412 
PHYSICAL GEOGRAPHY OE 
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 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 ac- 
curate information on this subject. In a Brazilian work 
on the Province of Para, the Madeira is stated to flow 
2970 bra^as, 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. 
Martins calculates that 500,000 cubic feet of water 
per second pass Obidos. This agrees pretty well with 
my own calculations of the quantity in the dry season ; 
when the river is full, it is probably much greater. If 
we suppose, on a moderate calculation, that seventy-two 
inches, or six feet, of rain fall annually over the whole 
Amazon valley, it will amount to 1,500,000 cubic feet 
per second, the whole of which must either evaporate. 
