Lakes which have no communication with the ocean are less numerous than fresh water lakes, but generally exceed the latter in point of extent. The largest lake in the world, of this description, is the Caspian sea ; which ex tends from south to north about 300 leagues, with a mean breadth of fifty leagues. This lake receives, as we already noticed, the Volga, besides several other rivers of consi derable extent. It is no longer a matter of doubt that the surface of this inland sea is nearly 200 feet below the me dium level of the ocean. Edin. Phil. Jour. vol. iii. p. 409. Hence the waters carried into the Caspian must disappear by evaporation. Lake Aral, which is situated in the same sandy region with the Caspian Sea, possesses a similar character, receiving the Gihon, and some other rivers of inferior note, without having any apparent communication with the ocean. The geological constitution of the soil seems to indicate, that, notwithstanding the difference of i level in these waters, the Euxine Sea, the Caspian, and Lake Aral, communicated with each other, at a period beyond the times of authentic history. See the article BLACK SEA.
America contains a few lakes of a similar character, but situated on a higher level. The lakes of Mexico, and of the valley of Aragua, are supplied by various streams, without having any outlet to the ocean.
Lakes which have no communication with the sea, are uniformly impregnated with some kind of saline matter. The lakes of Mexico are found to hold in solution the muriates and carbonates of soda. The same salts exist in the waters of the Caspian Sea and Lake Aral, as well as in the Natron lakes of Hungary.—Humboldt's New Spain, ii. 36. Edin. Phil. Jour. vol. vi. p. 260.
Lakes, in general, but especially such as have no outlet for the waters which flow into them, must gradually be come less deep, by the alluvions they receive. The pro tess may be slow, but it is never retrograde. Besides the instance of the Palus Mareotis, record furnishes several examples of the filling up of lakes by depositions. The Tigris and Euphrates, in the time of Nearchus, formed an extensive lake near the sea. That lake is no longer to be found. The upper part of the Paludis, into which the Pallacopa led, below Babylon, now forms a plain, although it retains its former name of Bahr Nedjuff; that is, the sea or lake of Nedjuff. It has also been remarked, that the site of the Lake Mareotis still bears the name of Ba heira, or the Lake.—Rennell's Herodotus, pp. 69. 542. Humboldt's New Spain, ii. 115.
The level of the waters of lakes is considerably affect ed by winds, as well as by fluctuations in their supplies. When the east wind blows with any violence, the water of the Lake Tezcuco, on which the city of Mexico stands, withdraws towards the western bank of the lake, and leaves an extent of more than 600 metres (1968 feet) dry. (Hum boldt's New Spain, vol. ii. p. 35.) The Lake of Geneva is affected in a similar manner. The waters of Lake On tario are subject to periodical elevations, to the extent of seven or eight feet perpendicular, which are, perhaps, partly owing to the same cause. (Howison's Upper Ca
nada, p. 50.) Mr. Dalton informs us, that the surface of Derwent lake is sometimes agitated, when no wind can be perceived, in so violent a manner that it exhibits large waves with white breakers. The phenomenon is denomi nated a bottom wind, but the cause of it is utterly un known. (Dalton's Meteor. Essays, p. 52.) Lake \Vet ter, in Sweden, is occasionally affected in a similar manner.
alte-Brun, Geog. i. 311. Phil. Trans. No. 298, p. 1938.) Some lakes are subject to periodical changes in their condition, which seem to imply the existence of vast sub terraneous excavations below their beds, which act upon their waters in the manner of reciprocating fountains, swallowing them up at one time, and forcibly rejecting them at another. One of the most remarkable lakes of this description is the lake of Circhnitz, in Carniola, already described under that article. We know not how far the explanation given in that article is satisfactory ; but the existence of the subterraneous cavity, into which the waters retire, seems to be proved by the fact, that fishes and aquatic birds, which descend with the water, are afterwards ejected alive. Some of the ducks, thus cast out with the water, are supposed by Dr. Brown, to be hatched under the mountain Javornick, as, when make their appearance, though they can swim well, they are quite blind, and without feathers. Phil. Trans. No. 53, p. 1083.
We cannot dismiss the subject of lakes, without taking notice of the floating islands which, in some cases, are to be found on their surface. In the Mexican lakes these singular islands are called Chinampos, and are of an arti ficial nature. " The ingenious invention of chinampos," says Humboldt, " appears to go back to the end of the 14th century. It had its origin in the extraordinary situ ation of a people surrounded with enemies, and compelled to live in the midst of a lake abounding with fish, who were forced to fall upon every means of procuring sub sistence. It is even probable, that nature itself suggest ed to the Aztecs the first idea of floating gardens. On the marshy banks of the lakes of Xochimilco and Chalco; the agitated water in the time of the great rises carries away pieces of earth covered with herbs, and bound to gether by roots. These, floating about a long time as they are driven by the wind, sometimes unite into small islands. A tribe of men, too weak to defend themselves on the continent, would take advantage of those portions of ground which accident put within their reach, and of which no enemy disputed the property. The oldest chinampos were merely bits of ground joined together artificially, and dug and sown upon by the Aztecs. These floating islands are to be met with in all the zones. I have seen them in the king dom of Quito, on the river Guayaquil, of eight or nine metres (26 or 29 feet) in length, floating in the midst of the current, and bearing young shoots of bambusa, pistia, stratiotes, pontederia, and a number of other vegetables, PHY