On Night-Frosts. 49 



of soil, even though equally lowly situated. The cause of tliis plienomenon 

 is souglit in flie condition of soil, considering bog-lands (mud soil, luouldering 

 nioss) to be exceptionally cold in comparison with meadow-ground. Tids is cor- 

 rect enough, because heat of the snn pénétrâtes deeper and in larger qnantity 

 into the latter, on account of its greater power of conducting. It is, however, 

 iincertain whether tliis quality of the soil is of so much intluence as to accoiint 

 for an incomparably greater sensitiveness to frost, foi', as shown above, the 

 heat conimunicated to the plants from the ground during a night of frost, is 

 unimportant. 



In order to place ourselves in circumstances fully comparable, we will 

 présume that both the bog-land and meadow-ground are well ditched and suf- 

 ticiently drained and sown e. g. with rye, of about the same development. We 

 will further imagine that both the tields were equally exposed to the sun during 

 the course of a day in June and that a temperate wind lias prevailed until 

 5 p. m. mixing the layers of air so as to make the conditions of humidity 

 in the air almost alike, but that the wind abates after that time. The eva- 

 poration from both flelds is constantly going on, but as the bog-land is presumed 

 to be much more moist, the evaporated quantity, there, is much largei', and, 

 as the vapour stays in the layer of air above the tield, the pressure of 

 moisture will be much greater above the bog-land than above the meadow- 

 ground. Furthei', the plants and the ground under them have, at the same 

 time, reached a lower température in the former than in the latter, because 

 at the evaporation a great (juantity of heat is absorbed in propoition to the 

 evaporated quantity of water, and because the heat from the earth is less in 

 the bog than in tho meadow, in conséquence of the less conducting power of 

 the former. Consequently, we have, shortlj' before sunset, a lower température 

 and a higher pressure of moisture above the bog-land than above the meadow- 

 ground. The quantity of both dépends on local circumstances and is difticult 

 to détermine. 



As pointed ont above, a rapid fall of température begins immediately be- 

 fore and particularly after sunset, when it is to be observed, that the dew- 

 point is reached earUer above the bog, both near the surface of the earth and 

 in the higher layers of air, and through this circumstance the bog receives a 

 sheltering screen of Condensed moisture earlier than the meadow does. The 

 absolute conséquence of this is, that the température, shortly after sunset, will 

 turn out alilie above both grounds, indeed, it may sometimes, at a cei'tain 

 point of time, prove higher above the bog than above the meadow. 



In this State of things, the phenomenon of the falling overcooled mist be- 



7 



