598 Joitriial of Agriculture. [10 Sept., 19; o. 



where the extreme dryness of the air causes evaporation to be active. In 

 some of our most northern districts, though the summer is very warm, 

 severe spring frosts during October and November ha\e occasionally to 

 b'._' reckoned with, visitations which may in a couple of hours destroy the 

 result of a whole year's work, and by damaging the pruning wood even 

 injuriously affect the following year's crop. Nor are vine-growers the 

 only sufferers ; orchardists, tomato-growers, and all culti\ating tender 

 plants are equally interested in the 'question. 



Anything that can be done to prevent, or e\en to reduce, such damage 

 is well worthy of consideration, and there is no doubt that much can be 

 done in various ways. In the Journal for September, 1902 (Vol. I., p. 

 877) and November, 1903 (Vol. II.. p. 342) much valuable information is 

 given concerning the question by Messrs. R. Dubois and G. H. Adcock 

 respectively — to these growers are referred. It is to be regretted that 

 more has not been done on the lines therein indicated, for with few excep- 

 tions our vine-growers and orchardists pay but little attention to fro.st 

 prevention anfl the closely allied question of frost prediction. 



Conditions Controlling the Occurrence of Frost. 



Before recalling the leading preventive and palliative mea.sures it is well 

 to briefly review the causes which contribute to the general fall of tem- 

 perature which constitutes the phenomenon known as a frost. 



Cold is the absence of heat. When the loss of heat is sufficient for the 

 tiiermometer to fall below 0° C. or 32° F. water freezes and we have a 

 frost. It is to the sun that we owe the heat that is manifest in every 

 object around us. The surface of the earth absorbs this during the day 

 and during the night it Icses it by radiation into space. The rapidity of 

 radiation varies enormously according to circumstances ; if sufficiently 

 active or, in other words, if it be not checked by adver.se atmospheric 

 conditions, enough heat is lost foi the temperature at the surface of the 

 soil to fall below freezing point. The following are the principal causes 

 which prevent a rapid fall of temperature: — 



WitiJ . — Spring frosts do not occur on windy nights. The soil loses heat 

 by radiation more rapidly than the air, the lower layers of which cool 

 down by direct contact. This cold air being heavier settles in low places 

 v.'here a frost occurs. Wind, by mixing the different layers, prevents this. 



Opacity of the Atmosphere to Heat Rays. — It seldom freezes when the 

 sky is overcast. Clouds constitute a screen which prevents radiation in 

 much the same way as the loss of animal heat is prevented by a rug or 

 blankt-t. Den.se clouds are not necessary for this result — a slight haze, 

 even, may be sufficient to prevent a frost. This is one of the ways in 

 which smudge fires act, as we shall see presently. 



Latent Heat given off by Water on Condensation and Congelaiioti. — - 

 Whenever water changes its state the change is accompanied by either 

 absorption or liberation of heat. The fall on evaporation is particularly well 

 known. We have practical illustrations of it in our water bags and butter 

 coolers — a like absorption takes place on liquefaction (freezing mixtures). 

 The inverse changes, viz., tho.se from the gaseous to the liquid and from 

 the liquid to the .solid are accompanied by an exactly equivalent heat 

 transaction ; only in the contrarv direction — instead of heat being ab.sorbed 

 and the temperature falling, heat is liberated. This cannot, for obvious 

 reasons, bring about a rise in temperature such as would be manifest to 

 our .senses. Hence, this change is not so readily realized as the fall of 

 temperature on evaporation ; nevertheless it constitutes a most valuable 

 check. Once dew commences to deposit, a further fall of any consequence 



