On N'niht- Frosts. 11 



In autumii 1892 the foUowing experiments were made: 



The flrst sei'ies shows a negative i-esnlt, i. e. the température of the co- 

 vered thei'mometer was below that of the uncovered one. In tlie last tliree 

 series the average température of the covered thermometer exceeds the unco- 

 vered only by 0",2; consequently a very small surplus and rather imcertain. 

 Considering that vapour Condensed to dew conveys a great amount of heat to 

 the surface of the earth, the conclusion of thèse experiments is that heat which 

 radiâtes from the surface of the earth after sunset, is scarcely perceptible on 

 a frosty night. 



The influence of this radiated heat is diminished by the cii'cumstance that 

 it meets the short grass, which generally covers the ground in places where 

 préventives against the damage of frost might be used. lliis source of heat 

 is without any influence and may therefore be negiected. 



4. Evaporation. Every hour of the day water évaporâtes from the 

 gi'ound and vegetation with more or less intensity, and hereby heat is con- 

 sumed in considérable quantifies. The intensity of evaporation détermines the 

 degree of humidity in the atmosphère. 



The aq. vapours thus formed are mixed mechanically with the sm-rounding 

 atmosphère, exercising a pressure which may be measured by the weight of a 



