28 Selim Lemström. 



Both thèse circiTinstances are of the greatest importance. 



The calm, of course, facilitâtes the destruction by radiation of heat, but it 

 also retains in the air that matter, which is introduced into it for its diminishing ; 

 the condensation counteracts the loss of heat and it retui'ns to the vegetables. 



If it were possible to put an end to the calm, i. e. could a breeze be 

 produced, the layers of air would be mixed and thus produce a warming effect 

 instead of a cooling. The same thing would happen if an increased condensation 

 of aq. vapour could be produced, in order that the radiation of heat from the 

 earth might be counteracted by radiation from this comparatively rieh soiu'ce 

 of heat. 



Both thèse ways hâve been tried. 



From times immémorial it bas been known that frost will not occur when 

 the sky is cloudy, and in many lands trials bave been made to produce arti- 

 ficial clouds of smoke by burning différent kinds of more or less cheap com- 

 bustibles. A rational method lias, however, not yet been created. This method 

 of protection against frost has therefore got into disrepute. In France, for 

 instance, the people say: it succeeds, but al ways at one's neighbours, thereby 

 indicating that smoke and vapour pass to their neighbour's field. They bave 

 not formed any association, — which is the case in many parts of Grermany, 

 — • where attempts hâve been made to protect vineyards and orchards against 

 both spring and autumn-frosts by the burning of coal-tar. 



It seems to us, however, that the exécution of the method has not been 

 well contrived and not well adapted to the ruling circum stances. 



From the preceding explanation it will be seen that night-frosts of any kind 

 may be prevented, if only artificial clouds or movements in the air are pro- 

 duced in a fit manner, or both these préventives are undertaken at the same time. 



Artificial clouds, consisting of smoke and vapour, may be produced by 

 every fuel possessing the foUowing qualifies: 



to be bandy and cheap, 



to be easily transportable 



to be easily kindled, 



to bui-n slowly (carbonize) and produce much smoke, vapour and heat, 



not to be spreading the fire. 



It is not easy to combine all these qualities in one combustible. Yet we 

 may hope that this has been effected by the so-called frost-torches in their 

 present form. 



