132 Principles of Plant Culture. 



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 containg rain water, 

 as shown. By means of a string attached to the board 

 or case at the end opposite the bulbs, 

 the instrument is whirled about in the 

 air a few times, after which the ther- 

 mometers are quickly read and the 

 difference in the readings noted. 

 When the air is comparatively dry, 

 evaporation from the muslin proceeds 

 rapidly, and on account of the heat 

 absorbed, the wet bulb 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 saturated air, evaporation 

 ceases and the two thermometers read 

 alike. By means of the following table, the dew point 

 for any ordinary out-door temperature and atmospheric 

 humidity during the growing season may be readily 

 determined. 



208. To Compute the Dew Point. Find the wet- 

 bulb depression by subtracting the wet-bulb reading 

 from that of the dry bulb ; locate this in the top line of 

 the table, and find the dry-bulb reading in the left hand 

 vertical column. Opposite the dry bulb reading, in 

 the column headed with the number indicating the wet- 

 bulb depression, is the dew point sought. 



FIG. 60. Sling psy- 

 chrometer, used 

 to foretell frost. 



