130 



The Principles of Fruit-growing. 



of cold produced by evaporation. Suppose the dry 

 bulb marks 65 F., and the wet bulb 56, then 



65 - 56 = 9 C 



or the 



cold produced by evapora- 

 tion. This swinging and 

 reading of the pyschrom- 

 eter are done in the shade 

 in the open air when the 

 temperature of dew-point 

 is sought ; it should be 

 done rapidly and the ther- 

 mometers read promptly 

 The dry -bulb gives the 

 temperature of the open 

 air, and dew-point is de- 

 termined by reference to 

 tabulated figures." A com- 

 mon form of psychrometer 

 is shown in Fig. 11, but 

 inasmuch as this has a cup 

 of water connected with 

 the wet -bulb, it is not so 

 handy for whirling. Such 

 an instrument may be 

 fanned instead of whirled. 

 Hammon gives the fol- 

 lowing directions and fig- 

 ures for determining the 

 dew-point : " To obtain 

 the dew-point from the wet -and dry -bulb hygrom- 

 eter or psychrometer, moisten the muslin on the 



i'ig. 11. One form of wet- and dry- 

 bulb thermometer. 



