OF INSECTS. 419 



the cast the mouth of the Idapa, or Siapa, which rises on 

 the mountain of Uuturan, and furnishes near its sources a 

 portage to the Bio Mavaca, one of the tributary streams of 

 the Orinoco. This river has white waters, and is not more 

 than half as broad as the Pacimoni, the waters of which are 

 black. Its upper course has been strangely misrepresented 

 on maps. I shall have occasion hereafter to mention the 

 hypotheses that have given rise to these errors, in speaking 

 of the source of the Orinoco. 



We stopped near the raudal of Cunuri. The noise of the 

 little cataract augmented sensibly during the night, and our 

 Indians asserted that it was a certain presage of rain. I 

 recollected that the mountaineers of the Alps have great 

 confidence in the same prognostic.* It fell before sunrise ; 

 and the araguato monkeys had warned us, by their lengthened 

 howlings, of the approaching rain, long before the noise of 

 the cataract increased. 



On the 14th, the mosquitos, and especially the ants, drove 

 us from the shore before two in the morning. We had 

 hitherto been of opinion that the ants did not crawl along 

 the cords by which the hammocks are usually suspended : 

 whether we were correct in this supposition, or whether 

 the ants fell on us from the tops of the trees, I cannot 

 say ; but certain it is that we had great difficulty to keep 

 ourselves free from these troublesome insects. The river 

 became narrower as we advanced, and the banks were so 

 marshy, that it was not without much labour M. Bonpland 

 could get to a Carolinea princeps loaded with large purple 

 flowers. This tree is the most beautiful ornament of these 

 forests, and of those of the Bio Negro. We examined 

 repeatedly, during this day, the temperature of the Cassi- 



* "It is going to rain, because we hear the murmur of the torrents 

 nearer," say the mountaineers of the Alps, like those of the Andes. The 

 cause of the phenomenon is a modification of the atmosphere, which has 

 jm influence at once on the sonorous and on the luminous undulations. 

 The prognostic drawn from the increase and the intensity of sound is 

 intimately connected with the prognostic drawn from a less extinction of 

 light. The mountaineers predict a change of weather, when, the air being 

 calm, the Alps covered with perpetual snow seem on a sudden to be 

 nearer the observer, and their outlines are marked with great distinctnesi 

 on the azure sky. What is it that causes the want of homogeneity in tb 

 vertical strata of the atmosphere to disappear instantaneously ? 



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