On Ntght-Frofifs. 47 



The heat produced hy the transformation of over-cooled mist into ice had 

 partly henetited the plant, preventing damage when later the ice was changed 

 into water, under the influence of the rays of Septemher sun, at which process 

 the plant had to émit part of the necessary heat. 



The results oi the ahove observations are such as to throw a light over 

 some phenomena hitherto not well explained. 



It is a fact well known by agricultuiists, that certain frosts never, or very 

 exceptionally, prove injurions. These frosts are generally characterized by the 

 fact that cereals, for instance, rye and barley, are coated over with ice, which 

 like a pod, surrounds the ear. For tins process there lias, of course, been 

 required a unich larger quantity of water than the layer of air inimediately 

 above the ears has been able to afford through condensation of its vapour. 

 This water must evidently bave originated elsewhere and the most natural 

 thing is to seek the cause in the fall of an over-cooled mist. 



This solution is further supported by the circumstance that such „pod- 

 frosts" — in Finnish „tuppi-halla" — occur at a time when the moisture of the 

 air is relatively high. 



From the following statement it will be easily understood that such frosts 

 can hardly cause damage. 



From the above it is evident that the over-cooled mist does not begin to 

 fall in a large quantity till late at night, or early in the morning. Hère, it 

 first causes a cooling of the layei' of air through which it falls and becomes 

 itself evidently a little warmer. 



This warming of the falling. over-cooled mist is, of course, considérable in 

 the beginning, but gradually diminishes, according as the lower layer of air is 

 cooled down. 



Towards morning the mist, upon l'eaching the ground, remains over-cooled 

 and, upon touching solid objects on the earth's surface, at once turns into 

 ice of 0". In this process, heat is given to the plant as well as to the sur- 

 roundings, in case their température has fallen below O". 



The conséquence of this process is that the layer of air which is above 

 the earth is cooled uniformly to a considérable height, probably to 20 or 30 

 m and its vapour falls to a pressure corresponding to the degree to which 

 the temperatui-e had fallen. As long as the mist continues to pour down, the 

 }'adiation of heat from the ground evidently diminishes in a high degree, for 

 the layer of air saturated by mist forms a sheltering screen, and if this con- 

 tinues un til sun-rise, it is self-evident that such a frost can do no damage. 

 Even though the plants be coated in a thick layer of ice, the latter will show 



