14 Additional Remarks dn the 



open air. The air being exhausted, to what other 

 invisible agent can the evaporation be attributed ? 



To no other than caloric, the subtle principle of 

 heat, which, uniting with the water, converts it into 

 an elastic vapour, lighter than air, which, therefore, 

 instantly ascends, and mixes with the atmosphere. 

 The principle of heat, then, seems to be the true 

 cause of this curious phenomenon. As agitation i« 

 also known to promote evaporation, so a brisk wind 

 jnay conspire with the heat of a clirqate in expediting 

 the process. 



Philadelphia, situated in north latitude 39° 57', 

 longitude west from London 75° 8' ; its mean heat 

 has been stated at 52 — 5 ; yet its extremes of heat and 

 cold considerably exceed those of London, or what 

 might be expected from its degree of latitude and 

 level surface. It seems also liable to still greater and 

 more violent transitions of temperature, and probably 

 , from similar causes to those which have been just , 

 mentioned. From the heat of the climate and vari- 

 able state of the winds, the quantity of rain, and con- 

 sequently of evaporation, must be much greater, patr 

 ticularly from the surface of the Atlantic Ocean, and 

 of the large lakes, marshes, and forests, according to 

 the point from which the wind blows. But the 

 greater the evaporation occasioned by the summer's 

 heat, the greater must be the absorption of caloric^ 

 or, in other words, the generation of cold, against thp 

 succeeding winter. 



