XVili INTRODUCTION. 
however, that the velocity of rivers depends not on the 
declivity of their beds alone, but chiefly on the mass and 
velocity of the water thrown into their channels at the 
spring head, and the supplies they receive from tributary 
branches as they proceed in their course. In the Amazons, 
the Ganges, the Senegal, the Gambia, and in every river 
whose course, in its approach to the ocean, lies through a 
low country, it will be found, that the rise of a few feet in the 
tide is sufficient to force back, up an inclined plane, by its 
mass and velocity, the whole current of the river to the 
distance of several hundred miles, and the farther in pro- 
portion to the narrowness and depth of the channel beyond 
its funnel shaped mouth. In estimating the probability, 
therefore, of the identity of the Zaire and the Niger, as far 
as the length of their course may be supposed to offer an 
objection, we should inquire rather into the supply of 
water than the declivity of the country through which it 
would have to pass. In this respect, the Niger would be 
placed under very peculiar circumstances; its course, 
lying on both sides of the Equator, and through a consi- 
derable portion of both tropical regions, would necessarily 
be placed, in one part or other, under the parallels of 
perpetual rains, and consequently receive a perpetual sup- 
ply of water. Now all the representations that have been 
given of the lower part of the Zaire, describe it as being 
nearly in a perpetual state of; flood, the height in the dry 
season being within nine feet of the height in the season of 
heavy rains; whilst the difference in the height of the Nile 
and the Ganges, at the two periods, exceeds thirty feet. 
The flooding of the Zaire is therefore periodical, its highest 
state being in March, and lowest about the end of August; 
