INTRODUCTION, 25 



ftood to rpeak of them, when ^e talk of the tempe- 

 rature of fuinmer and winter. It may be afked then, 

 Is the heat of the fun ftrfl: communicated to the air, 

 and thereby to the earth ? No : The air is fufceptible 

 of a very fmall degree of heat, from the rays of the 

 fun palling through it ; for it is well known they 

 produce no heat in a tranfparent medium, and con- 

 fequently, that the air is only fo far heated as it differs 

 from a medium that is perfedly tranfparent. 



The heat produced by the rays of the fun, bear a 

 proportion to their number, their duration, and thcijL' 

 falling more or lefs perpendicularly ; and it takes place 

 at the points where they ftrike an opaque and non- 

 refleding fur face. 



The furface of the earth may, therefore, be con- 

 fidered as the place from whence the heat proceeds, 

 which is communicated to the air above, and the 

 earth below. That this is really the cafe, is evident, 

 from the fuperior degree of heat produced by the 

 action of the fun upon an opaque body, which v;ill 

 often be heated to 150 (Fahrenheit), while the tem- 

 perature of the air is not above 90. It may feem, 

 therefore, that, to meafure the heat communicated tp 

 the earth, it fhould be done at the furface. where the 

 action of the rays immediately takes place. 



But though the heat be produced at the furface, 

 it is communicated freely to the air as well as to the 

 earth, from the rays of light acting for a longer time 

 upon the fame parts of matter : Yet there is little 

 doubt that much the greater part is carried ofF, 

 which as it is heated flies off, and allows a fref^ 



portion 



