72 HEAT. 



moisture in the air will differ from day to day, and 

 from hour to hour. This will depend a good deal on 

 the wind, whether it blows from the land or from the 

 sea. Sometimes it contains a cubic inch of water in 

 the form of vapor in every square yard, but generally 

 less. 



165. MIST AND FOG. These are 



What is the 1 . 



cause of mists aqueous vapor, rendered visible by the 

 and fogs? cooling of the air, as before explained. 

 When the air is saturated, the least cooling will pro- 

 duce a fog, as in the case supposed in paragraph 129. 

 When it is not saturated, more cooling will be required, 

 as in the case supposed in the subsequent paragraph. 

 The beautiful veil of mist, which forms in summer 

 nights over low places, is owing to the cooling of the 

 air below its point of saturation, which takes place 

 after sunset. 



166. MIXED CURRENTS OF AIR. The 



*" p henomena of mist > f g' clouds > and con - 



fogs by mixed sequently of rain, are more commonly 



currents of air . . . ., , , , 



owing to the mixture of cold and warm 

 winds or currents of air. When this admixture takes 

 place, the warm air becomes colder, and tends to de- 

 posit its moisture, and the cold air warmer ; and it 

 might be at first supposed that those two influences 

 would counteract each other. For example, if a cubic 

 yard of air at 100 mixes with a cubic yard at 50, they 

 both become 75, and it might be thought, that the 

 warming of the colder cubic yard would increase its 

 capacity for moisture, as much as the cooling of the 

 warmer cubic yard would diminish its capacity, and 



