the United Provinces, two small neighbouring rivers have, BOOK the one of an inch, the other of declivity for every 1000 feet.* The Marwede, between Herdinxveld and Dort, falls an inch along 1,125 feet; but between Dort and the sea, only one inch along 9000 feet.† Even the most rapid rivers have less declivity than is commonly imagined. The Rhine between Schaffhausen and Strasburgh, has a fall of 4 feet in a mile; and of 2 feet between Strasburgh and Schenckenschantz. Hence we see the reason why one river may receive another almost as large as itself, without any considerable enlargement of its bed; the augmentation of its body only accelerates its course. Sometimes one river falling into another with great rapidity, and at a very acute angle, will force the former to retrace its course and return for a short space towards its source. This has happened more than once to the Rhone, near Geneva; the impetuous Arva, which descends from the mountains of Savoy, being swollen beyond its usual size, has made the more gentle waters of the Rhone flow back into the lake of Geneva; causing the wheels of the mills to revolve backwards.t XIII. rivers. Some rivers have no stream whatever, and the cause is Confined easily discovered; the land baving scarcely any declivity, does not impart a sufficiently strong impulse to their waters, which are constantly retarded, and finally absorbed by the sand. Sometimes these waters are evaporated by the heat of the sun, as is the case with the rivers of Arabia and Africa; but they more commonly flow into pools, marshes, or salt lakes. and cas Rivers which descend from primitive mountains into the Cataracts secondary lands, often forms cascades and cataracts. Such cades. are the cataracts of the Nile, of the Ganges, and some other great rivers, which, according to Desmarets, evidently mark the limits of the ancient land. Cataracts are also Brahm. Principe d'Hydraulique, § 208. (en All.) + Velson, Rivierkundige Verhandeling, p. 126, (in Dutch.) Compare the l'Hydraulique Générale de Wiebeking, (in German.) BOOK formed by lakes: of this description are the celebrated falls XIII of the Niagara; but the most picturesque falls are those of Height of cataracts. rapid rivers, bordered by trees and precipitous rocks. Sometimes we see a body of water, which, before it arrives at the bottom, is broken and dissipated into showers, like the Staubbach; sometimes it forms à watery arch, projected from a rampart of rock, under which the traveller may pass dry shod, as the "falling spring" of Virginia; in one place, in a granitic district, we see the Trolhetta, and the Rhine not far from its source, urge on their foaming billows amongst the pointed rocks; in another, amidst lands of calcareous formation, we see the Czettina and the Kerka, rolling down from terrace to terrace, and presenting sometimes a sheet, and sometimes a wall of water.* Some magnificent cascades have been formed, at least in part, by the hands of man: the cascades of Velino, near Terni, have been attributed to Pope Clement VIII.; other cataracts, like those of Tunguska in Siberia, have gradually lost their elevation by the wearing away of the rocks, and have now only a rapid descent. The elevation of cataracts has generally been exaggerated that of Tequendama, formed by the Rio de Bogota, in South America, estimated by Bouguer at 1500 feet, is, according to Humboldt, not quite 600, and the highest fall ever known, that of Staubbach, instead of being 1100, as stated by some travellers, is only 900 according to trigonometrical measurement. When the ground does not form a steep, and almost perpendicular bank, but only a very rapid declivity; and when at the same time the bed of the river is confined by rocks, the waters acquire by compression an astonishing force. Winterbotham relates, Herbinis, Dissertatio de Admirandis Mundi Cataractis, suprâ et subterraneis. Amsterdam, 1678, in 4to. Voyages de Fortis, Carver, &c. &c. + Opere di Monsignor Claudio Todeschi, (Rome, 1779,) vol. ii. p. 77. ‡ Isbraud Ides, Voyages au Nord, viii. 54, sqq. Muller, Sketch of Russian History, viii. 100-118, sqq. (In German.) Bouguer, Voyages au Perou, p. 91. Humboldt, Vues des Cordilliers, p. 22. || Wyttenbach and Wolf, Storr, Travels in the Alps, i. 114-115, (in Ger man.) that the river of Connecticut, in the United States, at 40 leagues from its source, is so compressed by rocks, that it carries along on its surface, pieces of lead, as if they were so many corks; and that, notwithstanding the utmost efforts, it is impossible to insert an iron point in its waters: but this appears to be greatly exaggerated. BOOK XIII. increase of rivers. The periodical rise of the Nile was considered as a sin- Periodical gular phenomenon, and one of the greatest mysteries of nature, until modern Europeans, by penetrating into the torrid zone, which was almost unknown to the ancients, discovered that this wonderful property belonged to a great many other rivers besides the Nile. It is now well known that in all the countries situated between the two tropics, it rains incessantly during a certain season of the year. The period varies, according to local circumstances, but it is sufficient to know, that the torrid zone, deprived in a great measure of the benefit of snow and glaciers, has this deficiency supplied by copious torrents of rain, pouring down incessantly upon the ground, which has been almost burnt with heat