on the mechanical Expanjion cf Air. A 7 
ftance is defcribed at large, with a plate of the machine, in the 
Philofophical TranfaCtions for 1761, Vol. LII. 
5. From the four experiments already related; firft, of the 
mercury finking in the thermometer, by being expofed to the 
ftream of air from an air-gun ; fecondly, from its finking in 
the receiver of an air-pump, during the time of exhauftmg it ; 
t y , f 1 o m iti> finkm g when ex poled to a flream of air from 
the air-veflel of a water-engine; and, laftly, from the curious 
phenomenon of liiow and ice being produced by the flream of 
expanding air from the fountain of Hiero in an Hun " irian 
mine ; there is good reafon to conclude, that in all circum- 
flances, when air is mechanically expanded, it becomes capable 
of attracting the fluid matter of heat from other bodies in. 
contaCt with it. 
1 
Coldnefs of the Summits of Mountains. 
Now, as thevaft region of air which furrounds our globe is 
perpetually moving along its l'urface, climbing up the lides of 
mountains, and defeending into the vallies ; as it pafles along, 
it mult be perpetually varying its degree of heat, according to 
the elevation of the country ir traverfes . for in rifing to the 
fummits of mountains it becomes expanded, having fo much 
of the preffure of the luper -incumbent air taken away, and, 
when thus expanded, it attracts or ablorbs heat from the 
mountains in contiguity with it ; and when it defeends into 
the valley, and is again comprefled into lefs compafs, it again 
gives out the heat it has acquired to the bodies it becomes in 
contaCt with. 
The fame thing muff happen in refpeCt to the higher regions- 
*f the atmofphere, which are regions of perpetual frofr, as 
3 
was 
