Chap, i:.] Caujes of Change in the ' Atmofabere. 481 



and confequently the fun's rays were incapable of difTi- 

 pating it j and they were fo faint, that in pafling 

 through it, when collected in the focus of a burning 

 glafs, they would fcarcely kindle brown paper *. 



A principal caufe of the varieties and changes of 

 temperature, and a moft powerful agent in producing 

 cold, is evaporation. On this fubject it is remarked,- 

 firft, that in our climates the evaporation is about four 

 times as great between the vernal and autumnal 

 equinox as in the reft of the year. idly. Other cir- 

 cumftances equal, it is increafed in proportion to the 

 difference between the temperature of the air and the 

 evaporating furface ; it is confequently leaft when they 

 are nearly of equal temperature. The former part of 

 this propofition muft be underflood with fome reftric- 

 tion; for if the air is more than 15 colder than the 

 evaporating furface, there is feldom any evaporation at 

 all, and the air will more frequently, in that cafe, de- 

 pofit moifture than receive it. jdly. The degree of 

 cold produced by evaporation is much greater when 

 the air is warmer than the evaporating furface, than 

 when the latter is the warmer of the two ; for in the firft 

 cafe the dilation of the vapour is increafed, and in 

 fecond, it is checked. The more vapour is dilated, 

 the more fire or heat it abforbs ; and hence it is coldtft 

 in an exhaufted receiver, where it abforbs moft. 

 Hence warm winds, as the harmattan, firocco, &c. arc 

 more deficcatory than cold winds. 4thly. Evaporation 

 is always increafed greatly by a current of air flowing 

 over the evaporating furface. Hence a calm day is 

 always warmer than one in which there is a ftror.g 

 wind f. 



*See Dr. Franklin's Meteorological Conjectures, 

 f Kirwan on Climate, c. I . 



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