122 



PRINCIPLES OF PLANT CULTURE 



206. Foretelling frost. We know that sprinkling the 

 floor of a room cools the air, even though the water used 

 is no cooler than the air of the room. This is because the 

 air in taking up watery vapor absorbs heat, but this heat 

 is set free again when the watery vapor is precipitated. A 

 steam radiator gives out heat because the steam within 

 it is condensing into water. It follows that when the dew 

 point of the atmosphere is reached, a 

 very considerable amount of latent heat 

 is given off, which checks the fall of 

 temperature. The temperature of still 

 air, therefore, rarely falls much below 

 the dew point, and since the latter at 

 any given temperature depends upon 

 the amount of moisture in the air, if 

 we have an instrument capable of in- 

 dicating both the temperature and the 

 moisture of the air, we may compute 

 the lowest temperature to which the 

 atmosphere will be likely to descend 

 during any given night, and thus we 

 may foretell frost with some degree of 

 certainty. 



207. The sling psychrometer enables 

 us to determine both the temperature 

 and the moisture of the air. This instrument consists of 

 two accurately-graduated thermometers attached to a 

 board or case (Fig. 60). The bulb of one thermometer 

 is inclosed in thin muslin, which is wet just before using 

 the instrument by dipping the bulb in rain water, or is 

 connected with a small vessel containing rain water, as 

 shown. By means of a string attached to the board 

 or case at the end opposite the bulbs, the instrument 



FIG. 60. Sling psy- 

 chrometer, used to 

 foretell frost. 