during the dry season: Then all the lakes and rivers swell and overflow their banks. rivers pa If a river, under the influence of these tropical rains, Increase of flows along a plain, in a direction parallel to the equator, rallel to the its overflowing waters will spread with a certain degree of equator. equality over the whole extent of its banks. Such, in a great measure, is the case with the Oronoco, in America, the Senegal, and probably the Niger in Africa. rivers run ning from south, and If, on the contrary, such a river flows from a great ele- Increase of vation, from extensive mountains into deep plains and valleys; or, if its direction be perpendicular to the equator, north to that is, north and south, then, it is evident, that the ac- vice versa. tion of the tropical rains will be extremely unequal in different parts of this river; for it is manifest that the surplus of water will be carried almost entirely towards the lower parts of the territory of the river. This is exactly what happens in the floods of the Nile. This river, as the ancients have said, and notwithstanding the assertions of Bruce, comes from the mountains of the Moon, which pro BOOK bably form a central and very elevated plateau, occupying XIII. the middle of Africa, and extending especially towards the east and south. In Asia, the rivers Siam and Cambodja, flow in almost the same latitudes as the Nile, but in a different direction: being from north to south. These two rivers have floods resembling those of the Nile; the Indus, the Ganges, and in general all the rivers which flow between the tropics, present this very phenomena, with variations arising from local circumstances. No river beyond the torrid zone is subject to regular periodical swellings; the overflowings which occur in the temperate zones depend solely on the melting of the snow in the spring, and on the quantity of rain which has fallen upon the mountains.* Rivers dis- The rivers which disappear under ground have excited the appearing wonder both of ancients and moderns. The poets have ground. sung of the Alpheus, which, according to them, passes from under Causes of this pheno menon. Peloponnesus into Sicily, beneath the Ionian Sea, to mingle its amorous waves with those of Aretbusa. The ancients have mentioned a great number of rivers which lose themselves under ground, to reappear in a lower level; but this phenomenon, which most frequently is closely connected with that of subterraneous caverns, has been examined in a rational and sober manner only by the moderns. When a river is obstructed in its course by a bank of solid rocks, and finds beneath them a stratum of softer materials, its waters wear away the softer substance, and thus open up for themselves a subterraneous passage, more or less long. Such are the causes which have formed the sinking of the Rhone, between Seyssel and l'Ecluse; the bridge of Veja, near Verona, the arch of which has an elevation of more than 114 feet; and particularly the Varenius, Géographie Génerale, ch. 16. prop. 20. Lulof, Géographie Physique, ii. 262, (in German.) Bergmann, i. 387. sqq. (in Swedish.) + Pliny, ii. 103. Seneca Quest. Nat. iii. 26. Strabo, 1. ix. 424; xi. 519, edition Alm. Mæbius, Dissert. de Fluviis qui intercedunt et enascuntur. Annales des Voyages, iv. 81. Z. Betti, Descrizione d'un Maraviglioso ponte, 1766, Fortis in the Giornale d'Italia, vi. 241. magnificent Rockbridge in Virginia, an astonishing vault BOOK uniting two mountains, separated by a ravine of 270 feet in XIII. depth, in which the Cedar Creek flows. It is probable that' the fall of a rock has formed these natural bridges, like those of Icononzo, in Mexico. In Louisiana, trees, or rather whole forests, have been observed to fall on a river, covering it nearly with vegetable earth; and thus giving rise to a natural bridge, which, for leagues, has hid the course of the river from view; finally, the Guadiana sees its waters scattered and filtered in the sandy and marshy grounds, from which they re-issue in greater abundance. France affords very few examples of these disappearing rivers.t rivers. Rivers, in running into the sea, present a great variety of Mouth of interesting phenomena; many form sand banks, as the Senegal and the Nile; others, like the Danube, rush with such force into the sea, that one can for a certain space distinguish the waters of the river from those of the sea. The water of the little river Syre in Norway, are discernible for a considerable distance, (some say for two leagues) in the sea. It is only by a very large mouth, like those of the Loire, the Elbe, or the Plata, that a river can peacefully mingle with the sea. Rivers even of this nature, however, sometimes experience the superior influence of the sea, which repels their waters into their bed. Thus the Seine forms at its mouth a bar of considerable extent; and the Garonne, unable to discharge, with sufficient rapidity, the waters which it accumulates, in a kind of gulf between Bourdeaux and its mouth, exhibits this aquatic mountain, stopped by the flow of the tide rolling backwards, inundating the banks, and stopping vessels in their progress both up and down. This phenomenon, termed the " Mascaret," Mascaret, is only the collision of two bodies of water moving in oppo site directions. Jefferson's Notes upon Virginia. Guettard, Mém. de l'Academie, 1758. Pontoppidan, Histoire Naturelle de la Norwege, i. 145. VOL. I. 38 |