46 



Farmers' Bulletin 1096. 



FIG. 20. Sling psy - 

 chrometer used 

 to determine the 

 amount of mois- 

 ture in the at- 

 mosphere. 



scale to make the 

 readings agree with 

 the standard should 

 be attached along- 







side all thermome- 

 ters which are not 

 strictly accurate. 



MEASUREMENT OF 

 ATMOS PH ER I C 

 MOISTURE. 



On an earlier page 

 of this bulletin ref- 

 erence has been made 

 to the impor- 



tant influence of the water vapor in the atmos- 

 phere on the amount of fall in temperature during 

 the night. A knowledge of the amount of mois- 

 ture in the atmosphere is therefore of considerable 

 value to the orchardist. 



The temperature of the dew point is a direct 

 index to the amount of water vapor in the atmos- 

 phere and also indicates the point at which dew 

 or frost will begin to form as the temperature 

 falls. 



The simplest instrument for accurately deter- 

 mining the temperature of the dew point is the 

 sling psychrometer. (See fig. 20.) This consists 

 of two ordinary thermometers mounted side by 

 side on an aluminum strip and provided with a 

 handle for whirling. The bulb of the lower ther- 

 mometer is covered with thin muslin. When an 

 observation is to be made, the muslin is thor- 

 oughly moistened in clean water and the instru- 

 ment is whirled rapidly for a short time. Imme- 

 diately after the whirling is discontinued both 

 thermometers are read as quickly as possible, the 

 wet-bulb thermometer first. These readings are 

 kept in mind or noted on paper and the psychrome- 

 ter is immediately whirled again and more read- 

 ings are taken. This is repeated several times, 

 until two readings of the wet-bulb thermometer 

 agree closely or until the wet-bulb temperature 

 begins to rise. In other words, it is desired to 



