Scientific American Supplement, No. 620, November 19,1887 by Various
page 92 of 138 (66%)
page 92 of 138 (66%)
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curved surface which, commencing at low water, passes over the summit
of the tide down to the next low water--this is a wave of the first order. In waves of the second order, the force raising them acts only on the surface, and there the effect is greatest (as in the wind waves)--where one assists in giving to the water oscillating motion which maintains the next, and gradually puts the whole surface in commotion; but at a short distance down that effect entirely disappears. If the earth presented a uniform globe, with a belt of sea of great and uniform depth encircling it round the equator, the tide wave would be perfectly regular and uniform. Its velocity, where the water was deep and free to follow the two luminaries, would be 1,000 miles an hour, and the height of tide inconsiderable. But even the Atlantic is not broad enough for the formation of a powerful tide wave. The continents, the variation in the direction of the coast line, the different depths of the ocean, the narrowness of channels, all interfere to modify it. At first it is affected with only a slight current motion toward the west--a motion which only acquires strength when the wave is heaped up, as it were, by obstacles to its progress, as happens to it over the shallow parts of the sea, on the coasts, in gulfs, and in the mouths of rivers. Thus the first wave advancing meets in its course with resistance on the two sides of a narrow channel, it is forced to rise by the pressure of the following waves, whose motion is not at all retarded, or certainly less so than that of the first wave. Thus an actual current of water is produced in straits and narrow channels; and it is always important to distinguish between the tide wave, as bringing high water, and the tidal stream--between the rise and fall of the tide and the flow and ebb. |
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