On INi(fht-Frosts. 27 



heat must, clming the months of June and July, begin directly after sunset, 

 and in spring and autumn, later, and is to be continued till the danger is over. 

 As has been pointed ont before, the communication of heat may be effected 

 in the folio wing way: 



a) By a réduction of the radiation, by means of artifi- 

 cial clouds; 



l) By Condensed moistui'e that affords heat; 



c) By movements in the air, whicli mix iip the layers 

 of ah: 



It is self-evident that no damage by fi'ost can occur, if the occasional 

 cooled layer of air is gi-adually removed, so that instead of eveiy quantity dq 

 of heat lost, a corresponding quantity of cold air is removed. The work re- 

 quired for this proceeding, is of Aery httle sigiiificance, and if executed, the 

 plants will constant ly be suiTounded by ah-, the température of which \sill be 

 above O**, n. b. the whole of the cooled layer of air, is only to be re- 

 moved once. 



That this is the case, is proved by Nature herself in the clearest way, 

 inasmuch as even the faintest breath of wind can prevent damage by frost, as 

 soon as the température of the air is above 0'', though in that case the cold 

 ail" is not absorbed but the layers of ail- are mixed up together. 



In conclusion we beg to observe, that there is a factor, which most likely 

 will greatly contribute towards counteracting the radiation and which has not 

 been taken into considération, i. e. the circumstance, that the smoke and va- 

 poiu- induce and precipitate the condensation. 



The remarkable fact that the amount of heat, necessary to prevent 

 damage by frost, is so small, has been my chief incentive, in spite of many 

 difficulties, in working ont the discovery and perfection of a practical and 

 cheap method to prevent damage by frost. 



Natuie herself offers certain opportunities which we must use with care: 



a) the universal calm, wliich is reigning during a frosty 

 night and which is a necessary condition for ist occuiTence, 



b) the condensation of vapoiu' continually going on during 

 the night. 



