INTRODUCTION. XVil 
ments in favour of the identity of the Niger and Zaire were 
probably instrumental in bringing about the present expe- 
dition, in answer to this objection, has assumed the mo- 
derate height of 3000 feet for the source of the Niger 
above the surface of the ocean. This height, he observes, 
would give to the declivity or slope of the bed of the river, 
an average descent of nine inches in each mile throughout 
the course of 4000 miles. Condamine,” he adds, * has 
calculated the descent of the Amazons at six inches and 
three quarters per mile, in a straight line, which, allowing 
for its windings, would be reduced, according to Major 
Rennel’s estimate, to about four inches a mile for the 
slope of its bed.” And this descent is not very different 
from that of the bed of the Ganges; it having been as- 
certained from a section, taken by order of Mr. Hasi- 
ings, of sixty miles in length, parallel to a branch of the 
Ganges, to have nine inches of descent in each mile in 
a straight line, which, by the windings of the river was 
reduced to four inches a mile, the same as the bed of 
the Amazons: and this small descent gave a rate of mo- 
tion to that stream somewhat less than three miles an 
hour in the dry, and from five to six an hour in the wet 
season, but seven or eight under particular situations 
and under certain circumstances. If then, the Ganges 
and the Amazons flow at the rate of three miles in their 
lowest, and six miles in their highest state, with an average 
descent of no more than four inches a mile, while the 
Niger, according to the hypothesis, would have an average 
descent of nine inches a mile, the objection to the great 
length of its course in supposing its identity with the Zaire, 
would seem to vanish. It has been sufliciently proved, 
d 
