THE AMAZON VALLEY 285 



that at the sea-level, Barra can be but very little elevated above 

 the sea. 



For the height of the country about the sources of the Rio 

 Negro, Humboldt is our only authority. He gives 812 feet as 

 the height of Sao Carlos ; he, however, states that the determina- 

 tion is uncertain, owing to an accident happening to the 

 barometer; I may, therefore, though with great diffidence, 

 venture to doubt the result. The distance, in a straight line, 

 from the mouth of the Rio Negro to Sao Carlos, is rather less 

 than from the same point to Tabatinga, whose height is 670 

 feet. The current, however, from Tabatinga is much more rapid 

 than down the Rio Negro, the lower part of which has so little 

 fall, that in the month of January, when the Amazon begins to 

 rise, the water enters the mouth of the Rio Negro, and renders 

 that river stagnant for several hundred miles up. The falls of 

 the Rio Negro I cannot consider to add more than fifty feet to 

 the elevation, as above and below them the river is not very 

 rapid. Thus, from this circumstance alone, we should be 

 disposed to place Sao Carlos at a rather less elevation than 

 Tabatinga, or at about 600 feet. My observations up the Rio 

 Negro gave consistent results. At Castanheiro, about five 

 hundred miles up, the temperature of boiling water was 212-4, 

 at the mouth of the Uaupes 212-2, and at a point just below 

 Sao Carlos, 212-0. This would not give more than 250 feet 

 for the height of Sao Carlos above Barra ; and, as we have 

 estimated this at 200 feet above the sea, the height of Sao 

 Carlos will become 450 feet, which I think will not be found 

 far from the truth. 



The velocity of the current varies with the width of the 

 stream and the time of the year ; we have little accurate infor- 

 mation on this subject. In a Brazilian work on the Province 

 of Para, the Madeira is stated to flow 2,970 bracks, or about 

 three and a half miles, an hour in the wet season. At Obidos 

 I made an observation in the month of November, when the 

 Amazon is at the lowest level, and found it four miles an hour ; 

 but this by no means represents the current in the rainy season. 

 On descending to Para", in the month of June, 1852, I found 

 that we often floated down about five miles an hour, and as 

 the wind was strong directly up the river, it probably retarded 

 us, rather than helped us on, our vessel not being rigged in 

 the best manner. 



