THE AMAZON VALLEY. 
411 
This height I am inclined to believe quite great 
enough, from some observations I made with an accu- 
rate thermometer, reading to tenths of a degree, on the 
temperature of boiling water. This instrument I re- 
ceived from England, after leaving Para. The mean of 
five observations at Barra, some with river- and some 
with rain-water, gave 212*5° as the temperature of boil- 
ing water ; a remarkable result, showing that the baro- 
meter must stand there at more than thirty inches, and 
that unless it is, in the months of May and August, con- 
siderably more than that at the sea-level, Barra can be 
but very little elevated above the sea. 
Eor the height of the country about the sources of 
the Bio Negro, Humboldt is our only authority. He 
gives 812 feet as the height of Sao Carlos; he however 
states that the determination 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 
Bio 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 Bio 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 Bio Negro, 
and renders that river stagnant for several hundred 
miles up. The falls of the Bio 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 
