586 



sure. We could not guess what modification of 

 the atmosphere had caused this phenomenon, 

 which must not be confounded with the periodi- 

 cal replacing* of one species of insects by another. 

 The animated recital of the natives, however, 

 fixed our attention: we fancied we saw man 

 distrustful, uncertain with what he is menaced, 

 regretting his accustomed sufferings. 



The weather, when we left Esmeralda, was 

 very stormy. The summit of Duida was enve- 

 loped in clouds, but this mass of vapours, so 

 black, and so strongly condensed, still supported 

 itself at the height of nine hundred toises above 

 the surrounding plains. In judging of the mean 

 elevation of the clouds, that is of their lower 

 stratum in different zones, we must not con- 

 found the sporadic or solitary group with that 

 sheet of vapours, which, extending in a continued 

 body above the plains, terminates at a chain 

 of mountains. It is these sheets of vapours, 

 only, which can be considered as giving any 

 certain results ; solitary groups of clouds are 

 often ingulfed in the vallies by the sole effect of 

 descending currents. We saw clouds near the 

 town of Caraccas * at five hundred toises above 



* Below the Cross of la Guayra. (See vol. iii, p. 581 ; 

 and Obs. Astr., vol. i, p. 296.) I have entered into these 

 particulars on the height of clouds, to show how much it is to 

 be desired, that this height had been oftener determined by 

 aerostatic voyages. When the balloon ascends in the midst 



