16 Farmers' Bulletin 1096. 



temperature of the whole mass of surface air is raised slightly, al- 

 though remaining still relatively low. 



Another and probably the most important factor in protection by 

 heating is the amount of air movement near the ground. When the 

 air is calm the air warmed by the heaters remains over the fired 

 area and the maximum results in raising the surface temperature 

 are obtained. When the air is in motion, even though it is moving 

 only a few miles per hour, the heat is steadily carried away and a 

 greater quantity of fuel must be consumed to obtain the same effect 

 on the surface temperature. 



The matter of reinforcing the borders of an orchard with one or 

 two extra rows of heaters is of the greatest importance. When the 

 air is in motion, if there is no border row of fires, the heat from the 

 first two or three rows of heaters on the windward side is carried into 

 the orchard, leaving the outside rows practically unprotected. In 

 such cases the temperature in the outside rows may show a rise of 

 only 1 or 2 due to the firing, while the remainder of the orchard 

 may be benefitted by a 5 or 6 rise. To secure the maximum amount 

 of protection for border trees, a row of heaters, 10 feet apart, should 

 be placed about 40 feet to the windward of the outside row, with a 

 similar row about 20 feet to the windward. 



The smoke cover has very little influence on the* loss of heat by ra- 

 diation and the effect of smudge fires of damp straw or manure on 

 the temperature is practically negligible. However, a smoke screen 

 is of some value in shading the fruit and blossoms from the morn- 

 ing sun and preventing a too rapid thawing. 



Smudging and Pollination. In some deciduous fruit districts it 

 has been asserted that the smoke from the open oil heaters interferes 

 with pollination. However, the experience of a large number of 

 fruit growers, who for many years have smudged their trees while 

 in full bloom, does Hot bear out this contention. Pollination usually 

 takes place on the day the blossom opens and even if considerable 

 soot is deposited within the flower on the following night, no dam- 

 age results. As a matter of fact there is seldom enough soot de- 

 posited in a blossom to hinder pollination, even when firing is con- 

 tinued for several hours. 



During the seasons of 1917 and 1918 Mr. B. B. Lowry, of Medford, 

 Oreg., cooperating with the county pathologist, smudged six pear 

 trees, including practically all varieties grown commercially in the 

 district, every night from the time the buds began to open until 

 the fruit had set, in order to note the effect on pollination. Three 

 open lard-pail heaters were placed almost directly under each tree 

 and the blossoms were coated with soot to an extent that would never 

 be found in actual practice. The experiment was carefully checked 

 by the writer during the 1917 season. All the smudged trees bore 



