124 Principles of Plant Culture. 
Dry WET BULB DEPRESSION. 
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209. To Compute the Dew Point. Find the wet bulb 
depression, by subtracting the wet bulb reading from that 
of the dry bulb, and locate this in the top line of the table. 
Next, find the dry bulb reading in the left hand vertical 
column. Opposite this, in the column headed with the num- 
ber indicating the wet bulb depression, is the dew point 
sought. 
Example: Dry bulb reading.... 47° 
Wet bulb “* ... 40° 
Wet bulb depression. 7° 
Opposite 47°, in the left hand column, and under 7 in the 
top line, we find 28°,— the dew point. With these readings, 
toward sunset on a clear, still evening, we should expect 
frost, because the dew point is 4 degrees below the freezing 
point of water. A slight wind, a hazy atmosphere, or a few 
fleecy clouds would render the frost doubtful. With a dry 
bulb reading of 45°, and a dew point of 25°, a killing frost 
might be expected. 
