XX INTRODUCTION. 
prehend. In what way, it may be asked, can the tempera- 
ture of a stream in 10° N. lat. affect the temperature of 
the same stream in 6° S. lat.? There is no assignable ratio 
in which it ought to encrease or decrease in its long course ; 
it may change daily, and many times in the course of the 
day according to the temperature of the surrounding at- 
mosphere. Of the temperature of the Niger nothing is 
known, for Park does not appear to have noticed it; but 
that of the Zaire was repeatedly ascertained, in the present 
expedition, in different parts of its course, and was sel- 
dom found to differ more than 2° of Fahrenheit either way, 
from the temperature of the atmosphere ; remaining most 
commonly about.76, and 77°, which was pretty nearly the 
mean day temperature of the atmosphere. 
The hypothesis of Mr. Reichard, a German geographer 
of some eminence, which makes the Niger to pour its 
waters into the gulf of Benin, is entitled to very httle atten- 
tion. The data on which it is grounded are all of them 
wholly gratuitous. He proceeds on a calculation of the 
quantity of water, evaporated from the surface of the lakes 
of Wangara, and the quantity thrown into them by the 
Niger, without knowing whether the Niger flows into 
them or not, or even where Wangara is situated, much 
less the extent and magnitude of those lakes. The Rio 
del Rey, the Formosa, and the numerous intermediate 
branches that open into the gulf of Benin, are supposed to 
join in one great stream beyond the flat alluvial land which 
they seem to have formed ; the supposition, however, has 
never been verified by observation; but as far as it is 
known, the Rio del Rey proceeds from the northward, and 
