﻿and Fluctuations. 339 



an enormous lake or series of lakes, the river is many feet above 

 its level, and boats cross freely to and from the Madeira. Cas- 

 telnau's maps of this country are in this respect quite in accord 

 with Chandless^s description ; they both show that the land is, 

 for the time, virtually turned into an inland sea. Mr. Wallace's 

 description of the country adjoining the Rio Negro is to the 

 same effect ; the upper parts of the Amazon and of the Orinoco 

 exhibit the same peculiarity ; the very remarkable flatness of the 

 whole of this vast area retains the water, so that it runs off with 

 extreme slowness, and meantime much of it is flying off by evapo- 

 ration. The temperature is uniformly high, and the quantity 

 of aqueous vapour the air can contain is very great. Its elastic 

 force, indeed, tends to disperse it ; but aqueous vapour does not 

 easily diffuse itself through air that has already a high degree of 

 humidity ; it spreads itself into the superior strata slowly and 

 apparently with difficulty, even when the air above is far re- 

 moved from the point of saturation*. And here the only di- 

 rection in which it can spread is upwards : the Andes, to the 

 west, are impenetrable ; on the north the mountains of Vene- 

 zuela are nearly so; the ridge of elevated land on the south 

 offers an obstacle that checks, though it does not prevent, the 

 passage of air; and the trade- wind blowing fresh up the river, 

 not only drives it back on the east, but brings in new and com- 

 paratively dry airf, the elastic force of which the excessive eva- 

 poration rapidly increases. It would seem therefore probable 

 (and until we have more numerous and extended observations we 

 must rest on probability) that the barometer is high only during 

 this season of evaporation, that it is even low during the rainy 

 season, and that during the rest of the year, in the interval 

 between the draining off of the water and the return of the rains, 

 the barometer has a standard height concerning which, at pre- 

 sent, we have no reliable information; but now that steamers 

 run up the Amazon almost to the foot of the mountains, we 

 may hope that observations of the barometer and wet- and dry- 

 bulb thermometers will enable us before very long to speak with 

 more confidence J. 



It seems, however, as I have said, tolerably certain that the 

 barometric pressure in the upper valley of the Amazon is very 



* See a Note by the Astronomer Royal in Proc. of Brit. Met. Soc. 

 vol. i. p. 365. 



f At this season, too, the equatorial current over which the trade-wind 

 has passed is at its minimum of temperature, so that it supplies even less 

 vapour than usual. 



X The whole question, in its relation to the level of different stations on 

 the Amazon, is discussed at great length by Professor Orton in a paper on 

 " The Andes and Amazon " in Silliman's American Journal of Science, 

 2nd series, vol. xlvi. p. 203. 



