245 
Now, multiplying this evaporation with the surface of 
the lake, it will produce a mass of 73,102,500,000,000 
cubical feet of water, which ascend annually to vapour 
into the atmosphere. But having received in the same 
space of time by the rain a mass of 157,901,400,000,000 
cubical feet, there will still remain an overplus of 
84,798,900,000,000 cubical feet of water for the eva- 
poration of the rivers and lesser lakes, and for the 
decomposition of the water absorbed by the vegetables 
and other phenomena. This proves, that even in adopt- 
ing suppositions the most unfavourable to my system, 
yet a sea of an extent like the Caspian or the Red Sea, 
in the middle of Africa, would not by evaporation lose 
half the quantity of water which the rain annually sup- 
plies, and that more than one halt would be left for the 
other kinds of absorptions which have been mentioned 
already; so that if these latter do not consume it, which 
is most probable, our African sea ought to be much 
larger than I have been indicating. 
I shall not speak of the depth of this sea, as it must 
depend on the form of the ground; but whatever be this 
depth, the sea will keep without any alteration, the over- 
plus of the 20 feet of water carried off by evaporation. 
These calculations, proving how impossible it is that 
the Niger should lose itself in the marshes of Wangara, 
explain what becomes of so many rivers, which we see 
taking a direction towards the centre of Africa, with- 
out seeing the final part of their course. 
They also prove the impossibility of this immense 
mass of water getting out by way of the Guinea coast, 
as has been pretended by a learned German. In fact, 
the N : ger and the Senegal take their source in the mourn- 
