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May the 10th, Our canoe had received it's 

 lading during the night ; and we embarked a 

 little before sunrise, to go up the Rio Negro as 

 far as the mouth of the Cassiquiare, and to 

 devote ourselves to researches on the real course 

 of this river, which unites the Oroonoko to the 

 Amazon. The morning was fine ; but, in pro- 

 portion as the heat augmented, the sky became 

 obscured. The air is so saturated by water in 

 these forests, that the vesicular vapours become 

 visible on the least increase of evaporation at the 

 surface of the Earth. The breeze being never 

 felt, the humid strata are not displaced and 

 renewed by dryer air. We were every day 

 more grieved at the aspect of the cloudy sky. 

 M. Bonpland was losing by this excess of damp- 

 ness the plants he had collected : and I for my 

 part was afraid, that I should again find the 

 fogs of the Rio Negro in the valley of the Cassi- 

 quiare. No one in these missions for half a cen- 

 tury past had doubted of the communication, 

 which exists between two great systems of 

 rivers ; the important point of our voyage was 

 confined therefore to fixing by astronomical 

 observations the course of the Cassiquiare, and 

 particularly the point of it's entrance into 

 the Rio Negro, and that of the bifurcation of 

 the Oroonoko. Without a sight of the Sun and 

 the stars this object would be frustrated, and 

 we should have exposed ourselves in vain to long 



