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PLANT PHYSIOLOGY AND ECOLOGY 



two are designed to be moved or whirled in the air. The same 

 principle is applied in each, viz., that evaporation produces a 

 decrease in temperature proportional to the amount of moisture in 

 the air. The dry-bulb thermometer is an ordinary thermometer, 

 while the wet-bulb one is covered with a cloth that can be moistened. 

 The former indicates the normal temperature of the air, the latter 

 gives the reduced temperature due to evaporation. The relative 

 humidity of the air is ascertained by means of the proper tables, 

 from two terms, i.e., the air temperature and the amount of reduc- 

 tion shown by the wet bulb. 



37. Sling and cog psychrometers. For field work the sling 

 and cog psychrometers are much more convenient than the station- 



FiG. 8. The cog psychrometer. The wet bulb is the one covered 



with cloth. 



ary form. They are generally considered to be more accurate also, 

 since the movement prevents the accumulation of moisture about 

 the wet bulb. Of the two, the cog psychrometer is the more con- 



