128 SCIENCE AND FRUIT GROWING 



it resulted in but a slight lowering of their temperatures, caused 

 a considerable increase in the extent of the damage done, this 

 being probably due to the cells under the moister condition 

 becoming more full of water, and hence suffering more when 

 the water in them froze (cf. p. 131). 



Another view which is commonly held is, that much of the 

 damage done by spring frosts occurs during the thawing of the 

 frozen blossoms, a rapid thaw being the most harmful. That 

 this should be the case is not improbable, and harmonises with 

 what is known as to the necessity for very gradual heating in 

 the case of frozen meat, frost bites, etc. ; it also harmonises 

 with the practice of watering with very cold water plants (e. g. 

 chrysanthemums) which have been slightly touched by frost. 

 In the United States smoky fires, or smudges, are sometimes 

 started at dawn, to prevent the rapid heating of the frozen 

 buds by the sun. 1 On the other hand, evidence has recently 

 been adduced by W. G. Reed 2 which throws doubt on the 

 conclusion that rapid thawing causes damage. 



The three chief principles on which attempts have been made 

 to mitigate the damage done by spring frosts have been based 

 on (i) the formation of artificial clouds the smoke clouds, 

 or smudges, just mentioned which would impede radiation 

 from the ground, and hence cut off the supply of cold air; 

 (2) raising the temperature of the trees by means of radiant heat 

 or by hot gases, given out by fires in their vicinity ; and (3) raising 

 the temperature of the air by artificial heating. 



Direct experiments on the subject are difficult to make. 

 There are rarely two or three occasions in a season often not 

 one during several seasons when any experiment dealing 

 with the incidence of frost on the trees themselves could be 

 made, and on no two occasions are the conditions strictly similar, 

 so that comparisons are impossible, and the fallacious " post 

 hoc, ergo propter hoc " argument has often to be resorted to. 3 

 It is, at present, more profitable to examine the principles on 

 which such methods of prevention must be based; and, in the 

 first place, we have to admit some uncertainty as to how the 

 damage to the trees occurs : is it solely by the low temperature 

 of the air around them, or do they become cooled below the 



1 P. J. O'Gara, Farmers^ Bulletin 401, 1915. 



2 Pan-American Scientific Conference, Washington, 1917. 



3 For trials carried out in Gloucestershire and Kent, see F. G. Hooper, 

 loc. cit, and S. T. Parkinson, Journ. S. E. Agric. Coll., 1908, 235-281. 

 Of the numerous contributions on this subject in American Journals, a 

 comprehensive list up to 1917 is given by West and Edlefsen (loc. cit., 

 p. 126). See also U. S. Dep. of Agric., Weather Bureau, Publication 542. 



