184 French National Institute. 
has not been planted and watered by the hands of man. The 
evil is continually increasing, from the destruction of the 
canals which carry the water from the mountains; and the 
districts which are abandoned are impregnated with salt, 
which renders them for ever barren. 
But the meditations.of the sedentary naturalist may also 
contribute to the perfection of geography by views proper to 
direct the researches of travellers. 
M. Lacepede, by examining what is already known of 
Africa, by comparing the volume of the rivers which fall 
into the sea with the extent of ground upon which the rains 
of the torrid zone fall, and with the presumable quantity of 
evaporation, and, lastly, judging of the number and direc- 
tion of the chains of the interior by those which have been 
visited on the sea-coast of this great division of the globe, 
has offered some conjectures upon the physical disposition 
of the countries of the interior still unknown, and particu- 
larly upon the seas and large lakes which he presumes to 
exist there. He has also pointed out the routes which ap-- 
pear the most proper for conducting travellers most speedi i 
to the countries which remain to be discovered. 
There is another kind of conjectural geography, which 
endeavours to determine the antient state of countries by 
what we observe in them at present. 
M. Olivier with this view has examined what truth 
there was in the communication said to have formerly 
existed between the Black Sea and the Caspian. He thinks 
that it was in fact formed on the north af the Caucasus, 
and that it was the heaps of earth formed by the waters of 
the Conban, the Volga, and the Don which interrupted it. 
As the Caspian has not since then received from the 
rivers which fall into it enough of water to allow for its 
evaporation, it has always lowered jts level, and is at pre- 
sent 60 feet lower than the Euxine. 
It was in this manner that it separated itself Hae the 
great lake Aral, and left uncovered those immense plains 
of salt sand which surranund it to the north and the east. 
M. Dureau de la Malle, son of a member of the Insti- 
tute, has found in the Greek and Roman writers numerous 
testimonics 
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