MAGUE OP INSECTS. 
419 
the east the mouth of the Idapa, or Siapa, which rises on 
the mountain of Uuturan, and furnishes near its sources a 
portage to the Itio Mavaca, one of the tributary streams of 
the Orinoco. This river has white waters, and is not more 
than half as broad as the Paeimoni, 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 Cirnuri. 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 
bowlings, 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. W e 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 ; hut 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 hanks were so 
marshy, that it was not without much labour M. Bonplaud 
could get to a Carolinea prineeps loaded with large purple 
flowers. This tree is the most beautiful ornament of these 
forests, and of those of the Bio Negro. A 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 u modification of the atmosphere, which has 
an 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 distinctness 
on the azure sky . What is it that causes the want of homogeneity in the 
vertical strata of the atmosphere to disappear instantaneously ? 
2 e 2 
