419
PLAGUE OF INSECTS.
the east the m o u t h o f t h e I d a p a , o r Siapa, which rises on the m o u n t a i n o f U u t u r a n , and furnishes near its sources a p o r t a g e t o the Rio M a v a c a , one o f the tributary streams o f the O r i n o c o . This river has white waters, and is n o t m o r e than half as b r o a d as the P a c i m o n i , the waters o f which are black. I t s u p p e r course has been strangely misrepresented o n maps. I shall have occasion hereafter t o m e n t i o n the hypotheses that have given rise to these errors, in speaking of the source o f the O r i n o c o . W e s t o p p e d near the raudal o f Cunuri. T h e noise o f the little cataract augmented sensibly during the night, and o u r Indians asserted that it was a certain presage o f rain. I recollected that the mountaineers o f the A l p s have great confidence in the same p r o g n o s t i c . * I t fell before s u n r i s e ; and the araguato m o n k e y s had warned us, b y their l e n g t h e n e d b o w l i n g s , o f the approaching rain, l o n g before the noise o f t h e cataract increased. O n the 14th, the m o s q u i t o s , and especially the ants, drove us from the shore before t w o in the m o r n i n g . W e had hitherto been o f opinion that t h e ants did n o t crawl along the c o r d s b y which the h a m m o c k s are usually suspended : w h e t h e r w e w e r e c o r r e c t in this supposition, o r w h e t h e r the ants fell o n us from the t o p s o f the trees, I c a n n o t say; b u t certain it is that w e hud great difficulty t o k e e p ourselves free f r o m these t r o u b l e s o m e insects. T h e river b e c a m e narrower as w e advanced, and the banks w e r e so marshy, that it was n o t w i t h o u t m u c h labour M . B o n p l a n d could g e t t o a Carolinea princeps loaded with large p u r p l e flowers. This tree is t h e m o s t beautiful ornament o f these forests, and o f those o f t h e Rio N e g r o . W e examined repeatedly, during this day, t h e temperature o f 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 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