ULLOA's voyage to south AMERICA. 58^7 



above this fpace, that is, thofe which have the greatefl degree of altitude, move with a 

 much lefs velocity than the winds under them. Nor is it uncommon in other climates, . 

 befides that of Valles, for thefe clouds to move in a direction contrary to the more ^ '^ 

 denfe ones below it. Thus it appears to me, that without the danger of advancing 

 irregular fuppofitions, the fpace of the atmofphere, where the winds generally blow 

 with the greateft force; is that where the large drops, commonly called rain, are 

 formed. 



Now, in order to explain the fingularity of this remarkable phenomenon, I conceive' 

 that, in fummer, when the atmofphere is moft rarefied, the fun, by the influence of 

 his rays, proportionally elevates the vapours of the earth, and gives them a greater 

 degree of rarefaction ; for his beams being then in a more perpendicular direction to 

 the earth, they have the power of raifmg them to a greater height. Thefe vapours, 

 on their touching the lower part of the atmofphere, where the winds blow with the 

 greateft force, are carried away before they can rife to the height required for uniting 

 into drops, and, confequently, no rain can be formed. For, as the vapours ifTue from 

 the earth, they are wafted along the lower region of the atmofphere, without any ftop ; 

 and the winds blowing always from the fouth, and the vapours being rarefied propor- 

 tionally to the heat of the fun, its too great activity hinders them from uniting. 

 Hence, in fummer, the atmofphere is clear, or free from vapours. 



In winter, the rays of the fun being lefs perpendicular to the furface of the earth, 

 the atmofphere becomes confiderably more condenfed, but the winds from the fouth 

 much more fo, as being loaded with the frigorific particles from the frozen zone, which 

 particles it communicates to the vapours as they ilTue from the earth, and, confequently, 

 renders them much more condenfed than in fummer : hence they are hindered from 

 riling with the fame celerity as before. 



To thefe muft be added two other reafons ; one, that the rays of the fun for want 

 of fufHcient aftivity diffipates the vapours lefs, fo that they rife much flower. The other, 

 that the region where the wind has its greateft velocity being, in this feafon, near the 

 earth, will not admit of their rifmg to any height ; and thus they continue contiguous 

 to its furface, where they ftill follow the fame diredion, and form the moift fog then 

 felt : and having lefs fpace to dilate themfelves than at a greater height, they, confe- 

 quently, fooner come into contad, and when fufficiently condenfed, defcend in a 

 garua. 



In the middle of the day the garna ceafes, being then difperfed, which proceeds 

 from the fun's rarefying the atmofphere, whence the vapours afcend and remain fuf- 

 pended at a greater height, and thus they are rendered more tenuious ; and being 

 raifed to a region where they have more room to dilate, they are fo far difperfed as to 

 become imperceptible. 



After all, it muft be owned, that both in fummer and winter, fome vapours muft 

 furmount the difficulty of the wind in that region where its velocity is greateft, and, 

 getting above it, afcend to a greater height ; though not indeed in the very part where 

 they firft reached this current of wind, but at fome diftance from it ; fo that thefe va- 

 pours are to be confidered, on one hand, as yielding to the current of the air, and, on the 

 other, as afcending in proportion to the rarefa6lion they have received from the rays of 

 the fun. Hence it follows, that thefe vapours cannot be tkofe which are moft con- 

 denfed, as the difficulty of rifing is always proportional to the degree of condenfation ; 

 and, at the fame time, their magnitude would render them more fufceptible of the im- 

 pulfe of the wind. So that thefe confequently being the moft fubtile or tenuious, on 

 having paffed that region, the celerity with which they were before carried upwards is 



4 F 2 decreafed, 



