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Show 30! ItEPRODUCT IVE EFFECTS OF TIDES AND CURRENTS. estuary, because it holds in suspension, l~ke the ri~er, during certain seasons of the year, a large quantity of sediment; and when their waters, flowing in opposite directions, meet, this matter subsides. For this reason, in inland seas, and even on the borders of the ocean, where the rise of the tide happens to be slight, it is scarcely possible to prevent a harbour from silting up ; and it is often expedient to carry out a jetty to beyond the point where the marine current and the river neutralise each other's force, for beyond this point a free channel is maintained by the superior force of the current. The formation and keeping open of large estuaries are due to the combined influence of tides and currents ; for when the tide rises, a large body of water suddenly enters the mouth of the river, where, becoming confined within narrower bounds, while its momentum is not destroyed, it is urged on, and, having to pass throuO'h a contracted channel, rises and runs with increased velocity, just as a swollen river, when it reaches the arch of a bridD'e scarcely large enough to give passage to its waters, is preci~itated in a cataract, while rushing through the arch. During the ascent of the tide, a stream of fresh-water is flowing down from the higher country, and is at-rested for several hours; and thus a large lake of brackish water is accumulated which, as soon as the ebb causes the sea to fall, is let lo~se, as ·on the removal of an artificial sluice or dam. By the force of this retiring body of water, the alluvial sediment, both of the river and of the sea, is swept away, and transported to such a distance from t~1e mouth of the estuary, that a small part only can return with the next tide. In many estuaries, as in the Thames, for example, the tide requires about five hours to flow up, an~ about se:en to flow down ; so that the preponderating force 1s always m the direction which tends to keep open a deep and broad passage. But as it is evident that both the river and the tid.al curr:nt are ready to part with their sediment wl~enever theu: velocl~Y is checked there is naturally a tendency m all estuaries to silt up partiall'y, since the causes of retardation are very numerous, and constantly change their position. The new lands acquired within the mouth of an estuary~; only a few feet above the mean level of the sea, whereas c\s of great height are consumed every year· If, therefore, t e REPRODUCTIVE EFFECTS OF TIDES AND CURRENTS. 305 a· rea do fd l abn d · annually abandoned by th e sea were equa1 toth at mva e y ,It, there would still be no compensation in kind. . Many writ~rs have declared that the gain on our eastern coast, smce the earliest periods of history , has m ore th an counter b a-lanced the loss; but they have been at 110 p am· s t o ca1 c u 1a te t h e amount o.f .t h. e latter, and have often forgotten tha t , w 11 I'1 e th e new acqUisitions are manifest, there are rarely any natural monu-ments to attest the fo~mer existence of what is now no more. They have al~o taken mto their account those tracts, artificially recovered,, whiCh are often of great agricultural importance, and may rernam secm·e, perhaps, for thousands of years, but which are n. evertheless exposed to be overflowed aDo 'ain by a sma1 1 pr•o - portwn of th~ force required to remove the high lands of our ~h~res. It w1ll seem, at first sight, somewhat paradoxical, but 1t IS neverthele~s true, that the greater number of estuaries, alth~~gh pecuh~r]y exposed to the invasion of the sea, are usua ! c?~tractmg in size, even where the whole line of coast 1s givmg way. But the fact is, that the inroads made by the ocean . upon ~stuaries, although extreme] y great, are ~omp,Ie.ted durmg periOds of comparatively short duration; and I~ the mtervals between these visitations, the mouths of rivers, hke othe~ parts of the coast, usually enjoy a more or less perfect respite. All the estuaries, taken together, constitute but a small par~ of a great line of coast; it is, therefore, most probable, that If our observations extend to a few centuries onl we shall not see any, and very rarely all, of this small par~ e[~ posed. to the.fury of the ocean. The coast of Holland and Friesland, If studied for several consecutive centuries since the Roman era, would g~nerally have led to the conclusion that the ]and w?s ~ncroachmg fast upon the sea, and that the aggrandizement Withm the estuaries far more than compensated the losses on the.open coast. But when our retrospect embraces the whole period, an opposite inference is drawn; and we find that the Zuyder Zee, the Bies Bosch, Dollart, and Yahde are modern gulfs an dba ys, and that these points have been t'h e principal theatres of the retreat, instead of the advance, of the land. If ~e possessed records of the changes on our coast for several t ousand years, they would probably present us with similar r~sults i and although we have hitherto seen our estuaries for t e most part, become partially converted into dry land,' and Vot, I. X |