SPRING FROSTS 129 



temperature of the air by themselves radiating away their heat ? 

 That radiation occurs from the trees cannot be questioned, but 

 it is very doubtful whether such radiation would lower their 

 temperature to any appreciable extent; the blossoms are for 

 the most part -situated many feet above the ground, and any 

 air cooled by contact with them would at once sink down, and 

 be displaced by warmer air, thus preventing any continuous 

 fall of temperature, such as occurs with vegetation on the surface 

 of the ground ; it seems very improbable that the temperature 

 of any small, thin objects, such as the pistils or petals of a 

 blossom, could ever fall much below that of the air surrounding 

 them, for even with much larger objects, such as the bulbs of 

 a thermometer, it would appear that differences of about 2 

 are the greatest which can be obtained in this way. 1 The 

 fact, that damage by frost is more severe in the lower portions 

 of the ground, where radiation is least active, is a strong argu- 

 ment against the view of that damage having been caused by 

 radiation from the trees themselves. 



It is very doubtful, therefore, whether smudges will, from the 

 mere fact of their being smoke clouds, have any effect in 

 reducing damage, unless they can be extended sufficiently, not 

 only to cover with a pall the trees themselves in the lower portions 

 of the ground, but also to cover the higher ground in the neigh- 

 bourhood, where the cold air is being manufactured. To do 

 this effectively in a country such as England, with its broken, 

 undulating surface, would be a very difficult matter. 



It is more by the heat generated, than by the smoke produced, 

 that such smudge fires are likely to do good, and this seems to 

 be fully recognised in America, where the operation is described 

 as " orchard heating." A significant observation, bearing out this 

 point of view, has been made, namely, that the benefit pro- 

 duced increases with the area treated, 400 fires in ten acres 

 being more effective than 40 fires in one acre : if it were a 

 question of a smoke cloud, this would not be so, but if it were 

 a question of heating, it would be, for the larger the area heated, 

 the further removed will the bulk of it be from the surrounding 

 unheated area. Indeed, the number of fires used is such that 

 a considerable amount of heat must be generated : 40 per acre 

 is a minimum about one fire to every four trees and 50 to 

 100 are frequently lighted; in some cases even, where the fires 

 consist of burning oil in three-gallon containers, there may be 

 as many as one to three of these fires to every tree, the fires 



1 Wells, loc. cit., p. 116. 



