Oti Nif/ht-Frosts. 33 



by curves marked T/,, is in all parts of tlie field almost alike and pretty 

 regulär. 



The torches heing lighted at 10, 30 p. m., their etfect is not to he ohserved 

 till after that time. At point o, which was nearest the trench, the tempera- 

 tiu-e is, before the experiment, ahont 4" higher than that of point x: after 

 that, a rapid fall of température, lasting until midnight is ohserved, and then 

 the curves become almost parallel, with a différence of temperaturs, which, in 

 the tield of experiments, exceeds that of point o", by about 2V- 



It should be observed here, that, on account of the sloping of the field 

 towards the trench, the effect of the above-mentioned cold current of air was 

 niuch less, indeed, hardl}' worth mentioning, at point x. 



At 10 p. m. the température at point b was 2'',o lo wer than at point x at 

 the same hour, as is seen from Cm-ve I,,, Pl. II which clearly explains the 

 influence ot the cold current of air. The température in this point sinks only 

 very slightly. until 1,20 a. m. after which it begins to rise towards the end 

 of the experiment, showing at midnight an average surplus ot 1*',2. 



A rise of température is also observed in Curve T„, indicating that, in 

 point X too, some source of heat bas been acting, most likely some movements 

 in the layers of air, which is to be ascribed to the effect of the torches in the 

 field C of experiments. 



The changes of température at point c very nearly resemble those ot 

 point b, except the rise being here 1",6. 



At point cl, the curve T„ is very irregulär, indicating fluctuations in 

 the cold current of air which, on ifs way, lias passed the torches in the 

 field above. The average value of the lise of température here amounts to 



2V. ' 



The field was covered by thin timothj^-grass, which allowed the air to 

 move freely near the ground. Dm-ing the experiment, smoke and vapour was 

 offen carried away to the surrounding fields, but the torches constantly afford- 

 ed fresh smoke, so that diu'ing all the time of experimenting the field was 

 in a varying degree, covered with smoke. The smoke that bad left the field 

 of experiments, was not dispelled, but settled över the surrounding fields in a 

 layer, which seemed to be of a depth of 2,3 m. to 3,5 m. 



' Tho thcrmometers e, f and (/ aro mentioiieJ here, in order to show the changes of température 

 near the trench, hut as the tirst-namcd was placed between a and d, and the two latter hy the trench 

 near the tield, they are not marked here by curves. 



