Page:Science vol. 5.djvu/515

 Jonx 12, 1886.]

��479

��The suggestion ' that water in motion rolls rather than aUdes,' is valuable as esplaioing the '' inner movements of the particles oT water among themselves,' which are aggregated in the indirect currents found even in carefully pre- pared beds, which are aymmetrical. smooth, and straight. These movements were well illustrated by Mr. Francis, in a scries of ex- periments at Lowell in 1867, by mixing white- wash with the clear water of an artificial channel. But the movements of mass inaugu- rated by the relative movements of the particles of water among themselves appear too feeble to account for ihc immense quantities of sedi- ment observed in suspension in great alluvial rivers. It also seems clear, aa concluded by Mr. Herschel, in his paper on the erosive and abrading power of water, " that direct friction tends to drag materials along the bed, or down the banks, if these have a sufficiently steep side-slope," and that "the effect of the simple friction of a stream upon its bed and banks is not a source of danger : its action is very slow, and it has never been shown to be of a danger- ous character in any instance." Certainly the sertiraentarj- grains have no power of motion independent of the water surrounding them : and friction against a smooth bed could not impart such vertical movement to the water as is necessary to lift these grains, except in the slight degree that may result from the movement of the particles of water among themselves. The conditions that prevail in natural stream-beds are necessary for a great suspension of sediment. The projections, in- equalities, and sinuosity of such beds expose the material composing tbem to the impact of the current rather than to simple friction, and also cause those extreme indirect movements of large masses of water that, in great rivers, develop whirls, boils, and eddies, and which alone are capable of lifting numerous and coarse grains of silt, sand, and even gravel. When a ' boil ' rises in the Mississippi River, the surface may be raised many inches, and the charge of sediment so dense, that it is seen to roll away from the crater in cloud-shaped masses.

The suspension of sediment is, then, only an indirect result of the velocity, depending more directly ujxjn the character of the lied — its symmetry, smoothness, and straightness, — than upon the velocity, or the relative depth of its different reaches and stages.

The amount of sediment suspended through- out the Mississippi River appears, however, more controlled by the tributary moat largely supplying its volume at the time than upon

��any or all other causes. It was noticed by Capt. Brown, U.S. corps of engineers, that ''the Missouri was one of the greatest con- tributors of sand to the Mississippi River," observably even in the South Pass, 1,300 miles below its mouth.

The apparent anomalies in the following table are largely explained by the relative discbai^e of the Missouri and the clear-water ti'ibutaries. It will be observed that frequent- ly the aggregate bulk or weight of aediment passing per second (2 X sediment in each foot) is greater at or below a mean stage than it is at the maximum.

In this table the velocity is di\'ided into half-feet per second ; and in the other columns are given the number of periods during the series of observation in which each rate pre- vailed, the number of separate measurements taken in each perio<i, and the mean quantity in each period at each rate of velocity.

Fulton, Nov. 27. 1879, to Oct. 12, 1880, Miasiaaippi Biver commission.

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��From the theory of Mr. Login, before quot- ed, the inference is drawn, that when flowing water is saturated, or loaded in pro|>ortioD to its velocity with sediment, the erosion and caving of banks will cease. In this is involved the assumption that the erosive power is that of friction rather than of impact. Surveys have been made by the Mississippi River com- mission which show the relative amount of caving on the right and left banks below Cairo. In this part of the river the tine of demarrn-

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