On Night-Frosts. 19 



Q, means the quantity of heat which in 1 hour radiâtes from a sm-face 

 of 1 m^, K is a constant which dépends on the nature of the surface, K' is 

 another constant which dépends on the character of the surrounding gas; 

 «=1,0077; O- the température of the siUTOundings ; t the surpkis of tempéra- 

 ture in the radiating surface. 



We will now, Avith regard to our purpose in view, examine the quanti- 

 fies given in the formula, in order to lind out wliether it is possible to make 

 a direct calculation of the quantity of heat, which during a frosty night, 

 radiâtes from the plants. 



First we have to explain how the radiating siuface, in the formula de- 

 noted by 1 m-, is to be understood. To give an exact estimation of tliis siu- 

 face, i. e. to the sum of all the upper surfaces of every individual part of the 

 plants, from the root to the top, including all leaves etc. seeras impossible. 

 Should we even contine oiu'selves to the topmost, délicate parts of the plants, 

 which here would be the correct thing to do, the détermination will, howe- 

 ver, be very doubtful, as attention must likewise be paid to the position 

 every part occupies with regard to the horizontal plane. It is the same with 

 constant K: it dépends on the character of the surface, and there are no au- 

 thentic experiments for the détermination of the power of emission of plants 

 in general. 



As to e, or the température of the surroundings, the matter stands some- 

 what differently, for here we have the experiments of Pouillet among others. 

 By the température of the surroundings during a dear night, Ave must under- 

 stand the température of an artificial sky above the thermometer. This sky 

 must be of the same effect as the atmosphère with the space above. The 

 température of it is evidently not constant, but varies every moment. 



In his låter experiments Pouillet used an artificial sky of constant tempe- 

 rature -20", and his experiments Avith the actinometer led to an équation 



Z^t- I a 



Avhere 3, Avhich by Pouillet Avas called the zénithal température, corresponds to 

 0; Ms the surrounding température of the air in the vicinity of the actino- 

 meter, and ffl is a constant, the wortli of Avhich Avas proved, by the experi- 

 ments, to be about 7" to 8°. 



This formula can, however, not be applied during a frosty night, because 

 we do not knoAV Avhat Avorth we must give t, for the température of the sur- 



