Mr. wilson^ Account of a mqfi 
expofed to the open air during the night; but upon re- 
flecting afterwards on the fnow at the Obfervatory being 
fo much below o, the greateft cold of the air at the Col- 
lege, and having on other occafions found a difference of 
only 4 0 at moft in air at thefe two ftations, I was led into 
a fufpicion that the fnow might perhaps have been fo far 
cooled down by an evaporation at the furface. With a 
view to this opinion I projected the experiment with the 
bellows defcribed below, by which I was not without 
expectations of producing; a ftill more remarkable fall of 
the thermometer when lying on the fnow. Before din- 
ner this day I met with Dr. irvine, to whom I commu- 
nicated the above obfervations, and the intention of try- 
ing the evaporation ; for that fnow and ice did actually 
evaporate in fome circumftances feemed fufficiently 
proved by an experiment made here by my father in. 
1 7 6 8, which is related in the Philofophical Tranfations, 
vol. lxi. p. 3.26. Dr. irvine approved much of the 
propofal, and moft obligingly agreed to fpend the 
night at the Obfervatory, that we - might,., by a regular. 
courfe of obfervations,.. afcertain the difference of tem- 
perature, and try whether evaporation was really the. 
caufe of it.. 
3 
Alb 
