THE AMAZON VALLEY 2S5
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 21 2*4°, at the mouth of the Uaupes 2i2'2°, and at a point just below Sao Carlos, 2i2*o°. 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 bracas, 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.
