CLIMATIC FACTORS : THE AIR 8 1 



Measurement of Atmospheric Moisture.— The psychrometer 

 and the hygrometer are the two instruments most useful to ecol- 

 ogists for this purpose. The former consists of two thermometers, 

 one of which has the bulb wrapped with a wet piece of cloth, and 

 both of which are aerated in some fashion, usually by whirling. 

 Evaporation from the wet cloth is controlled by the moisture in 

 the atmosphere, and the bulb is cooled in proportion to the evap- 

 oration. The dry bulb gives the temperature of the atmosphere, 



FlG. 31. A sling type of psychrometer for determining relative humidity 

 by the difference in temperature of the wet and dry bulb after whirling.— 

 Courtesy Friez Instrument Division, Bendix Aviation Corporation. 



and the difference between the dry and wet bulb readings gives 

 the wet bulb depression. Knowing the barometric pressure and 

 these two values, the relative humidity can be quickly determined 

 from standard tables or from nomograms. 108 The necessity for 

 aeration of the thermometers, usually accomplished by rapid ro- 

 tation, has led to the design of several "sling" type psychrometers. 

 Because these must be whirled, they require considerable space for 

 operation. The "cog" psychrometer, functioning like an egg beat- 

 er, can be used in much smaller spaces. 



The hygrometer is usually a continuously recording instrument 

 in which an arm marks on a rotating drum the stretching and con- 

 tracting of a strand of hairs, which respond to relative humidity. 

 The drum is so calibrated that relative humidity is recorded di- 

 rectly. Often the device is equipped to record the temperature 

 simultaneously and is then called a hygrothermograph. Naturally, 



