YOLUME OF THE AmAZON. 273 



atmosphere, fewer musquitoes, gi-eater salubrity, and ab- 

 sence of crocodiles, as also of fish, mark the region of these 

 black rivers." This is not altogether true. The Amazon 

 throughout is healthy, being swept by the trade - winds. 

 The branches, which are not so constantly refreshed by the 

 ocean breezes, are occasionally malarious ; the " white- 

 water" tributaries, except when they have a slack current 

 in the diy season, have the best reputation, while intermit- 

 tent fevers are nearly confined to the dark-colored streams. 

 Much of the sickness on these tropical waters, however, is 

 due to exposure and want of proper food rather than to the 

 climate. The river system of South America will favora- 

 bly compare, in point of salubrity, with the river system of 

 its continental neighbor.* 



As we might expect, the volume of the Amazon is be- 

 yond all parallel. Half a million cubic feet of water pour 

 through the narrows of Obidos every second, and fresh 

 water may be taken up from the Atlantic far out of sight 

 of land. The fall of the main easterly trunk of the Am- 

 azon is about six and a half inches per mile, equivalent to 

 a slope of 21' — the same as that of the Xile, and one third 

 that of the Mississippi. Below Jaen there are thirty cata- 

 racts and rapids ; at the Pongo de Manseriche, at the alti- 

 tude of 1164 feet (according to Humboldt), it bids adieu to 

 mountain scenery. Between Tabatinga and the ocean the 

 average current is three miles an hour. It diminishes to- 

 ward Para, and is every w^here at a minimmn in the dry 

 season ; but it always has the " swing" of an ocean cm-rent. 



* The average temperature of the -svater in the Lower Amazon is 81°, that 

 of the air being a little less. The temperature of the Huallaga at Yurimaguas 

 was 75° when the air was 88° in the shade ; in another experiment both the 

 river and air were 80°. The Maranon at Iquitos was 79° when the air was 

 90°. At the mouth of the Juima, Herndon found both water and air 82°. 

 In the tropics the difference between the temperature of the water and air is 

 proportionally less than in high latitudes. 



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