128 



Principles of Plant Cidtun 



we have an instrument capable of indicating both the 

 temperature and the moisture of the air, we may com- 

 pute the lowest temperature to which the atmosiihere 

 will be likely to descend during any given night, and 

 thus we may foretell frost with some degree of certainty. 



208. The Sling Psychrometer (psy- 

 clinim'-e-ter) enables us to determine 

 both the temperature and the moisture 

 of the air. This instrument consists of 

 two accurately-graduated thermometers 

 attached to a board or case (Fig. 60). 

 The bulb of one thermometer is inclosed 

 in thin muslin, which is wet just before 

 using the instrument by dipping the bulb 

 in rain water, or is connected with a 

 small vessel containing rain water, as 

 shown. By means of a string attached 

 to the board or case at the end opposite 

 the bulbs, tlic instrument is whirled 

 about in the air a few times, after which 

 the thermometers are quickly read and 

 the difference in the readings noted. "When the air is 

 comparatively dry, evaporation from the muslin pro- 

 ceeds rapidlj', and on account of the heat absorbed, the 

 wet buli> indicates a lower temperature than the dry one. 

 When the air is damp, evaporation is slower, and the 

 difference between the readings of the two thermometers 

 is less. In saturate<l air, evaporation ceases and the two 

 thermometers read alike. By means of the following 

 table, the dew point for any ordinary out-door tempera- 

 ture and atmospheric humidity during the growing sea- 

 son mav be readily determined. 



Fig. UO. Sling psy- 

 chroinetpr, used ti.) 

 foretell frost. 



