250 MOISTURE OF THE AIB,; ITS FALL. 



vents the fall of the temperature below the dew- 

 point. 



During the short nights of the early summer in 

 the inland parts of Germany, the temperature 

 seldom falls below the clew-point of the air. But 

 as soon as the nights begin to increase in length 

 towards the end of July, and still more hi August 

 and September, there is never a cloudless night 

 without a plentiful fall of dew. In October, 

 radiation often cools our soil even down to 0° C. 

 (32° I\) ; the vapour set down by the air then 

 solidifies ; and we have hoarfrost. You will often 

 see, a short time before the winter sets in, and at 

 the beginning of spring, the grass in meadows 

 covered with rime — a proof of the strong radiating 

 power of the grass — while the temperature has 

 fallen nowhere else below the freezing-point. 



Under the clear skies of the Tropics, the fall of 

 dew is often so plentiful that its effect is almost 

 like that of a smart shower of rain. 



The settlements of moisture from the air, of 

 which I have been treating, are brought on only 

 by means, and on the surface, of solid bodies which 

 are surrounded by the air. If a large mass of the 

 air is cooled down throughout, below the dew- 

 point, the water that is withdrawn from it does not 

 generally run together into drops, but forms 

 little bubbles (vapour-vesicles or cloud-bubbles), 

 which float in the air like those of soap-water, con- 



