On Niffht-Frosts. 41 



During the expeiiment a cold current of air from tlie southei'ii side of 

 the field was observed to descend from the sandhill and advance towards the 

 trench. This current was particularl}- noticeable in the ditclies, but conld, for 

 the moment, not be stopped, as the torches, in order to avoid tlie trampling 

 down of the corn, were not placed across the plots of gronnd. At the end 

 of the experiment, ice, or lioar-frost, had again been tormed on the ears of corn 

 the in surrounding field, but in the tield of experiment there was not the hast 

 vestige of Jioar- frost to be observed, not even on the low situated parts of it. 

 On the lawn outside the slieltered tield, the formation of hoar-frost began very 

 early, and could even be traced, though in a small degree, in the middle ot 

 some open spots of the protected tield which were not overgrown with rye. As 

 at the preceding experiments, a doud of smoke was drifting to and fro in the 

 field, from time to time över the protected field, and, at other times, far away. 

 A warm breath of air was also observed towards the end of the experiment. 



V. 

 THE EFFECT OF TORCHES. 



In order to give a closer détermination of the effect of torches on the 

 general course of the Avhole phenomenon, we will again summarize the acting 

 and counteracting causes of the night-frost. 



The radiation from the earth is counteracted by the following causes: 



1. By Condensed aq. vapour of air, both in the higher layers of air 

 and in tliose near the surface of the earth. 



2. By motions of air, under the influence of a re-al breath of wind. 



3. By radiation of heat and conducting ot heat from the surface of 

 the earth. 



At any hour of the night plants will show a température depending on 

 thèse causes. Shortly before and afier sunset, a rapid fall of température will 

 occur, which in clear and calm weather, an hour aftei- sunset, will reach, near 

 the surface of the earth, almost the worth, to which the température will sink 

 during the night. For 3 or 4 hours the différence may amount to 2" or 3°, 

 or 0",7 per hour, though, on the othei' hand, the température befoi'e and aftei" 

 sunset may easily fall as much as 10" in 1 ^, or 6",- per hour. Un June 12"" 

 the temperatui'e feil 



