128 The Principles of Fruit-growing. 
are. Some day the country will have charts of iso- 
phenal lines as well as of its isotherms. 
Local studies of this type must eventually come 
to be an important province of meteorological 
bureaus. Every state must ultimately be completely 
charted not only in respect to liability to frosts, but 
to other incidents of local climate and weather. 
The most reliable prediction of frost is given by 
readings from the wet- and dry-bulb thermometer, 
which measures the moisture in the air. Kedzie 
gives the following description of this thermometer :* 
“The sling psychrometer is a formidable name, but 
a simple instrument. It consists essentially of two 
thermometers, the bulb of one being left naked and 
kept dry, the bulb of the other being covered with a 
thin layer of cloth which is kept wet (‘wet-bulb’) 
during the time of an observation. By placing these 
‘dry-bulb’ and ‘wet-bulb’ thermometers side by 
side and comparing their readings, we may determine 
the amount of cold produced by evaporation, and thus 
measure the relative dryness of the air. If there is 
no evaporation the two thermometers will show the 
same temperature, but any evaporation will produce 
cold, and the more rapid the evaporation the greater 
the reduction of temperature. The drier the air the 
more rapid the evaporation, and the greater the cold 
caused by evaporation. The psychrometer, or the 
‘wet- and dry-bulb thermometer,’ affords the means 
for determining the amount of moisture in the air, 
* See also Horticulturist’s Rule-Book, 4th ed., 222. 
