64 Ferrel on the Cause of the Inundation of the Nile. 
about the last of March, or about two months after the middle of 
the rainy season. 
The annual inundation of the Nile, itseems to me, can be very 
satisfactorily accounted for in the same manner. Wherever the 
source of this river may be, it can have little effect in causing 
the inundation, for it must be a very small part of all the trib- 
utaries which make up the Nile; and it is to the sources of the 
principal tributaries that we must look for the cause of the inun- 
ation. We have seen that at the southern part of lake Nyanza 
therefore, from May to November, must be between the parallels 
of about 5° and 17° north latitude. If now we examine a map 
of this region, it is seen that the great water-shed drained by the 
Blue Nile, and its tributaries, embracing nearly all of Abysinia, 
and also several important tributaries of the White Nile, is situ- 
ated principally between these latitudes. Hence the immense 
amount of rain falling in this region during the rainy season, 
must cause an inundation of the Nile, just as it does of the Oro- 
noco or of the Amazon. From what has been stated, the mid- 
dle of the rainy season here, must be about the first of August, 
and the greatest height of the lower parts of the Nile is about the 
first of October, so that the flood would have about two months 
to descend. From what we know’of the usual velocity of the 
currents of other rivers generally, this would be just about the 
time required. 
The rainy belt from November to May is perhaps mostly south 
of the equator, and the source of the Nile or some of its tributa- 
ries must extend into this belt during this season, else the Nile, 
flowing more than 1000 miles through a rainless region, from 
which it does not receive asingle tributary, however small, could 
not be supplied with water. This isan argument in favor of the 
hypothesis that the Nile has its source in lake Nyanza; but I 
think the water-shed of that lake, would not be more than su 
ficient to supply the Nile at low water, and that if ever the Ge- 
ography and Meteorology of this region shall be well understood, : 
the cause of the inundation of the Nile, will be found in lati- 
tudes farther north, as stated above. 
' Cambridge, Mass., Dec, 12, 1862, Nautical Almanac Office. 
