THE EARIH. 14,3 



that velocity which maintains their future current. At first 

 thek course is generally rapid and headlong ; but it is retarded 

 in its journey, by the continual friction against its banks, by the 

 many obstacles it meets to divert its stream, and by the plains 

 generally becoming more level as it approaches towards the sea. 



ing-, 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. The Niger, if it does not touch the sea, most probably falls into a 

 lake of this kind, and not into a marsh. South America contains the lake 

 Titicaca, which has no efflux, althougli it receives another very consider, 

 able one into it. In short, these lakes appear to belong to the interior of 

 great continents ; they are placed on elevated plains, which have no sensible 

 declivity towards the sea, and which do not permit these collections of water 

 to open for themselves a passage through which they may flow out. But 

 why do these lakes, which are always receiving supplies of water, but have 

 no outlet, why do they not overflow their lianks ? We may answer, that 

 with respect to tliose which are situated in a hot climate, evaporation, as 

 Halley observes, is sufficient to carry off their excess of water. It remains 

 to be determined, whether the reasonings of this philosopher can with just- 

 ness be applied to a climate so cold for example as that of the Caspian Sea 

 Let us, in the first place, observe, that the quantity of water which the rivers 

 pour into this basin, has been exaggerated : there are no other great rivers 

 except the Wolga, the laik, and the Kur, which flow into it; the remainder 

 consists only of .small rivulets. We must add, that the whole of the eastern 

 iviast scarcely furnishes one rivulet to this extraordinary sea. And let us 

 al.to remark, (for in physical geography every fact is worthy of attention,) 

 that the Wolga, by no means a deep river, seems to be in part absorbed by 

 the ground which borders its course ; and it is this humidity which renders 

 these lands so distinguished for their fertility, when compared with the 

 neighbouring soil. Finally, were we determined to suppose th.it there is a 

 dUproportion between the extent of the Caspian .Sea and its evaporation, on 

 the one side, and the volume of water that it receives on the other, (which 

 we are far from blowing,) we have still to take into account the absorption 

 of its waters by the calcareous mountains whicli border it towards the south 

 and south west We know how porous and -spongy land of this kind is. Ail 

 accounts agree in describing the mountains to the south of the Caspian a i 

 being still more penetrated with moisture, and more abounding in springs 

 than those of Mingrelia, which proves either absorpticm, or (what is ot more 

 consequence,) a very strong evaporation. The insalubrity of the air near 

 tiiese lakes, is another circumstance which still farther confirms the opinion 

 of Halley. The physical phenomena which certain lakes present, have 

 always excited the astonishment of the multitude. 



The periodical lakes are the most common. Those which are formed by 

 excessive rains, and which are again dried up by the rays of the sun, by eva- 

 poration, or infiltration, appear to be scarcely worthy of our attention. In 

 Kurope these are nothing but pools, but between the tropics, these pools 



