On Xight-Frosts. 15 



The température has fallen to - 6" and the whole matter is turned to ice 

 and for tliis tranformation must be added 



0,4 Cal. 

 hence we sliall have 



3,:i Cal. per ur. 



Part of tlie dew lias its cause in the moisture evaporated from tlie layer 

 of the earth beneath the surface, l)ut the (juantity resulting of its condensation 

 caiuiot at present l)e exactly determined, the necessary expérience being want- 

 ting. (See Note III.) 



Summing up all the acting and counteracting causes of lowering of tem- 

 pérature in a clear night we get among the former: 

 in the tii'st place 



Radiating of heat; 

 in the second place 



Movements in the air, 



caused by the cooling of air through its touching the plants and its run- 

 ning down into the lowest places. 



As counteracting causes we have in the tirst place 

 Condensation of aq. vapour in the atmosphère 

 in general, by Avhich the radiation is lessened; in the second place, 



Condensation of aq. vapoiir near the surface of the earth 

 by which lii'st dew an then hoar-frost is created; 



and in the tliii'd place. 



Movements in the air in form of slight hreezes 

 or faint draughts which mix the layers of air. 



AU the other causes of the fall of température during a clear night may 

 be regarded as of so small influence that they scarcely need be taken into 

 considération. 



All these causes put an obstacle against the loss of heat of vege- 

 tables by radiation, making the fall of température, produced by it, more and 

 more slow. At last it reaches a limit which cannot be surpassed, i. e. the heat 

 emitted by the plants is hence restored to very nearly the same amount. 



