Upon a REMARKABLE COLD. 157 



lowed by a flow evaporation, the fnow turned quickly as warm 

 and fometimes warmer than the air. 



How difficult foever it may be, upon known principles, to 

 account for the connection between the excefs of cold at the 

 furface of the fnow and its attracting hoar-froft, it is yet eafy 

 to underftand how the fnow may become as warm or warmer 

 than the air above, notwithflanding an evaporation takes place. 

 For though, from Dr BLACK'S general laws, a certain wafte or 

 abforption of fenfible heat muft thereby infallibly enfue, it is 

 evident, that a much drier ftate of the air, and a much more 

 rapid evaporation, may be necefTary to prevent the vippermoft 

 ftratum of fnow from being heated by the much warmer fnow 

 immediately beneath : For, on December 30. when the froft 

 was very intenfe, the fnow was fo warm as + 24, at a little 

 more than three inches below the furface. And this I take to 

 be the reafon why the excefs of cold, at the furface of the fnow, 

 fo readily difappears when the attraction of hoar-froft is fu- 

 fpended, and, along with it, that active cooling procefs which 

 feems, fome how or other, to be its concomitant. 



WHEN the atmofphere, after having been, for fome time, 

 very ferene, becomes fuddenly clouded, it is certain alfo, that 

 this change muft be attended with the extrication of much fen- 

 fible heat in the higher regions, where thofe vapours are con- 

 gregated. A ftore of heat, fo produced, muft foon affect the 

 mafs of air which lies below. It is well known alfo, that the 

 lower air, thus riling in its temperature, will be lefs difpofed 

 than before to give out any matter in the form of hoar-froft, 

 or may be even enabled to abforb this from the furface of the 

 fnow, and of other bodies, in the way of evaporation. Hence 

 may be underftood, how, upon the fudden formation of clouds 

 or fog, the thermometer, expofed to the air, always rofe fome 

 degrees ; and how, in confequence of this increafe of tem- 

 perature, the fnow-fcale, and other bodies, no longer attracted 

 any hoar-froft ; and, finally, how the excefs of cold at the fur- 

 face 



