on the mechanical Expanfion of Air. 49 
in the fame quarter, viz. N.E. or S.W.) the air became many 
degrees warmer. A fimilar fa£t is related from Musschen- 
broek, in Mr. Kir wan’s ingenious work on the Temperature 
of different Latitudes ; viz. that in winter, when the mercury 
in the barometer defcends, the cold increafes. More accurate 
obfervations on this fubjeft, when the air is ftationary, or 
when the wind continues in the fame quarter, might lead to 
the difcovery of the quantity of heat fqueezed out of the air 
by a certain p re flu re. 
The D evaporation of aerial Moijlure . 
As heat appears to be the principal caufe of evaporation, as 
well as of folution, and of fluidity in general, the privation of 
heat may be efteemed the principal caufe of devaporation : for 
though the air may, by its own power of attraction, or by 
means of the electricity it may contain, diflolve and fuf- 
pend a portion of water, as water diffolves and fufpends a por- 
tion of fait ; yet, by the application of cold, thefe are reflec- 
tively precipitated ; and therefore heat mav be aflumed as the 
immediate caufe of thefe folutions. Add to this, that water 
boils in vacuo with lefs heat; that is, it evaporates in vacuo 
fafler or eafier than in the open air, and therefore the attractive 
power of the atmofphere does not feern neceflary to evapo- 
ration. 
Now, when the barometer finks (from whatever caufe not yet 
underflood this may happen) the lower ftratum of air becomes 
expanded by its elafticity, being releafed from a part of the 
fuper- incumbent prefl'ure, and, in confequence of its expan- 
fion, robs the vapour which it contains of its heat; whence 
Vol. LXXVI1I. H that 
