2S6 



PHYSICAL GEOGRAPHY OF 



Martius 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, or flow out of the mouth of the Amazon ; so 

 that if we increase the amount given by Martius 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 easy to understand how the tides 

 can flow 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 current 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 bottom, 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 current still forcing its waters onward ; 

 but it is not easy to see how the stream could be thus elevated 

 to a higher level than the waters of the ocean which caused 

 the rise, and we should therefore suppose that at Obidos, where 

 the tidal rise ceases to be felt, the river is just higher than the 

 surface of the ocean at the highest spring-tides. 



