170 EXPERIMENTS and OBSERVATIONS 



latent principles, the quantity of hoar-frofl given out, com- 

 pared with the correfponding cold, may be very different. 



IN the paper of the year 1781, it is faid, " That laft year 

 " there was a much more copious giving out of hoar-frofl at 

 " times when the difference of temperature was not more re- 

 " markable ;" and, from this fact, I was then difpofed to con- 

 clude, that the cooling procefs had no immediate dependence 

 upon the feparation of hoar-froft. But, from the facts which 

 have been afcertained this winter, it feems neceffary to correct 

 this opinion in the extent above explained. It ought alfo to be 

 confidered, that the comparifon above qxioted between the 

 years 1780 and 1781, had refpect to the temperature of the 

 fnow upon the ground only ; and it is evident, that this may,' 

 cceteris paribus, differ more or lefs from that of the air, accord- 

 ing as the fnow has fallen more or lefs recently, or is more or 

 lefs deep, upon warm or upon frozen ground. 



IT may alfo be proper here to take notice of what may be 

 called a fpurious kind of hoar-frofl, which is attended with no 

 cold whatever at fettling upon bodies. I mean that which trees 

 and buildings are fo apt to be decorated with during a mifl in 

 frofly weather ; in which cafe it is obvious, that the hoar-frofl 

 was previoufly formed in the air, and floated in it, and was not 

 formed in the act of being depofited upon the body. 



SECT. III. 



THOUGH the night-obfervations of this and the former win- 

 ters afforded fo many examples of cold thus depending upon 

 the feparation of hoar-froft from the air, I come now to men- 

 tion one, and the only one inflance wherein, during the night, 

 this condition of things was totally reverfed, and when the 

 fame phenomenon, of an excefs of cold at the furface of the 

 fnow, took its rife from a moft manifefl evaporation. 



JANUARY 



