FIG. 5. 



therefore cause a condensation of its watery vapour. This 

 may be imitated by filling a large glass goblet with very cold 

 water, and bringing it into a very warm room, the outside 

 of the glass vessel will become covered with dew, although 

 previously quite dry ; the same may be shown by filling a 

 similar glass with water at the ordinary temperature, taking 

 care that the outside is dry, then stirring in an ounce or so 

 of nitre or Rochelle salt, which will cause the water to 

 become cold, and the outside of the glass to be covered, as 

 before, with dew. Dew falls fastest when there are no 

 clouds in the sky, as radiation then takes place from the 

 earth into space, and is not compensated for by a return of 

 radiation from the clouds to the earth. 



Clouds are formed by the partial condensation of vapour, 

 and are borne along by the wind, instead of falling as rain ; 



