130 The Principles of Fruit-growing. 
of cold produced by evaporation. Suppose the dry 
bulb marks 65° F., and the wet bulb 56°, then 
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Fig. 11, One form of wet- and dry- 
bulb thermometer. 
65° — 56° = 9°, 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 
