Jb INTRODUCTION^ 



As to foils : A ftoney, chalky, or fandy earth, it is 

 known, reflefts moft of the rays into the air again^ 

 and retains but few, by which means a confiderablc 

 acceflion of heat is derived to the air ; as, on the con- 

 trary, black, loofe, and fwampy foils, abforb moft 

 of his rays, and return few into the air. 



It is certain, that heat, communicated by the fum 

 to bodies on this earth, depends much upon other 

 circumftances, befides the dire£t force of his rays ; 

 thefe muft be modified by our atmofphere, and vari- 

 oufly refleded and combined by the adion of the fur- 

 face of the earth itfelf, to produce any remarkable 

 effe£ls of heat : So that, if it were not for thefe addi- 

 tional circumftances, it is much to be queftioned, 

 whether the naked heat of the fun would be very 

 fenfible. 



To this purpofe it is obferved by Ulloa, in his 

 Voyage to Peru, that on the weftern fhore of that 

 country, from Santa Maria de la Purilla to Lima, 

 it is winter on the mountains from January or 

 February to June, whilft it is fummer in the vallies ; 

 but from June to November or December, it is 

 winter in the vallies, and fummer in the mountains. 



It is found, by the fcieace of aftronomy, that 

 the fun is farther from the earth in fummer, than in 

 winter ^ but as the eccentricity of the earth's orbit 

 bears no greater proportion to the earth's mean 

 diftance from the fun, than 17 do to 1000, thi^ 

 fmall difterence of diftancc cannot occafion any great 

 difference of heat or cold. But the principal caufc 

 ©f the difference between the heat of fummer and 



that 



