28 farmers' Bulletin 1096. 



until they become hot and the residue is loosened, when they are re- 

 moved with tongs one by one and rapped against a post, causing the 

 residue to fall out. They are then examined for leaks by holding 

 them upward toward the sun. The work can be handled very rapidly 

 in this way and there is little danger of accident. 



Coal Heaters. Coal is burned in open piles on the ground, in wire 

 baskets and in specially designed sheet-iron heaters. Coal heaters 

 possess some advantages over oil heaters in that the heat can be 

 applied near the ground, there is no strong up-draft and usually not 

 much soot or smoke after the first few minutes of burning. On 

 the other hand, coal fires are often difficult to light and hard to regu- 

 late; the fires often have a tendency to smolder slowly or burn out 



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FIG. 13. Plan of orange grove where temperature records shown in figure 14 were 

 obtained. Dots represent orange trees spaced 20 feet apart ; crosses represent low-stack 

 oil heaters. The square marked II shows location, of thermometers in fired orchard. 

 The check station was located about 800 ifeet to the eastward to avoid the influence of 

 the fires. The air drift on cold nights was normally from a northerly direction. 



with a rush. When firing is required for several hours it is neces- 

 sary to replenish the fuel in the heaters. The labor costs are rela- 

 tively high, as a large number of men are required to attend to this 

 work. In California reserve supplies of coal for each heater are 

 kept in wooden boxes set under the trees. At the end of the sea- 

 son the heaters are placed on top of these boxes and are left out all 

 the year. 



The number of coal fires to the acre should be about the same as 

 when the lard-pail oil heaters are used. 



Lighting Equipment. Orchard heaters of all kinds can be lighted 

 easily and rapidly with torches burning a mixture of gasoline and 



