EVAPORATION AND CONDENSATION. 297 



to decrease from below upwards.* Hence it is, 

 that the lower atmosphere is transparent, even 

 when the sky is overcast with clouds,! the ele- 

 vation of which is in proportion to temperature 

 that is, the plains of condensation are higher 

 within the tropical than middle latitudes ; while 

 in the latter their elevations vary with the season. 

 From the observations of Mr. Crosthwaite it 



* It was supposed by Leslie and Dalton, that the diminution of 

 temperature in the atmosphere, on ascending above the earth's 

 surface, was owing to its increased capacity for heat, caused by 

 augmented rarefaction from diminished pressure. But this is not 

 the only cause ; for it is well known, that large portions of Asia, 

 Africa, and South America, are from eight to ten thousand feet 

 above the level of the ocean ; whereas their average temperature is 

 much higher than that of isolated mountains of equal elevation 

 in the same latitudes. Mount Lebanon is not much higher than 

 the Plateau of Mexico ; yet the former is covered with snow 

 nine months in the year, while there is perpetual spring on 

 the Table lands of Mexico, about the same latitude and eleva- 

 tion. The truth is, that the greater warmth of the lower 

 atmosphere over extensive plains, than that of mountains, is 

 owing to the obstruction of the sun's rays by the earth's surface. 

 The stratum of air in contact with the earth becomes heated and 

 rises, when the next stratum is heated and rises, until the whole 

 atmosphere becomes warmed to a considerable height. Hence 

 it is, that the plain of perpetual congelation varies in height in 

 different parts of the world in nearly the same latitudes. 



f The dryness of the atmosphere is in proportion to the eleva- 

 tion of its temperature above the dew point, which decreases from 

 the surface of the earth to the region of clouds. This explains why 

 rain often ceases about mid-day, when the clouds are redissolved 

 by the increasing power of the sun ; and why the lower atmos- 

 phere is so often obscured in England by a misty rain after sun- 

 set during winter. 



