Adding Moisture to the Air. 107 
cable only in cranberry bogs, and in places where 
vineyards are arranged to be flooded for the de- 
struction of phylloxera. 
The following remarks by Hammon indicate the 
nature of the problem :* 
“In places where irrigation can be used, it will 
be found of great value in protecting against frost. 
Let the water be turned on until the soil is 
thoroughly moistened. The evaporation of the water 
from the damp soil will tend to raise the dew- 
point. Since evaporation takes place near the sur- 
face this method is especially valuable in protecting 
low plants and shrubs, but has also been found 
very valuable in protecting citrous groves from freez- 
ing weather. The irrigating should be done at as 
early an hour as possible, preferably on the day 
preceding the night when frost is anticipated, and 
the ground kept thoroughly wet until danger from 
frost is passed.” 
“Moist soil, or localities that can be easily 
flooded for the purpose of protection, are to be 
preferred to dry sections of otherwise similar loca- 
tion; for the evaporation of the moisture from the 
sol, on dry, cold nights, will tend to raise the 
dew-point of the air and thus diminish the proba- 
bility of frost. 
“The irrigation of the hillsides about a valley 
in which protection is desired, and the growing 
* W. H. Hammon (Forecast official United States Weather Bureau), in 
“Frost, How and When to Prevent Injury Thereby;” also pub. in Cal. Frt. Gr., 
Feb. 8, 1896. 
