Frost and the Prevention of Damage by It. 



47 



obtain readings of the two thermometers after the wet-bulb ther- 

 mometer has reached its maximum depression. 



If the wet-bulb temperature falls to 32 F. and remains at that 

 point, the whirling should be continued for some time later, even 



FIG. 21. Improved high-stack heaters in place in orange grove. 



though two or three successive whirlings fail to cause it to read lower. 

 When the water in the muslin begins to freeze, sufficient latent heat 

 is liberated to keep the temperature at 32 until all the water on 



FIG. 22. Short-stack oil heaters in place in an orange grove. 



the thermometer bulb is frozen. After this occurs, the evaporation 



from the ice may cause the wet-bulb thermometer to read below 32. 



After the final readings have been made the wet-bulb temperature 



is subtracted from the dry-bulb temperature and the temperature of 



