be given to the sails which will impel the vessel. In this case the required course itself is resolved into two, in which the vessel sails alternately, a process which is called tacking. Thus, suppose the vessel is required to move from A to E, fig.16., the wind setting from E to A. The motion AB being resolved into two, by being assumed as the diagonal of a parallelogram, the sides A a, aB of the parallelogram are successively sailed over, and the vessel by this means arrives at B, instead of moving along the diagonal AB. In the same manner she moves along B b, b C, C c, c D, D d, d E, and arrives at E. She thus sails continually at a sufficient angle with the wind to obtain an impelling force, yet at a sufficiently small angle to make way in her proposed course.