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- ajid dry-bulb thermometer,' affords the means 

 for .determining the amount of moisture in the air, 



*See also Horticulturist's Rule-Book, 4th ed., 222. 



