242 
that the Sahhara occupies one half of the space of the 
above surface; and if so, the rain on the other half will 
flow into the great interior lake. I hope that these con- 
cessions will meet with no opposition. We are, there- 
fore, to establish our calculation in the following man- 
ner: The supposed surface amounts to about 240,000 
square leagues, at 20 on the degree; but as we shall 
leave half of the above surface to the deserts, we have 
120,000 square leagues remaining, which furnish rain 
water to the great lake, and which, at the rate of 
292,410,000 square feet a-mile, will produce the sum of 
35,089,200,000 square feet, on which the rain deposits 
a mass of 157,901,400,000,000 cubical feet of water in 
the year. 
Allowing to our interior African sea 250 leagues in 
length by 50 in breadth, it will be as large as the Cas- 
pian, or the Red Sea, and form a surface of about 12,500 
square leagues, which are equal to 3,655,125,000,000 
square feet. 4 
The annual evaporation in Europe, according to 
Dobson, upon an average calculation of the temperature 
at 11°, amounts to 30 or 38 inches. Mr. Humboit has 
in America observed it in Cumana, at 2 8 0 temperature, 
to be 2780 millimeters per annum. It has been found 
at Guadeloupe to be 4 to 6 millimeters a-day; and the 
learned traveller believes, that it may be fixed at 80 in- 
ches per annum for the tropics. But in order to avoid 
all possible objections which might be started by the 
antagonists of this system, I will take a valuation against 
myself, by trebling the quantity adopted by Mr. Hum- 
bolt, and assume the evaporation to be 240 inches, or 
vO feet per annum, 
