On Night-Frosts. 61 



The most extraordinaiy thing, in this case, is, however, that this small 

 quantity of heat is, by no means, the sniallest necessary, hut it is moie than 

 probable that about \ of it Avould be sufticient. 



But as sm-ely as tliis quantity of heat, transformée! into work and ap- 

 plied in such a manner that a layer of air not thicker than a fraction of a 

 millimeter, is raised 3 m high a second, would be sufticient to make the tem- 

 pérature, in a field of 500 hectares, stand above O" round the upper parts of 

 the plants, just as impossible does it seem to eftect this result directly, and 

 therefore this must be done by heating or by sucking away the whole of the 

 cooled layer of air to a height of about 2 m. 



Each torch thus absorbs l,t>00 m'' of air in three hours or (),09269 m' a 

 second, while on the square meter, immediatety surrounding the torch, the speed 

 must amount to 0,oo m a second. 



We limit ourselves to these tew calculations, whicli only prétend to be 

 approximate and serve but to give some idea of the forces anil (juantities in 

 the case in question. 



ïhe constantly ascending currents of air absorb the cooled air from the 

 surface of the earth, their place being supplied by warmer air, which again is 

 cooled by contact with the ground, i. e., its heat is gÏA'en to the vegetation 

 and is, in its turn, absorbed. 



Thus, if the foUowing facts are taken into considération: 



that, in conséquence of the labile equilibrium of the layers of air (a warm 

 layer of air between two colder), currents of air, in the shape of 

 breezes of gyratory motion, will probably arise, blemling the layers 

 of air; 



that clouds of smoke and warm moisture •\\ill spread in all directions, di- 

 minishing the radiation and diflfusing heat; 



that this smoke and moisture leads to a more rapid condensation of the 

 aq. vapour in the air; 



there is every probability that this method Avill be successful. 



But care must be taken that strong currents of cooled air are kept out 

 or eut of by a number of torches, i. e., that local circumstances guide all the 

 measures taken. 



The expenditure for 500 hectares will be 500 Fm if we count each torch 

 at 5 penni, including tlie cost of the torches and of the labour attending 

 their placing and burning. According to the calculation above, 500 hect. 



