2G 



ELEMENTS OF GENERAL SCIENCE 



the cup is cooled by the cold metal until its capacity for water 

 vapor is less than the amount of such vapor present. Some 

 of the water vapor therefore condenses into the liquid form 

 and is seen on the surface of the cup. At the moment when 

 the first trace of moisture appears upon the surface of the cup 

 the adjacent space is evidently saturated ; that is, the tempera- 

 ture near the cup is that at which the water vapor present 

 saturates the space. By reference to the table (sect. 22) 



FIG. 18. Average relative humidity in the United States 

 The figures show relative humidity in percentages. After diagram by H. J. Cox 



the amount of water vapor present at saturation may be 

 ascertained when the temperature corresponding to the dew 

 point is known. In this case the temperature is secured by 

 observation of a thermometer which is used to stir the water 

 during the progress of the experiment 



Having determined the dew point and the temperature of 

 the air, it is necessary only to refer to the table in order to 

 learn both the amount of water present in the air and the 

 amount required for saturation. From these figures the rela- 

 tive humidity may be calculated as in the preceding problems. 



