130 SCIENCE AND FRUIT GROWING 



being placed actually under the trees, so that the hot gases from 

 them pass between the branches. 1 In such a case direct heating 

 of the trees by contact with the products of combustion occurs, 

 and this must also occur to a limited extent in other cases where 

 the fires are placed near, instead of actually under the trees, 

 though under those circumstances, the bulk of the hot gases will 

 rise beyond the trees without having come into contact with them, 

 and such heating as has occurred will have been caused mainly 

 by the heat radiated from the fires. 



Heating by radiation, however, is a very inefficient and un- 

 economical method of heating : according to West and Edlefsen 

 (loc. cit.j p. 126) the column of hot air from these heaters rises 

 to a height of 50 to 75 feet, that is, far above the tops of 

 the trees, and with 100 heaters to the acre, thermometers 

 suspended from the branches of the trees show a rise of 

 only 4 F. 



As to whether oil or other materials, such as coal, brush wood, 

 etc., is best for making such fires, opinions seem to differ in 

 America, 2 and, of course, in this country oil is not likely to be 

 used for such a purpose, at any rate, at present. Where coal is 

 used, 50 to 60 Ibs. are required for each fire which has to last for 

 six hours, and about half a gallonjof oil is required to light it. 

 The uneconomical nature of the process may be estimated by the 

 fact that the heat from the burning of the oil used in lighting 

 three fires, if it were fully utilised, would suffice to raise the 

 temperature of a layer of air 20 feet deep over an acre of land 

 by about 10 F. 



The larger the fire, the less will be the direct heating of the 

 air at the level of the trees ; one large fire in a plantation would 

 result in the formation of a large volume of heated gases, which 

 would rise vertically to a great height, and involve a large in- 

 draft of cold air at the ground level, probably doing more 

 harm than good. For any direct heating of the air, the smaller 

 the fires, the greater their efficiency. The ideal form of heating 

 would consist of a large number of fires or flames, so small that 

 the heated gases from them would not rise above the tops of 

 the trees; they should be placed, not at the lower portions of 

 the ground, but near the higher portions, where the cold air is 

 being manufactured, and should be so arranged that this cold 

 air, as it flows downwards, passes the heaters, and has its tem- 



1 Church and Fergusson, loc. cit., p. 124. 



2 See West and Edlefsen, loc. cit., p. 126, and O'Gara, loc. cit., p. 128; 

 also Lewis and Browne, Oregon Expt. Station, Bulletin, no, 1911, and 

 Greene, Iowa State Coll., Bulletin, 129, 1912. 



