GREAT EXTENT OF EVAPORATION. 
186 
■Besides, filtration extends in a lateral direction far beyond 
tfie bed of tbe river. The shore, which apears dry to us, 
imbibes water as far up as to the level of the surface of 
the river. We saw water gush out at the distance of fifty 
toises from the shore, every time that the Indians struck 
their oars into the ground. How these sands, wet below, 
but dry above, and exposed to the solar rays, act like 
sponges, and lose the infiltrated water every instant by 
evaporation. The vapour that is emitted, traverses the 
upper stratum of sand strongly heated, and becomes sensible 
to the eye when the air cools towards evening. As the beach 
hries, it draws from the river new portions of water ; and it 
may be easily conceived that this continual alternation of va- 
lorization and lateral absorption must cause an immense 
mss, difficult to submit to exact calculation. The increase 
°f these losses would be in proportion to the length of the 
c °urse of the rivers, if from their source to their mouth they 
'vere equally surrounded by a flat shore ; but these shores 
being formed by deposits from the water, and the water 
having less velocity in proportion as it is more remote from 
source, throwing down more sediment in the lower than 
111 the upper part of its course, many rivers in hot climates 
undergo a diminution in the quantity of their water, as they 
!l Pproach their outlets. Mr. Barrow observed these curious 
e ffeets of sands in the southern part of Africa, on the banks 
°f the Orange Biver. They have also become the subject 
a very important discussion, in the various hypotheses 
that have been formed respecting the course of the Niger.* 
Near the Yuelta de Basilio, where we landed to collect 
Prints, we saw on the top of a tree two beautiful little 
pmnkeys, black as jet, of the size of the sai, with prehensile 
Their physiognomy and their movements sufficiently 
s howed that they were neither the quato (Simia beelzebub) 
o Geographers supposed, f.„' a long period, that the Niger was entirely 
(1 sor bed by the sands, and evaporated by the heat of the tropical sun, as 
® embouchure could be found on the western coast of Africa to meet 
(, e ^'I'firements of so enormous a river. It was discovered, however, 
tli 16 handers, in 1830, that it does really flow into the Atlantic; yet 
b r e cause mentioned above is so powerful, that of all the numerous 
j 8 ,inc bes into which it separates at its mouth, only one (the Nun River) 
ii„ n 1 a , vigahle even for light ships, and for half the year even those are 
™* bl e to enter. 
