107 



about 2 p.m. ; rising again till 8 p.m. and falling by slow degrees 

 until morning. 



The degree of humidity determines the dew-point. When 

 the temperature has fallen so low that the air is saturated, it 

 cannot remain in form of vapour if the temperature is still 

 falling, but turns into water. 



The evaporation ceases as soon as the dew-point is reached, 

 as it probably does long before the temperature attains 32 deg., 

 F., and instead of an absorption of heat by producing vapour, 

 heat is now created by condensation. 



When summing up all the acting and counteracting causes 

 of lowering of temperature on a clear night, we get among the 

 former in the first place, radiation of heat ; in the second place, 

 movtvients in the air, caused by the cooling of air through its 

 touching the plants and its running down into the lowest places. 



As counteracting causes we have in the first place rondensa- 

 tion of aqueous vapour in the atmosphere in general, by which the 

 radiation is lessened ; in the second place condensation of aqueous 

 vapour near the surface of the earth by which first dew and then 

 hoar-frost is produced ; and in the third place movements in the 

 air in the form of sliiiht breezes or faint draiu/hts which mix the 

 different strata of the air. 



All the other causes of the fall of temperature during a 

 clear night may be regarded as of so small influence that they 

 scarcely need to be taken into consideration. 



All these causes prevent the loss of heat from vegetation by 

 radiation, making the fall of temperature produced by it 

 slower and slower. At last it reaches a limit which cannot be 

 exceeded, i.e., the heat euiitted by the plants is then restored to 

 very nearly the same amount. 



The causes of night-frosts have been the subject of special 

 study not only by Lemstrom and Homen in Finland, but also by 

 Hamberg and Juhlin in Sweden. 



We have now to consider the questioa — To what temperature 

 can plants be exposed witlioat dainaf/e .■ In this matter the 

 experience is still very limited, especially as the general climatic 

 conditions of a country influence the vegetation, and consequently 

 the question has to be made a separate study for each country. 

 ■ Some important conclusions might still be derived from what is 

 known at the present time. 



Numerous but by no means final researches have shown 

 that temperatures between the freezmg point of water on the 



