These facts prove, that evaporation alone carries off a very considerable quantity of water, and it is from this great evaporation from the Mediterranean that the ocean continually flows thither through the straits of Gibraltar. It is difficult to estimate the quantity of water any sea receives; we should be acquainted with the breadth, depth, and rapidity of all the rivers which enter therein, how much they increase and diminish in the different seasons of the year, and how much it loses by evaporation; the last of which is most difficult; for even supposing it proportional to the surfaces, it must be more considerable in a hot than in a cold climate; besides, water mixed with salt and bitumen, evaporates more slowly than fresh water; a troubled sea more quickly than one that is tranquil; and the difference of depth has also some effect: in short, so many circumstances enter into this theory of evaporation that it is scarcely possible to calculate any exact estimations on it. The water of the Black Sea appears to be less clear and less saline than that of the ocean. There are no islands in it, and its tempests are more violent and more dangerous than in the ocean, because the whole body of its waters being contained in a bason, which has but a small outlet, when they are agitated, they have a kind of whirling motion which strikes the vessels on every side with an insupportable violence. Next to the Black Sea the greatest lake in the universe is the Caspian Sea, whose extent in length from north to south is about 300 leagues, and scarcely more than fifty broad. This lake receives the Wolga and some other considerable rivers, as the Kur, the Faie, and the Gempo; but what is singular, it does not receive any on its eastern side; the country on that side being only a desert of sand almost unknown. Czar Peter I. sent some engineers there to design a chart of the Caspian Sea, who discovered that its figure was quite different from that given by former geographers, who had represented it to be round, whereas it is very long and narrow. The eastern coasts of this sea, as well as the neighbouring country, were unknown: even the existence of lake Aral, which is 100 leagues distant from it towards the east, was doubtful, or at least thought to be a part of the Caspian Sea, so that before the discoveries of the Czar there was unknown land in this climate upward of 300 leagues long by 100 or 150 broad. Lake Aral is nearly an oblong, and may be 90 or 100 leagues long, by 50 or 60 broad; it receives two very considerable rivers, the Sideroias and the Oxus, but as well as the Caspian has no outlet for its waters; and it bears the further resemblance, for as the Caspian receives no river on the east, so lake Aral receives none on the west, from which we may presume, that formerly these two lakes were but one, and that the rivers having, by degrees, diminished, left a great quantity of sand and mud, and which forms the country that now divides them.