CLOUDY WEATHER. 169 



On the lOtli of May, their canoe being ready, they em- 

 barked to go up the Eio Negro as far as the mouth of 

 the Cassiquiare, and to devote themselves to researches 

 on the real course of that river, which united the Orinoco 

 to the Amazon. The morning was fine ; but, in propor- 

 tion as the heat augmented, the sky became obscured. 

 The air was so saturated by water in these forests, that 

 the vesicular vapours became visible on the least increase 

 of evaporation at the surface of the earth. The breeze 

 being never felt, the humid strata were not displaced and 

 renewed by dryer air. The travellers were every day 

 more grieved at the aspect of the cloudy sky. Bonpland 

 was losing by this excessive humidity the plants he had 

 collected ; and Humboldt, for his part, was afraid lest he 

 should again find the fogs of the Eio Negro in the valley 

 of the Cassiquiare. No one in these missions for half a 

 century past had doubted the existence of communica- 

 tion between two great systems of rivers ; the important 

 point of their voyage was confined therefore to fixing by 

 astronomical observations the course of the Cassiquiare, 

 and particularly the point of its entrance into the Kio 

 Negro, and that of the bifurcation of the Orinoco. With- 

 out a sight of the sun and the stars this object would be 

 frustrated, and they would have exposed themselves in 

 vain to long and painful privations. Their fellow-travel- 

 lers would have retupned by the shortest way, that of 

 the Pimichin and the small rivers ; but Bonpland and 

 Humboldt persisted in the plan of the voyage, which 

 they had traced for themselves in passing the Great Cata- 

 racts. They had already travelled one hundred and 

 eighty leagues in a boat from San Fernando de Apure to 

 San Carlos, on the Rio Apure, the Orinoco, the Atabapo, 



8 



