Extensive accumulations of water, surrounded on all sides by the land, and having no direct communication with the ocean, or with any sea, are called lakes. Lakes are of four distinct kinds. The first class comprehends those which have no issue, and which do not receive any running water. These are generally very small, and do not merit much attention. The second class comprises those lakes which have an outlet, but which do not receive any running water. These lakes are fed by a multitude of springs; they are naturally on great elevations, and are sometimes the sources of great rivers. The third class of lakes is very numerous, consisting of all such as receive and discharge streams of water. Each of the lakes of this class may be looked upon as forming a basin for receiving the neighboring waters; they have in general only one opening, which almost always takes its name from the principal river which flows into it. These lakes have often sources of their own, either near the borders, or in their bottom. The great lakes of North America are of this class, which in point of extent resemble seas, but which, by the flow of a continual stream of fresh river water, preserve their clearness and sweetness. The fourth class of lakes present phenomena much more difficult to explain. We mean those lakes which receive streams of water and often great rivers, without having any visible outlet. The most celebrated of these is the Caspian Sea; Asia contains a great many others besides. South America contains the Lake Titicaca, which has no efflux, though it is the receiver of another lake. These collections of water appear to belong to the interior of great continents; they are placed on elevated plains, which have no sensible declivity towards the sea, and thus afford no outlet. With respect to those situated in a hot climate, evaporation is sufficient to carry off their excess of water.