GREAT EXTENT OF IVAPOBATiOW. 1G5 



Besides, filtration extends in a lateral direction far beyond 

 the bed of the 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. Now 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 

 dries, it draws from the river new portions of water ; and it 

 may be easily conceived that this continual alternation of va- 

 porization and lateral absorption must cause an immense 

 loss, difficult to submit to exact calculation. The increase 

 of these losses would be in proportion to the length of the 

 course of the rivers, if from their source to their mouth they 

 were 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 

 its source, throwing down more sediment in the lower than 

 in the upper part of its course, many rivers in hot climates 

 undergo a diminution in the quantity of their water, as they 

 approach their outlets. Mr. Barrow observed these curioua 

 effects of sands in the southern part of Africa, on the banks 

 of the Orange River. They have also become the subject 

 of a very important discussion, in the various hypotheses 

 that have been formed respecting the course of the Niger.* 



Near the Vuelta de Basilio, where we landed to collect 

 plants, we saw on the top of a tree two beautiful little 

 monkeys, black as jet, of the size of the sai, with prehensile 

 tails. Their physiognomy and their movements sufficiently 

 showed that they were neither the quato (Simia beelzebub) 



Geographers supposed, to* a long period, that the Niger was entirely 

 absorbed by the sands, and evaporated by the heat of the tropical sun, as 

 no embouchure could be found on the western coast of Africa to meet 

 the requirements of so enormous a river. It was discovered, however, 

 by the Landers, in 1830, that it does really flow into the Atlantic; yet 

 the cause mentioned above is so powerful, that of all the numerous 

 branches into which it separates at its mouth, only one (the Nun River) 

 is navigable even for light ships, and for half the year even those art 

 unable to enter. 



