22 Farmers' Bulletin 1096. 



depends on the distance from the heater. With increasing distance 

 the radiation intercepted decreases very rapidly. 



Experiments conducted by Prof. H. H. Kimball show that when 

 the high-stack heater is burning at a moderate rate with only the 

 lower section of stack red-hot, the heat radiated directly to the tree 

 is sufficient to counterbalance outgoing radiation at a distance of 10 

 feet. When the entire stack is red-hot the outgoing radiation will be 

 counterbalanced at a distance of about 15 feet. 



With the short stack and lard-pail heaters radiation is not of so 

 much importance unless the plants to be protected are very near the 

 heater. 



A large percentage of the radiant heat given off by an orchard 

 heater is lost directly to the sky without appreciable effect on the 

 temperature of the air or of the plants. As radiant heat travels in 

 straight lines and is completely absorbed or reflected by fruit and 

 leaves, any fruit shaded from the heaters by leaves or branches can 

 receive practically no direct benefit from the radiated heat. It is 

 plain, therefore, that to obtain the greatest amount of protection 

 from the same amount of fuel, heaters which are most efficient in 

 raising the temperature of the air should be used if possible, rather 

 than those which radiate most of their heat and are not so effective 

 in raising the temperature of the air. 



Distribution of Heaters. For the best distribution of the heat 

 throughout the orchard it is better to have the heaters placed in 

 every row, if possible, instead of concentrating them in every fourth 

 or fifth row. This makes for a more general intermixing of the 

 warmed air from the heaters with the cold air surrounding them. 

 If rows of heaters some distance apart are lighted through an orchard 

 on a calm morning, from the edge of the fired area it is possible to 

 note " arches " in the smoke over the fired rows, with depressions in 

 between. If the air is moving steadily from one direction, even 

 slowly, the heat will be spread out and mixed so that this " chimney " 

 effect will not occur, even if rows some distance apart are lighted. 



In some parts of southern California, where the air drift is prac- 

 tically always continuous from the same direction during a cold 

 night, firing along " check lines " is practiced. Mr. Willis S. Jones, 

 of Claremont, Calif., is the originator of this plan and has had great 

 success with it on his own 40-acre orange grove. His plan is as 

 follows (see fig. 8) : 



The air movement in his grove is generally steady and is normally 

 from the north. It sometimes shifts to the northeast and east, but 

 practically never blows from a southerly or westerly direction. On 

 the northern and eastern borders of the grove three short-stack citrus 

 heaters are placed to each tree, and on the line immediately inside 



