Of lakes which receive affluenta without having any visible outlet, the largest is the Caspian. The Aral, and the Dead Sea, or Lake Asphaltites, are also examples of this kind of lake, which is very com mon in Asia. Some of them are of vast extent, such for instance is the lake Terkiri in Tibet, 27 leagues long and 9 leagues wide, and the lake Hobo-nor, or Koko-uor, in the same country, whose surface is 240 square leagues. It was at one time thought that the saltness of certain lakes was due to the circumstance of their receiving the saline impuri ties of their affluents, which impurities could not escape for want of an outlet ; but on the one hand, the Dun-als in Segistan, which receives the IIelmund and has no outlet, is perfectly fresh; and on the other, there are many salt lakes which have no affluents, hence the saltness of lakes must have some other cause. The question has sometimes been asked, what becomes of tho excess of water brought into lakes having no outlet 1 Halley thought evaporation was all-sufacient to carry it off, and his opinion is highly plausible. If, however, it shall be found by actual experiment that a greater quantity of water is brought into a lake without apparent issue than can be carried off by evaporation, the natural conclusion will be, that the surplus is lost by infiltration or sub-aqueous drainage. Several of these lakes have for merly had outlets, but water has ceased to flow from them, because the lakes have sunk in consequence of receiving now a much smaller quantity of water than formerly. There are many lakes in Europe at the present day whose outlets are diminishing; such among others are the lakes Balaton and Neuaiedel in Huugary. The extent of surface of the former is very great compared with the quantity of water which it receives, so that the evaporation is rapidly diminishing the lake, and the river Sehio, which used to carry off its superabundant waters and pour them into the Danube, is now nothing more than a slip of bog; and as for the lake Neusiedel, it appears formerly to have communi cated with the Danube by the Raab, into which it emptied its waters, and with which it boa now no other communication than by a swamp. The Aral also, it is generally believed, once communicated with the Caspian.
Those lakes which have an outlet without any apparent affluent are fed by subaqueous springs, which, bursting out in a hollow, must fill it up before the waters can flow off in a stream. These lakes are generally situated at considerable elevations above the level of the sea.
Thus there is one on Monte Rotondo in Corsica, at an elevation of 9069 feet. From lakes of this kind some of the largest rivers take their rise; the Volga, for instance, springs from such a lake in the government of Tver in Russia. The lake Titicaca in the Bolivian Andes, is 12,000 feet above the sea-level, and is 120 fathoms deep, while the Caspian is below the sea-level, according to some, 100 feet. The sur face of Lake Superior is 600 feet above the sea-level, and in certain places the lake has a depth of 1200 feet; while not only this but all the other Canadian lakes show evident marks of their level having been originally considerably higher, and that they have been lowered by degrees. Lake Erie has a medium depth of only 60 to 70 feet,
and the shallowness there is little doubt is owing to the detritus brought down from the lakes above, and there deposited.
Lakes'which receive one or more tributary streams and have a visible outlet for their superabundant waters are the most common and the largest ; such are the lakes of Switzerland and of the north of Italy, the lakes Ladoga, Onega, Peipus, and Ilmen in Russia ; the Saima in Finland, the Wener in Sweden, the Enara in Lapland, &e. In Asia there are the Nor-Zaissan and the Baikal, &c. In Africa the existence of large lakes in the interior has been well established ; first, by the precise verification of lake Tehad, whose outfall is the Niger ; and more lately by the discoveries of Dr. Livingstone of the Ngami, of which the great outfall is the Zambezi ; by Captain Speke of the lake Nyassi, or Nyanza, which is considered by him as the true source of the Nile. In North America, Lake Superior, Lake Huron, Lake Erie, and Lake Ontario are examples of this kind of lake ; each of them receives several affluents; and the grand outlet of the whole is the river St. Lawrence.
Lakes owe their origin to different circumstances ; some from the sinking of the soil by the falling in of subterraneous caverns—such is the supposed origin of the Baikal ; others are caused by earthquakes— such a lake was formed in the province of Quito in 1797; some by tho fall of mountains, as the Oschenen-see in the canton of Berne ; or by lava currents damming up the stream, as the lakes Aidat and Cassiere in Auvergne, in France. Many are supposed to be the remains of the universal ocean which once covered the earth, and their waters origi nally salt, have become fresh from their receiving constant supplies of fresh water while the salt was continually let off by their outlets.
Almost all lakes are in progress of diminution, although this is not everywhere apparent. The detrital matter brought in by their affluents is imperceptibly filling up their beds ; and if regular observations were made, many provinces which owe much pf their prosperity to their lakes, would find the time fast approaching when these pieces of water will become mere pestilential marshes.
Certain lakes exhibit remarkable phenomena : thus some have float ing islands in them, as is the case with a small lake near St. Omer. The lake Gerdau, in Prussia, has a floating island, on which a hundred head of cattle may be seen pasturing. In the lake KoIk, in Osnabruck, there is a floating island, on which fine elms are growing. Some of those floating islands sink and rise again ; thus in the lake Mang in Smoland, a province of Sweden, there is a floating island which appeared and disappeared ten successive times between the years 1696 and 1766. Other floating islands are found in East Gothland and many other places. Some subterranean lakes are supposed to have become so by the formation and subsequent fixing of floating islands, which succes sively uniting have finished by forming a solid crust over the water.