Lnyury from Cold During Growing Period. 123 
or case (Fig. 59). 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. 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 thermometers are quickly read 
and the difference in the readings noted. 
When the air is comparatively dry, evapo- 
Ml ration 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 thermom- 
eters is less. In saturated air, evaporation 
_& ceases and the two thermometers read 
Fie. 59. Sling psy- 
chrometer, used to alike. By means of the following table, 
foretell frost, the dew point for any ordinary out-door 
temperature and atmospheric humidity may be readily 
determined. 
Table for computing the Dew Point in Degrees Fahrenheit. 
Dry WET BULB DEPRESSION. 
Bus 
1 2 3 4 5 6 7 8 9 10 11 12 13 
36 32 30 27 24 21 18 134+ 8 —1—12 —32 
37 34 32 29 25 21 19 15 9 +3—7 —23 
38 35 33 30 26 23 19 17 11 +6—8 —16 sas 
39 36 34 31 28 24 20 16 14 8 0 —11l —3l1 
40 37 35 32 29 26 22 18 12 10 +3 —6 —22 
41 39 36 33 30 27 23 19 14 8+6—2 —15 wins 
42 40 37 34 31 28 25 21 16 10+3 —2 —9 —29 
48 41 38 35 33 30 26 22 18 13 +6 —3 —5 —20 
44 42 39 37 34 31 27 24 20 15 9 —1 —12 —18 
45 438 40 38 35 32 29 25 21 17, 11 +4—7 —27 
