3^ 



THE APPLE 



to a large extent. The sling psychrometer is used for measuring the 

 dew point, or the temperature at which dew will form. There is a 

 close relationship between the dew point and the minimum temper- 

 ature of the ^gj^ night, providing the observations are made 



late in the evening and the 

 sky remains clear, with but 

 little wind. 



The sling psychrometer. 

 This instrument consists of a 

 pair of thermometers provided 

 with a handle which permits 

 the thermometers to be whirled 

 rapidly, the bulbs being thereby 

 strongly affected by the tem- 

 perature and the moisture in 

 the air. The bulb of the lower 

 of the two thermometers is cov- 

 ered with thin muslin, which is 

 wet at the time an observation 

 is made. 



How to make an observa- 

 tion. The so-called wet bulb 

 is thoroughly saturated with 

 water by dipping it into a 

 small cup or wide-mouthed 

 bottle. The thermometers are 

 then whirled rapidly for fifteen 

 or twenty seconds, stopped, 

 and quickly read, the wet bulb 

 first. This reading is kept in 

 mind, the psychrometer im- 

 mediately whirled again and a 

 second reading taken. This 

 is repeated three or four times, 

 or more if necessary, until at least two successive readings of the 

 wet bulb are found to agree very closely, thereby showing that it 

 has reached its lowest temperature. A minute or more is generally 

 required to secure the correct temperature. These readings are then 



el^ 



FlG. io. A psychrometer 



An instrument that has proved of much value 

 in predicting frosts. (After Iowa State College) 



