BOUGUER's voyage to PERU. 2^1 



well as Lima, are never roofed ; they content themfelves with mats by way of a cover- 

 ing, over which they throw a fprinkling of foil or afhes, to abforb the dew and damps 

 of the night. 



That the extreme differences in the conftitution of th§ atmofphere, and the quality 

 of the foil of thefe countries, have a connexion with each other, cannot be doubted. 

 The nature of the foil has an influence upon the lower region of the air. Forefls, in 

 all hot countries, notwithftanding the iky may be ferene, and the air pure without, are 

 almofl always involved in a thick atmofphere. The fafl: is certain, becaufe it is vifible ; 

 befides, it is not difficult to explain the reafon of it. Trees, like the earth, which is 

 covered with corrupted vegetable and animal matter, and always expofed to exceffive 

 heat, is fubjeO: to a perpetual wafting. The evaporation takes the appearance of a fog 

 which rifes to no height, feldom above that of the trees, if we notice only the denfe 

 part of it J but the imperceptible parts afcend to a great height, fufficient, fuch is the 

 attraflive virtue of the wood, to conftitute a fpecies of communication between the 

 foreft and the clouds. The exhaled particles unite themfelves with the vapours which 

 form the clouds, which becoming, as it were, very fuddenly heavy, lofe their equili- 

 brium on the bed of the air in which they are fufpended. While it is raining, they are 

 in the midft of a fog ; that is to fay, the rain falls not ufually as it does here, breaking 

 from a cloud apparently high over our heads : more frequently in the forefts of the 

 torrid zone, all the upper and lower regions of the atmofphere are equally thick, or 

 furcharged. 



Whatever tends to the progrefs of phyfics, is certainly not foreign to the relation of 

 a voyage which has been undertaken to improve that fcience. I, therefore, fhall not 

 be afraid of joining to the recital of fa£ls, certain refledions arifing therefrom, from 

 which fome benefit may be derived. There is every reafon to induce a belief, that the 

 fmall particles of which the fogs and clouds are formed, are not fmall folid fpheres j 

 but that they are rather, fimply, bubbles of air. Were this not fo, it would be im- 

 pollible that the clouds ftiould rife, and that they (hould afcend to a greater height in 

 fummer than winter, when the air, lefs condenfed, is lefs capable of fupporting them. 

 Attention to every other circumftance, and even to the manner the evaporation of liquors 

 is accomplifhed, will confirm this truth. In fad, whatever internal agitation may be 

 imagined in any liquor that evaporates, the fmall particles which are thrown out, quickly 

 lofe all their motion by the refiftance of the air, if they were but fimply thrown out, 

 and ai e not pofleflTed of a lightnefs difpofing them to float and rife. 



Thefe fmall bubbles fufpended in the air may be diflblved in various ways to rain. 

 The wind, by driving them one againft the other, may jumble and break them. The 

 heat may become fo violent, that the bubbles, by too great an inflation occafioned 

 by it, may burft. A very contrary caufe will produce a very fimilar efi'eft, when the 

 air contained in thefe fmall hollow fpheres fliall experience a too great condenfation, ' 

 which will caufe fuch a diminution in the bulk of thefe fmall bubbles, that they cannot 

 buoy themfelvfe. When the wind coming from the fea, bringing a cloud along with 

 it, conducts it over a coaft covered with wood, it can affect but little alteration in the 

 heat. A wood reflets the rays of the fun but in a very trifling degree ; and it is cer- 

 tain that the heat, at a certain height above it, cannot be more intenfe than over the 

 fea. But the continual evaporation, we have obferved, of the woods, further the de- 

 fcent of the clouds and their diflblution ; whereas, in the environs of Lima, and to the 

 fouth of Guayaquil, nothing like this happens. 



The winds, which prevail moft in thefe parts, ordinarily blow from the fea and fouth- 

 v/eft : but when a cloud driven by this wind reaches the land, it becomes exppfed to a 



VOL. XIV. o o n^vr 



