24 Farmers' Bulletin 1096. 



the eighth rows east and west are fired. This has been necessary on 

 only two occasions -in four years. During the four seasons from 

 1914 to 1917, more than 580 heaters were never lighted at one time 

 on the 40 acres. The heaters are burned at their maximum rate at 

 all times. Mr. Jones advocates this system of firing on account of 



1. Easy and rapid lighting; one man can light 250 to 300 heaters 

 per hour. 



2. Easy and rapid refilling the next day. 



The firing on the 40- acre grove is easily handled by two men. 



In experiments carried on during the winter of 1918-19 in co- 

 operation with the Pomona Valley Frost Protective Association and 

 Mr. Jones it was found that the temperature 5 feet above the 



FIG. 9. Outer check line of fires on the north of Willis S. Jones orchard, photographed 

 about 3.30 a. m. on a cold morning. An exposure of about 15 minutes was required. 

 The absence of any flame on the right (north) side of the heaters indicates the steadi- 

 ness of the air drift. 



ground and TO feet inside the two outside check rows can be raised 

 from 2 to 4. With several check rows across the line of drift burn- 

 ing it is probable the temperature at the leeward side of the orchard 

 is raised a somewhat greater amount. In adopting this system of fir- 

 ing, the space between the check lines should contain at least one 

 heater to every two trees for use in an emergency, when the fires on 

 the check lines may fail to hold the temperature above the danger 

 point. 



The heated air from a fired orchard often drifts through neighbor- 

 ing orchards which are not fired, affording them in some cases even 

 more protection than the fired orchard itself. (See figs. 10 and 11.) 



Number of Heaters per Acre. The number of heaters to the acre 

 necessary for protection depends on the location of the orchard with 



