THE WATER OF THE AIR 25 



saturation at the observed temperature. In the problems and 

 illustrations which we have discussed in the two preceding 

 sections both of these factors have been given, but if we were 

 to attempt to ascertain the relative humidity experimentally, 

 we should have to ascertain both of them for ourselves. 



FIG. 17. A cumulus cloud 



This kind of cloud is formed by the condensation of moisture in columns of rising 



air. Note the characteristic lower edge of the cloud, indicating the level at which 



the dew point is reached 



A common experimental procedure for ascertaining the 

 amount of water in the air is as follows : Water is placed in 

 a bright metal cup, ice is added, and the whole is stirred in 

 order that the water may be evenly cooled. In a short time 

 water will be deposited on the outer surface of the cup in a 

 thin film, and as time passes it may accumulate in drops, as it 

 does more familiarly on a pitcher containing ice water. This 

 water has been deposited from the air because the space about 



