V. EXPERIMENTS and OBSERVATIONS upon a REMARK- 

 ABLE COLD which accompanies the S E p A R A T i o N o/* H o AR- 

 FROST/TO? # CLEAR AIR. By PATRICK WILSON, M. A. 

 F. R. S. EDIN. and Profeffor of AJlronomy in the Univerfity of 

 GLASGOW, 



[Communicated by Dr BLACK ; and read by Dr WdLKER^ Secretary , 

 Julys- 1784-] 



I 



SECT. I. 



MACFARLANE Obfervatory, Glafgow College, Feb. 14. 1784. 



N the feventieth and feventy-firfl volumes of the London 

 Philofophical Tranfactions, the reader will find an account 

 of fome Experiments and Obfervations made here upon cold in 

 the years 1780 and 1781. 



THOUGH, at firft, I had no other view but that of keeping a 

 regifter of the very cold weather which fet in on the i3th Ja- 

 nuary 1780, yet I was foon led to extend the plan of my obfer- 

 vations, upon meeting with a new phenomenon, which appear- 

 ed to me to deferve fome attention. This phenomenon confid- 

 ed in a conflant difference of temperature of the fnow which, 

 at that time, covered the fields,- and that of the air at a few feet 

 above : the fnow being the coldefl. 



HAVING, by careful and repeated trials, fully affured myfelf 

 of the fad, I was infenfibly drawn to form fome conjectures as 

 to the caufe. The moft obvious fuggeflion was, that fuch an 

 excefs of cold depended upon an evaporation from the fnow ; 

 efpecially when it was confidered, that this fubftance, from its 



fpongv 



