PROPERTIES OF GASES 91 



of the heat. When such unequal distribution of heat occurs 

 the air is called wind. By exposing a large canvas surface 

 to the wind, boats may be propelled. The farmer utilizes 

 the wind to turn a large wheel or windmill, and thus pump 

 his water from a well. 



92. Moisture in Air. Absolutely dry air is a thing un- 

 known in the natural world. The atmosphere is like a great 

 sponge. It greedily takes up water and gives it back only 

 when it has more than it can hold. Very few people realize 

 that water in the form of vapor is much lighter than air, 

 and that air containing a large proportion of water vapor 

 weighs considerably less than the same bulk of perfectly dry 

 air. 



The amount of water vapor in the air varies of course 

 with the temperature. In every cubic foot of air at 40 

 below the Fahrenheit zero, there is 

 oV of & gram of water. When the 

 atmosphere contains as much mois- 

 ture as it will hold it is said to be 

 saturated, and its humidity is 100%. 

 If it contains only one-fourth of what 

 it can absorb the humidity is 25%. 

 The average humidity of the United 

 States varies from 80% along the 

 coasts to less than 40% in Arizona and 

 New Mexico. A relative humidity of FlG - 55. Hygrometer, 

 less than 50% indicates a comparatively dry climate, 

 while a humidity of only 35% indicates the dryness of 

 the desert. 



The percentage of water in the air is measured by an 

 instrument called a hygrometer (Fig. 55). 



