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Show 246 PROPORTION OF SEDIME~T ment t.s constan t 'pto. vi'n g ' as that geo(oJ 'rapher remarks, that t.h de waters 0f t h e M.I SS.l SS·i p p1· and the Red River onc. e ocf cup.1 e alternatel y cons1' de t.a b le tracts below their pr. esent pom.t o umon. Such al terna tI. ons ar· e probabl. y common m submarm. e spaces s:i tuated b etween two convergm(oJ' deltas. For before. the t.w o ri·v ers um' te , there must almost always be a certam· pde nodd w l1 en an m· t e1. m e d1'ate tract will be alternate.l y oc.c up1e an a b an d onecl b Y tlle waters of e.a ch stream ; sm· ce I.t c·a n1 rare1 .y h appen, tl1 a t the season of htghest flood w· 1ll p~re Cise y co1i - respon d m· eac1 1 . In the case of the Red Rtver, 10r ·c xamI p ed, and Ml.S S.I SS. lpp.l , W h'IC h carry off the waters f.r om. count·n esh p ac•e under widely distant la~itudes, .an exact comctdence m t e time of greatest inundation IS very Improbable. CONCLUDING REMARKS ON DELTAS. Qu a. n t t' t yoi f Sediment in River Wat·e r.-Very few sdat isfact·o ry expen·m en ts have as yet been made, to enab·l e usf to 1e termmt e, with any degree of accuracy' the mean quantity o eart 1Y ~a. ter discharged annually into the sea by some one of. the prmCI~al n·v ers of th e ea1· th · Ha' rtsoeker computed the Rh.m e to con.t am*, wh en mos t fl oo ded ' one part in a hundred of mud· m suspensdiO tln t. B several observations of Sir George Staunto.n, It appeare la tl!c water of they ellow River in China contamed earthy matter . the roportion of one part to two hundred, and he calculated ~~at it trought down, in a single hour, two million feet of earth, or forty-eight million daily; so that, if t~e Yellow Sea ?e taken to be one hundred and twenty feet deep, It would req.mr: seve'n.ty da s for the river to convert an English square mtle mto film laid and twenty-four thousand years to turn the whole :• . nto 'terra £rma assuming it to be one hundred and twenty- v~ ~housand squa:e miles in extent t. Manfredi, th~ cele~ra~. Italian hydrographer, conceived the average propor~wn ~ :~1e~ ment in all the running water on the globe, whiC~l Ie\ th to be l__ and he imagined that it would reqmre at ou-1 e sea, T'fo' . d d · the genera nd Years for the sediment carne own to raise sleav el of the sea about one_fo.ot. Some wrt' te rs, on the contrary, * Comment, Bonon., vol. ii., part i., P· 237• .. 408 . t Staunton's Embassy to China.. London, 1791, 4to, vol.n., P· IN RIVER WATER. 247 as De Maillet, have declared the most turbid waters to contain far Jess sediment than any of the above estimates would import; and there is so much contradiction and inconsistency in the facts and speculations hitherto promulgated on the subject, that we must wait for additional experiments before we can form any opinion on the question. One of the most extraordinary statements is that of Major Rennell, in his excellent paper, before referred to, on the Delta of the Ganges. " A glass of water," he says, "taken out of this river when at its height, yields about one part in four of mud. No wonder, then, that the subsiding waters should quickly form a stratum of earth, or that the delta: should encroach on the sea*!" The same hydrographer computed with much care the number of cubic feet of water discharged by the Ganges into the sea, and estimated the mean quantity through the whole year to be eighty thousand cubic feet in a second. When the river is most swollen, and its velocity much accelerated, the quantity is four hundred and five thousand cubic feet in a second. Other writers agree that the violence of the tropical rains, and the £neness of the alluvial particles in the plains of Bengal, cause the waters of the Ganges to be charged with foreign matter to an extent whoUy unequalled by any large European river during the greatest floods. '\Ve have already alluded to the frequent sweeping down of large islands by the Ganges; and Major R. H. Colebrooke, in his account of the course of the Ganges, relates examples of the rapid filling up of some branches of the river, and the excavation of new channels, where the number of square miles of soil removed in a short time (the column of earth being one hundred and fourteen feet high) was truly astonishing. Forty square miles, or ~5,600 acres, are mentioned as having been carried away, in one locality, in the course of a few years t. ~ut although we can readily believe the proportion of sediment m the waters of the Ganges to exceed that of any river in northern latitudes, we are somewhat staggered by the results to w?ich we must arrive if we compare the proportion of mud, as giVen by Rennell, with his computation of the quantity of water discharged, which latter is probably very correct. If it • Phil. Trans., 1781. t Trans. of the Asi!l.tic Society, vol. vii., p. 14, |