294 



BOUGUER's voyage to PERU. 



fage to the rays, is evidence that its fmall particles is fcarcely affeded by them. Indeed, 

 what impreilion can it receive, oppofmg no obftacle to their paffage ? According to 

 the obfervation I have formerly made, the light, when formed of parallel rays, lofes not 

 here below one hundred thoufandth part of its force traverfing a foot of free air. One 

 may judge from this, how few of the rays are deadened, or can aft upon this fluid, in 

 pafTmg through a bed whofe thicknefs (I will not confine myfelf to fay) is not an inch 

 or a line, but I will even fay, is not the fimple diameter of a molecule. In the mean- 

 time, fubtilty and tranfparency are the greater as more elevated : the natural eye, in 

 looking at obje£ts at a diftance in the Cordelier, will fometimes perceive this. Finally, 

 the groffer air heats below, by its contad with, or vicinage, to denfer bodies that it fur- 

 rounds, and upon which it creeps ; and the heat may communicate itfelf with it, nearer 

 and nearer, to a certain diftance. The lower part of the atmofphere, by this means, 

 daily contrafts a very confiderable heat ; and it will be the greater, to its degree of denfity. 

 But this, we know, is not the cafe at a league and a half, or two leagues above the fur- 

 face of the earth, although the light, when it pafles there, fhould be more vivid. The 

 air and the wind muft neceflarily then be very cold ; and the more elevated the fitua- 

 tion in the atmofphere, the more penetrating it muft be. 



Further, the heat we ftand in need of to exift, is not fimply that we receive every 

 inftant immediately from the fun. The momentary degree of this heat correfponds but 

 with a fmall portion of that which all the bodies which prefs upon us have contracted, 

 and by which ours is pretty nearly regulated. The adion of the fun does but merely 

 maintain in the fame ftate the aggregate of total heat, by fupplying the diminution it conti- 

 nually fuffers from the night. If the degrees added are greater than thofe loft, the 

 body of heat, as in fummer, will augment, and it will increafe more and more to a cer- 

 tain line ; but, conformably to what we have feen, this addition, or this total, thus to 

 exprefs it, of accumulated degrees, can never reach far up the fummit of a high moun- 

 tain, the moft elevated point of which is generally but of fmall fize. This is the 

 caufe why the changes in the thermometer were fo great upon Pichincha j while at Quito 

 they were fo little, and ftill lefs on the fea fide. The loweft ftate of the thermometer, 

 at every place, has always relation to the degree of heat imbibed by the foil, and this 

 quantity being very fmall upon the fummit of the mountain, the portion f«pplied by the 

 fun during the day muft necelTarily be found relatively greater. 



It is certain one may compare the heat the earth contrads by the conftant action of 

 the fun, with the moft part of other phyfical effeds, which augment by degrees, and 

 are comprehended in limits they cannot pafs. The degrees of augmentation which 

 refult from the complication of the whole, are never continually equal : thefe degrees, 

 principally, if confidered in the middle of their progrefs, go on diminifhing, till they 

 become nothing, or till the effed ceafmg to augment, reaches the utmoft verge of 

 accretion. .Now it follows from hence, that the leiTer the accumulated heat is, or the 

 further diftant from its maximum, the more augmentation it will admit in an equal tirnje 

 by the adion of even the fame agent. 



There is yet another Angularity peculiar to the elevated parts of the Cordelier, and 

 which arifes from the fame caufe ; and that is, when you pafs out of the ftiade into 

 the fun, a greater difference or alteration is felt in the temperature of the air than 

 here during our fineft days : there are times when every thing confpires at Quito to 

 render the fun exceedingly piercing ; one ftep only is neceflary into the ftiade, and we 

 are almoft fenfible of the cOld, a circumftance that could not be, were the body of 

 heat acquired by the earth much more confiderable. This explains why the fame 



thermo- 



