Lakes differ very much in temperature, transparency, and in the colour of their waters. Lakes fed by the water of melted snows in summer are generally much colder than would be thought conformable with the season ; but the difference is principally in waters, which, being cold, remain at the bottom by reason of their greater density. Some lakes never freeze, which is owing to their great depth. This is the case with Loch Ness in Scotland, which is 810 feet deep in the deepest part. Lakes are not subject to tides ; at least the amount of tide, so far as observation goes, seems not to be ascertained.
The remarkable transparency of certain lakes is truly astonishing ; thus the waters of Lake Superior are so pellucid, that, according to Mr. Ileriot, the fish and rocks may be seen at a depth incredible to persons who have never visited these regions. The density of the medium on which the vessel moves appears scarcely to exceed that of the atmosphere, and the traveller becomes impressed with awe at the novelty of his situation. Elliot, in his Letters from the North of Europe,' says, " Nothing appears more singular to a foreigner than the transparency of the waters of the Norwegian lakes. At the depth of 100 or 120 feet, the surface of the ground beneath is perfectly visible; sometimes it may be seen wholly covered with shells, sometimes only sprinkled with them ; now a submarine forest presents itself to view, and now a subaqueous mountain." In the lake Wetter, in Sweden,
it is said a farthing may be seen at the depth of twenty fathoms.
With regard to the colour of lakes, It may be observed that it is sometimes very diffienit to account for the tints of large masses of water. The colour of the bottom, the depth, the shadows and reflected oolours of surrounding bodies, subaqueous vegetation, springs, and many other affect the colour of lakes.
Lakes perform a very important function in the economy of the earth. Rain does not always fall, and wore it net for lakes, both visible and subterranean, those great natural reservoirs, the greater number of rivers would be dried up in summer, and canals could not have a constant supply of water. The freshness and humidity which them sheets of water occasion by their evaporation are also eminently favourable to the vegetation in their environs. Many lakes are of sufficient extent to be navigated, and thus facilitate commerce and industry. The fisheries of some are very valuable, and others, by the salt obtained from them, are a mine of wealth. Finally, they most agreeably diversify the surface of the earth by the various appearances which they present of the beautiful and the sublime.