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660 Additions and Correffions

the back water when the tide ebbs; but the pofition and confiruction of thefe de- pend fo much on local circumftances, that the engineer who conducts the work ought to be the beft judge where and how to form them. A great deal depends on a fkilful engineer in works of this kind, who will contrive many ways to facilitate the work, and to overcome all difficulties in the execution of it.”*

The following“ fpecies of fea-embankment is perhaps the moft important of any; for there aré few efiuaries, or mouthsof rivers, where large and valuable tracts of Jand may not be gained by fuch means. The fhoals, or flats formed at the en- france of fuch rivers, are often compofed of the richeft manures or moft fertilif- ing particles, brought down by the ftream from the towns above, or from the ad- jacent country through which the river flows. Thefe fhoals may therefore, by pro- per management, be in general very eafily converted into the moft fertile plains. Where fuch a fituation happens, the firft object is to collect the whole river into one fiream, and to prevent its overfpreading a wider extent than juft fufficient for its difcharge; or, perhaps, it may be better to alter the courfe of the river altogethers and to make it difcharge itfelf at fome other place. The principle upon which that idea is founded, is this It has been obferved, and proved by experience, that if the courfe of a river or ftream be altered in fuch a manner as to make it difcharge itfelf into the fea at a different place than it did before, the former place will in a few years, by the conftant accumulation of fand or mud brought in every tide, be fo

-choked up or raifed above its former level, as to form of itfelf, in procefs of time,

a bank that, witha very little affiftance, will quite exclude the fea: for as the cur- rent of the river before carried away all that fediment which the motion of the waves naturally ftirred up, the current being removed, all or moft of the muddi- nefs will not only be carried further up the old channel of the river, but a great part of it be depofited there as the tide recedes. It has even been obferved, that in {pring-tides and particular winds this fediment is depofited in greater quantities than at othér times; and Mr. Beatfon has been informed that on making a perpendicular eut inthe land fo gained, the different ftrata or layers. were fo diftinct, that thofe- made at fpring-tides could eafily be diftinguithed from the reft. This is a very curi- ous fad, and well worth the attention of all who have lands fituated at the mouths of rivers, as there may, in many fuch places, be confiderable tracts gained in this manner at a very fmallexpenfe. Although this fact may be proved by experience in feme places, neverthelefs he fhould imagine the effect would not be the fame in all fituations. Where there is a great extent of flat or muddy thores, the motion of the waves.

+ Beatfon in Communieations to the Board ef Agriculture, Vol. II.