MOISTURE IN THE AIR. 
97 
tlie temperature is below freezing, it falls as snow or 
hail. When the saturated air is cooled by grass 
blades or by other objects that radiate their heat 
freely, the vapor condenses on them as dew ; in freez¬ 
ing temperature, as frost. 
Fig. 53.—Various forms of snow crystals. 
To compute how much vapor there is in a cubic 
foot of air requires properly constructed tables, which 
the interested reader can obtain in any good work on 
Meteorology. My hygrometer gives very interesting 
results without showing the number of grains of 
water in a cubic foot of air. 
