2 BULLETIN 821, U. S. DEPARTMEITT OF AGRICULTURE. 
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, sho-«ing 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, in 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 
