No. 49 ([page 404, first paragraph of second note]). The length of the lower course of the Po having been considerably increased by the filling up of the Adriatic with its deposits, the velocity of the current ought, prima facie, to have been diminished and its bed raised in proportion. There are grounds for believing that this has happened in the case of the Nile, and one reason why the same effect has not been more sensibly perceptible in the Po is, that the confinement of the current by continuous embankments gives it a high-water velocity sufficient to sweep out deposits let fall at lower stages and slower movements of the water. Torrential streams tend first to excavate, then to raise, their beds. No general law on this point can be stated in relation to the middle and lower course of rivers. The conditions which determine the question of the depression or elevation of a river bed are too multifarious, variable, and complex to be subjected to formulæ, and they can scarcely even be enumerated. See, however, note on p. 431.
No. 50 ([page 406, first paragraph]). The system proposed in the text is substantially the Egyptian method, the Nile dikes having been constructed rather to retain than to exclude the water. The waters of rivers which flow down planes of gentle inclination, deposit in their inundations the largest proportion of their sediment as soon as, by overflowing their banks, they escape from the swift current of the channel, and consequently the immediate banks of such rivers become higher than the grounds lying farther from the stream. In the "intervals," or "bottoms," of the great North American rivers, the alluvial banks are elevated and dry, the flats more remote from the river lower and swampy. This is generally observable in Egypt, though less so than in the valley of the Mississippi, where, below Cape Girardeau, the alluvial banks constitute natural glacis descending as you recede from the river, at an average of seven feet in the first mile.—Humphreys and Abbot's Report, pp. 96, 97.
The Egyptian crossdikes, by retaining the water of the inundations, compel it to let fall its remaining slime, and hence the elevation of the remoter land goes on at a rate not very much slower than that of the immediate banks. Probably transverse embankments would produce the same effect in the Mississippi valley. In the great floods of this river, it is observed that, at a certain distance from the channel, the bottoms, though lower than the banks, are flooded to a less depth. See cross sections in Plate IV. of Humphreys and Abbot's Report. This apparently anomalous fact is due, I suppose, to the greater swiftness of the current of the overflowing water in the low grounds, which are often drained through the channels of rivers whose beds lie at a lower level than that of the Mississippi, or by the bayous which are so characteristic a feature of the geography of that valley. A judicious use of dikes would probably convert the swamps of the lower Mississippi valley into a region like Egypt.
No. 51 (second note). The mean discharge of the Mississippi is 675,000 cubic feet per second, and, accordingly, that river contributes to the sea about eleven times as much water as the Po, and more than sis and a half times as much as the Nile. The discharge of the Mississippi is estimated at one-fourth of the precipitation in its basin, certainly a very large proportion, when we consider the rapidity of evaporation in many parts of the basin, and the probable loss by infiltration.—Humphreys and Abbot's Report, p. 93.
No. 52 ([page 423, first paragraph]). Artificially directed currents of water have been advantageously used in civil engineering for displacing and transporting large quantities of earth, and there is no doubt that this agency might be profitably employed to a far greater extent than has yet been attempted. Some of the hydraulic works in California for washing down masses of auriferous earth are on a scale stupenduous enough to produce really important topographical changes.
No. 53 ([page 435, first note]). I have lately been informed by a resident of the Ionian Islands, who is familiar with this phenomenon, that the sea flows uninterruptedly into the sub-insular cavities, at all stages of the tide.
No. 54 ([page 438, note]). It is observed in Cornwall that deep mines are freer from water in artificially well-drained, than in undrained agricultural districts.—Esquiros, Revue des Deux Mondes, Nov. 15, 1863, p. 430.
No. 55 ([page 441]). See, on the Artesian wells of the Sahara, and especially on the throwing up of living fish by them, an article entitled, Le Sahara, etc., by Charles Martins, in the Revue des Deux Mondes for August 1, 1864, pp. 618, 619.
No. 56 ([page 444, first note]). From the article in the Rev. des Deux Mondes, referred to in the preceding note, it appears that the wells discovered by Ayme were truly artesian. They were bored in rock, and provided at the outlet with a pear-shaped valve of stone, by which the orifice could be closed or opened at pleasure.
No. 57 ([page 447, second note]). Hull ingeniously suggests that, besides other changes, fine sand intermixed with or deposited above a coarser stratum, as well as the minute particles resulting from the disintegration of the latter, may be carried by rain in the case of dunes, or by the ordinary action of sea water in that of subaqueous sandbanks, down through the interstices in the coarser layer, and thus the relative position of fine sand and gravel may be more or less changed.—Oorsprong der Hollandsche Duinen, p. 103.