18 Selim Lemström. 



we will proceed from tlie following facts, wliich hâve partlj'^ been explained 

 by science, partly proved by expérience. 



To simplify tliings, we présume the field to be very nearly a plane sur- 

 face and the plants in question to rise above the surface to a quantity, the 

 average worth of which may be indicated by h in meters. 



Expérience shows, that only the upper parts of plants (in cereals, the 

 ears; in esculent roots, the last sprouts), are in need of protection, because the 

 other parts are hardy and cannot be damaged. 



Shortly before and after sunset, the ground begins, in conséquence of the 

 radiation, to lose heat, which is proved by a rapid fall of température. The 

 rays of heat chietiy proceed from the surface of the earth and the objects on 

 it, among others, the plants, through the atmosphère to cosmical space. On 

 their way through the atmosphère they suffer an absorption, the intensity ot 

 which dépends on its physical State for the moment, or, to speak from the 

 point of view of the moveable equilibrium of température: the atmosphère ra- 

 diâtes back to the ground one part of the radiated quantity of heat. 



The whole of the quantity of heat Q departed during the time r, may be 

 divided into three parts: 



Qs, the quantity of heat, radiating from the surface of the earth. 



Q„, the quantity of heat, radiating from the plants on the earth's sur- 

 face and 



Q„, the quantity of heat radiating from the layer of air nearest the 

 ground. 



We will, for the present, put aside Q„ or the quantity of heat radiating 

 from the earth's surface, as it is very slight during the interval of the night 

 in question, and because the raising of température of the uppermost layer of 

 earth during a frosty night never needs to be thought of. 



The quantity Q„, on that account of the insignificant power of emission ot 

 the air and vapour, and considering the thinness of the layer of air neare the 

 surface of the earth to be taken into considération here, is so little, that, on 

 a first approximation, it can always be neglected. 



It is, consequently, Ç„ which we have to détermine. 



The formula proposed by Dulonö and Petit for the réfrigération of a 

 warm body lias, on the basis of experiments made, received from Peclet the 

 following form: 



<?„ :- 124,72 Ku" («' - 1) + 0,552 Kt''""^^; 



