4 BULLETIX 821, U. S. DEPARTMENT OF AGRICULTURE. 
to provide a more complete combustion of fuel with a consequent 
small amount of smoke or smudge^ were tried during this period, 
but they were abandoned later, owing to mechanical difficulties and 
other causes. 
Usually one heater for every tree was provided, and in some cases 
two rows of heaters were arranged on the north and west sides of the 
orchards. The heaters were usually placed in the center of the 
square made by four trees, as shown in figure 3, so as to avoid as far 
as possible injuries to the foHage and fruits from the flames or from 
Fig. 2.— Thetypeofheaterinmost common use in the Corona district. From left to right in the fore- 
ground are sho^vn a gasoline torch for lighting the orchard heaters, a can for distributing oil to the 
heaters, and a style of heater of about 9 gallons' capacity. In the background stands a small oil 
tank. The oil is dra-svn off from the tank through a valve at its base, sho-mi at the right. 
close contact with the heaters. They were Ughted, as a rule, when 
the temperature in the orchards dropped to 31° F., although the 
temperatures at which lighting began depended somewhat upon other 
conditions than those of temperature and the time of night at which 
the low temperature was reached. For example, if the temperature 
dropped to 32° F. before midnight the heaters were usually lighted 
at this temperature. If the temperature did not drop below freezing 
until the early morning, as was frequently the case, the heaters were 
usually lighted at 30° F. In some cases when the temperature did 
not drop too near the danger point until 5 to 6 a. m. the heaters were 
not hghted until the thermometer reached 29° or 28° F. 
