cuL rrv ATioN. 189 



robs the vapor in the atmosphere of its heat, and 

 causes it to be deposited on its own surface. It looks 

 as though the pitcher were sweating^ but the water 

 all comes from the atmosphere, not, of course, through 

 the sides of the pitcher. 



If we breathe on a knife-blade, it condenses in the 

 same manner the moisture of the breath, and becomes 

 covered with a film of water. 



Stone houses are damp in summer, because the 

 inner surfaces of the walls, being cooler than the 

 atmosphere, cause its moisture to be deposited in the 

 manner described. By leaving a space, however, 

 between the walls and the plaster, this moisture is 

 prevented from being troublesome, and if the space 

 is closed against the circulation of air containing 

 moisture there will be no vapor bi'ought in contact 

 with tlie cool surface of the wall, and therefore no 

 deposit of moisture. 



Nearly every night in the summer season, the cold 

 earth receives moisture from the atmosphere in the 

 form of dew. 



A cabbage, which at night is very cold, condenses 

 water to the amount of a gill or more. 



The same operation takes place in the soil. When 

 the air is allowed to circulate among its lower and 

 cooler particles, they receive moisture from the same 

 process of condensation. Therefore, when, by the 

 aid of under-drains, the lower soil becomes sufficient- 

 ly open to admit of a circulation of air, the deposit of 

 atmospheric moisture will keep the soil supplied with 

 water at a point easilv accessible to the roots of plants. 



