On X) c/ht- Frosts. 



45 



As 011 tlie 24"' 8epteml)er tlie ground was not _yet covered with hoar- 

 frost at 3,45 a. ni., with tlie exception of a boanl near the place of observa- 

 tion, and the thernionieters not haviiia,- fallen lower than (f,i and 0",i, it is 

 evident that the radiation from the eartli liad beeii verv slight during the 

 night. Tliis is seen still niore clearly from the fact that the température near 

 the very surface of the earth had proved to be exactly what it was 6 hoiu's 

 earlier or, at î),so p. m. the 23'^ September, i. e. the evening before. This 

 fact proves moieover, something eise: that the continued falling of température 

 near the ground, is not caused only by radiation from it, liut by other cir- 

 cumstances, which we will now analyse more closely. 



The wliole iieighbourhood being covered with a light mist, the radiation 

 took place in such a nianner that the heat of the ground radiate<l to tlie mist 

 above, which of course gave back the greater part of it, so the falling of tem- 

 pérature was very slow on the ground. This i)henoineiion was augmented by 

 the constant formation of fresh mist, i. e. the mist become deiiser, the moisture 

 passed from vapour to liquid form and radiated heat. In the uppermost layer 

 of the mist however, tliere occurred on intense radiatinn to spacc, only faintly 

 interrupted by the moisture of the air, the présence of which was announced 

 by the greyish-blue colour of the sky, (luite distinct, even wlien seen from 

 higher points. It now seems very probable that, in conséquence of the calm 

 weather, this upper layer of mist was cooled below zéro, forming over-cooled 

 water, which, in conséquence of its greater specific weight, feil down to the 

 surface of the earth, giving way to fresh layers of mist, wliicli, in tlieir turn, 

 suffered the same over-cooling. This process had evidently been going on all 

 night, but not till morning did the traces of this over-cooled mist appear on 



' Slight fog. Hoar-frost was ohserved on a lioard on tlie ground. 



' Denser fog. The wholc ground, higher places as well as lower, covered with ice; the glasses 

 round the thermometer-tubes also covered with ice. 



