32 CHANGES IN THE ATMOSPHERE. 



Those things which have the roughest surface, like the 

 stems and leaves of grass, cool most rapidly. The heat 

 thus radiated is sent out into the thin air, and, if there 

 are no clouds, is lost in vast space. The air which is 

 near to these blades of grass imparts its heat to them and 

 grows cold. The air thus becomes incapable of holding 

 in solution all the water it had dissolved, and deposits it, 

 in minute particles, upon the surface of the grass. The 

 radiation- goes on, and the moisture continues to be 

 deposited, till the blades of grass are covered with drops ; 

 and these drops are drops of dew. 



Now, just as, by placing a screen before a fire, we pre 

 vent the heat from being radiated into the room, and send 

 it back to the fire, so a screen of clouds stretched over 

 the earth prevents the heat received from the sun from 

 being rapidly radiated into the empty air, and thus 

 prevents the formation of dew. We find, accordingly, 

 that dew is formed only on clear evenings. 



99. Hoar-frost is formed in precisely the same manner 

 as dew, but at so low a temperature that the moisture 

 freezes as it collects on the radiating surface, and, instead 

 of forming round drops, shapes itself into slender needles 

 of ice. 



100. The Climate of a Country is the general effect of 

 the combined action of all the causes just spoken of, viz., 

 heat, moisture, wind, and of others still.* The husband- 



* Humboldt says : &quot; Tho expression climate signifies all those states and 

 changes of the atmosphere which sensibly affect our organs temperature, 

 humidity, variation of barometric pressure, a calm state of the air or the effects 

 of different winds, the amount of electric tension, the purity of the atmosphere 

 or its admixture with more or less deleterious exhalations, and, lastly, the degree 

 of habitual transparency of the air and serenity of the sky, which has an impor 

 tant influence not only on the organic development of plants and the ripening of 

 fruits, but also on the feelings and the whole mental disposition of man.&quot; 

 Cosmos, I. 313. 



