THE AMAZON VALLEY. 
413 
or flow out of the mouth of the Amazon ; so that if we 
increase the amount given by Martins by one-half, to 
take in the lower part of the Amazon and to allow for 
the whole year, we shall have the evaporation as one- 
half of the rain falling annually. 
It is a fact which has been frequently stated, and 
which seems fully established, that the Amazon carries 
its fresh waters out into the ocean, which it discolours 
for a distance of a hundred and fifty miles from its 
mouth. It is also generally stated that the tide flows up 
the river as far as Obidos, five hundred miles from the 
mouth. These two statements appear irreconcilable, 
for it is not easv to understand how the tides can flow 
tf 
up to such a great distance, and yet no salt water enter 
the river. But the fact appears to be, that the tide never 
does flow up the river at all. The water of the Amazon 
rises, but during the flood as well as the ebb the cur- 
rent is running rapidly down. This takes place even at 
the very mouth of the river, for at the island of Mexiana, 
exposed to the open sea, the water is always quite fresh, 
and is used for drinking all the year round. But as salt 
water is heavier than fresh, it might flow up at the bot- 
tom, while the river continued to pour down above it ; 
though it is difficult to conceive how this could take 
place to any extent without some salt water appearing 
at the margins. 
The rising of the water so far up the river can easily 
be explained, and goes to prove also that the slope of 
the river up to where the tide has any influence cannot 
be great ; for as the waters of the ocean rose, the river 
would of course be banked up, the velocity of its cur- 
