2 BULLETIN 821, U. S. DEPAKTMENT OF AGKICULTURE. 



in figure 1. On nights when minimum temperatures of 31° F. or 

 lower were recorded, firing was practiced in all orchards that were 

 provided with heaters. 



Fig. 1. — Thermometer shelter, showing the arrangement of the maximum and minimum thermometers, 

 and the thermograph from which the data presented in Table I were secured. The shelter faces the 

 north, and the instruments are placed about 3 feet from the ground. 



The Corona district consists of an area of about 5,000 acres of 

 citrus fruit, m an almost unbroken planting, upon a mesa which slopes 

 to the north and west. The orchards in which the studies discussed 

 in this bulletin were made are located in the center of this area. The 



