Ferrel on the Cause of the Inundation of the Nile. 63 | 
the water in the lower part of the Nile, does not begin to rise 
until the latter part of June. Hence the water would be about 
seven months in flowing from this lake to the lower part of the 
Nile, notwithstanding this lake is about 4000 feet above the level 
of thesea. This would give a velocity for the flow of the water 
considerably less than one mile per hour, which is far less than 
the velocity of rivers generally, especially at the times of in- 
undation. 
In order to account for the Nile’s inundations, it is necessary 
to understand the causes of the rainy seasons, and the laws which 
govern them, in the region of the sources of the Nile, and its 
principal tributaries. Although we know but little of these 
from direct observations in the region itself, yet I think we may 
have a pretty correct idea of them from the observation of the 
laws which prevail generally at other places in the same latitude. 
It is well known that there is a belt surrounding the earth near 
the equator where the northeast and southeast trade winds meet, 
nearly one half the surface of the globe, very little rain falls; 
ut the vapor is carried to the latitude where the trades meet, 
where the ascending currents carry it up to a point where it is 
condensed, and hence nearly all the rain which would otherwise 
fall over the whole regions of the trade winds; falls in a narrow 
belt only a few degrees wide. This belt is not stationary, but vi- 
brates with the seasons nearly 1000 miles in latitude, having its 
Coming flooded by the immense amount of rain, an inundatior 
follows in that sivie which is at its maximum toward the mouth 
