48 



Farmers' Bulletin 1096. 



the dew point is found by referring to special tables, usually fur- 

 nished with the psychrometer. 



The psychrometer should be whirled and read in the shade. 



FIG. 23. A corner of a frozen orange dump after a cold season. Thousands of dollars 

 worth of otherwise perfect fruit a total loss through frost damage. 



Considerable heat is required to evaporate water, and the heat 

 removed from the bulb of the wet thermometer for this purpose 

 causes its temperature to fall. The less moisture already in the at- 



FIG. 24. Frozen oranges dumped in lemon grove to be plowed under for fertilizer. 

 Very little frozen fruit is used in this way, as most growers believe too much acid is 

 added to the soil. 



mosphere the more rapidly the evaporation goes on and the lower is 

 the temperature of the wet bulb. When the atmosphere is saturated 

 the readings of the dry and wet bulb thermometers are the same. 



o 



