48 Selim Lemström. 



a température of almost O" and therefore, on account of tlie slioii time of the 

 plienomenon, it cannot cause a réfrigération intense enougli to make the cell- 

 water of the phvnt freeze. On the ice changing into water and moisture, some 

 moments, critical for the plant, will indeed occur, on account of the great quan- 

 tity of heat consunied, hut all our expérience tells us that the mere change 

 of ice into moisture cannot become injurions, if the température is not too 

 much below O" ( — 2'-,o) near the ground. 



It is, however, not decided that the humidity of the air is so great 

 as to cause the over-cooled mist to fall continually until sunrise. It may 

 liappen that the supply of mist, or Condensed vapour in general, already comes 

 to an end, for instance, an liour hefore sunrise, and it is clearly understood 

 that the radiation of heat from the ground will, iii this case, increase in in- 

 tensity to such a degree that the température may fall considerahly in a i-ela- 

 tively short time. Provided that this over-cooled mist should, for the most 

 pai't, have fallen from 2 to 3 hours hefore sunrise and that the air he con- 

 tinually stagnant, there will have heen formed, ahove the tield exposed to such 

 a phenomenen, a clear and transparent air, which very slightly prevents the 

 radiation of heat, and under such circumstances the température may fall so 

 much as to cause damage by frost. 



This result derived from observations will, if applied, give a very pro- 

 bable explanation of the following phenomena, partly difficult, partly impossible 

 to explain. 



a) Of a clear night the température is lowest shortly before sunrise, even 

 for that reason, that it is the result of radiation of heat during the whole 

 of the preceding night. If considering that a condensation of moisture has 

 been going on at the same time, it is easily understood that radiation of heat 

 must increase in intensity if this Condensed vapour has fallen to the ground 

 in one form or other, for by this pi"Ocess the layer of air has become more 

 transparent for heat, i. e. it has lost its character of a sheltering screen. 

 Should a calm prevail shortly before sunrise, we must admit that every agri- 

 culturist, who considers the time immediately before sunrise to be the most 

 dangerous during a night of frost, has made a correct observation. In every 

 special case, the greater or smaller influence that must be attributed to the 

 phenomenon described above, of course dépends on the quantity of moisture in 

 the air. It is probable that in any case it is of some influence, but also that 

 it is of the greatest effect only during the early part of spring and in autumn. 



h) It is a well-known fact that damp, boggy lands, low-situated, 

 are more sensitive to frost than meadow-ground consisting of another kind 



