252 MOISTURE OF THE AIR; ITS FALL. 



vapours do not become visible at all, although the 

 evaporating process is going on more quickly than 

 ever. If, however, the air is already moist, the 

 mist is formed in dense masses, and spreads to a 

 considerable distance. When, therefore, we see 

 mists hanging over the tops of hills, and con- 

 tinually increasing in bulk, we conclude that we 

 shall soon have rainy weather. 



In Germany mists are formed plentifully in 

 spring, but most so in autumn, when the cool 

 nights, following warm days favourable to evapo- 

 ration, bring on a general cooling of the air below 

 the dew-point. This withdrawal of the moisture 

 from the air in the form of cloud-bubbles, and the 

 running together of these into real drops, which 

 then quickly fall by reason of their greater weight, 

 takes place the more perfectly, the sharper the cool- 

 ing has been during the night. But that this cool- 

 ing by radiation may go on briskly, it is necessary 

 that the air be very pure, and free from vapours in 

 the higher regions, which in Germany occurs chiefly 

 with a north-east wind. There then it would be 

 concluded from the formation of such morning 

 mists that there will be a continuance of fine 

 weather. When the nightly cooling in autumn is but 

 slight, this is generally the result of the atmosphere 

 being already more or less bedimmed with vapour. 

 The south-west wind has perhaps already set in in 

 the upper regions, while the weather-vane is still 



