BEACH TRAVEL
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is cycloidal, and this approximates to an ellipse in approaching shoal water. The force conveyed in an oscillatory wave revolves in a circular movement at the crest. As may be readily observed, an object floating on an oscillatory wave is not impelled forward by that wave—it drifts with the current. From the point when the wave becomes translatory in form its whole mass travels onward in the manner of a flowing stream. When its height attains one-third of its length the wave breaks. The translatory waves that affect a foreshore plough up the beach constituents, and the problem of defence works is to hold the shingle and sand up to their work, so that the shore shall not be denuded of its natural protection.
   Obviously the shape of the shore in section is a factor of first-rate importance. The normal forces of attack and the contours of a sea marge are complementary. If a shore be neglected its contour changes, its gradient grows dangerously steep, and the coast-line suffers erosion.
   The mechanical action of the off-shore wind in heaping up a foreshore is somewhat obscure. Presumably by checking the onrush of the crest it upsets the equilibrium of the wave. The motion of the crest being retarded, the base of the wave sets up an undertowing action. The beach materials are thus pushed up the under-water slope, and, the scouring action of the crest being absent, they accumulate.
   It must be borne in mind that the flood tide is both longer and stronger than the ebb, and therefore that, so far as the action of the tidal current is effective, it operates by driving the beach constituents in the direction of the flood. Wave action, when oblique to the shore, causes a series of unsymmetrical impulses. The impinging wave drives a column of shingle up the slope of the foreshore, the impelling force becomes exhausted, and the water escapes through the beach. The recoil of the spent water follows the most direct line of travel, i.e. that at right angles to the coast-line. It has pushed a certain quantity of beach material beyond the power of the recoil and dragged the balance down again, and thus the march of that material follows a zigzag, saw-tooth line of travel. The operative wind-waves in the English Channel are those produced by the