LAKES are in the land what islands are in the sea ; they are sur rounded on all sides by land, as islands are by water. They are sheets of water of greater or less extent, and differ from lagoons in their origin, and from tanks and reservoirs by their being naturally formed, whereas the Latter are the works of man. From ponds and pools it is not m easy to distinguish them, it being difficult to draw the line between a large pond and a small lake. The feature by which perhaps they would be best distinguished is this, that a lake is fed by streams either flowing at time surface of the soil or eubterraneous, while a pond, though large, In only the accumulation of rain-water in sonic hollow. nue ponds are usually dried up in hot weather, while true lakes aro only temporarily diminished by heat.
Lakes have eometimes been divided into fresh-water lakes and salt , water lakes; though here again it is not easy to draw the lino between the two, as from the freshest to the most salt the degrees of saltness are very various.
The principal difference in lakes is this : some have no apparent affluents nor outlet, others have affluents without any visible outlet, some have an outlet without any visible affluents, and others again have both affluents and an outlet. Lakes without outlets have the level of their waters horizontal, that is, parallel to the curvature of the earth ; while those with affluents and outfalls are on the contrary more or less out of the horizontal level, sometimes, as in the Lago Maggiore, as much as three inches in a mile.
Lakes without apparent affluents or outlets are comparatively small, and yet they are, relatively speaking, more permanent than larger lakes, because, being fed chiefly by subterraneous springs, they are not liable to be filled by those deposits of earth and sand which are the main cause of the rapid desiccation of such lakes as receive the troubled waters of torrents and rivers. If we follow the usual custom and call all natural sheets of water lakes, then there are many lakes without affluents or outlet. Thus they are very numerous to the northward of the Caspian and in the plains which extend between the Ural Moun tains and the Irtish, as also in the great Steppe of Baraba between the Irtish and the Ob. But in truth the greater part of these are more
properly ponds, formed of the accumulated waters from rain and melted snow. The largest of them are not more than ten or twelve miles in circumference and six or seven feet deep ; indeed many of them are quite dried up towards the end of summer. Some are salt and yield considerable profit. Their saltness is not easily accounted for ; the more particularly as among and close to those that are salt there are many whose waters are quite fresh. The opinions of natural ists on the subject of salt lakes are very various, and no satisfactory theory has perhaps yet been offered. Small lakes of the kind of which we have been speaking, that is to say, such as have neither affluents nor outlet, sometimes occur in hollows resembling the craters of extinct volcanoes. There has been much controversy as to the existence of lakes in volcanic rocks. Dolomieu, Spallanzani, and others, asserted their existence, while M. Desmarest absolutely denies the possibility of lakes existing in the craters of extinct volcanoes. But near the present active volcano of Antueo (in the state of La Plata, not far from Mendoza), there is a lake of immense depth, at a height of from 4000 to 5000 feet above the sea, which has such an intimate relation with the volcano, that on every eruption the lake pours out a stream of muddy water, wasting the adjacent district. The little lake of Nemi, about 20 miles from Rome, is unquestionably formed in an old volcanic crater, and that it has increased since its first formation, is shown by its having engulphed a building of the time of Tiberius. The cele brated lake of Averno is, according to Ferber and Breislak, situated in an ancient crater ; as were those probably of Bolsena and Bracciano. The subject of lakes, considered geologically, has not yet received the attention it deserves. The connection of lakes with the main ocean is an important but little known question.- They are all basins enclosed by certain obstructions that act as dams, and which certain phenomena, such as earthquakes or volcanoes, may destroy, and allow the water to run oft London, there is every reason to suppose, stands on the site of what was once a lake of large size.