VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 37^ 



it; 3dly, from its sinking when exposed to a stream of air from the air-vessel of 

 a water-engine ; and, lastly, from the curious phenomenon of snow and ice 

 being produced by the stream of expanding air from the fountain of Hiero in 

 an Hungarian mine ; there is good reason to conclude, that in all circumstances, 

 when air is mechanically expanded, it becomes capable of attracting the fluid 

 matter of heat from other bodies in contact with it. 



Coldness of the summits of mountains. — Now, as the vast region of air which 

 surrounds cur globe is perpetually moving along its surface, climbing up the sides 

 of mountains, and descending into the valleys ; as it passes along it must be 

 perpetually varying its degree of heat, according to the elevation of the country 

 it traverses : for in rising to the summits of mountains it becomes expanded, 

 having so much of the pressure of the super-incumbent air taken away, and 

 when thus expanded it attracts or absorbs heat from the mountains contiguous 

 to it ; and when it descends into the valley, and is again compressed into less 

 compass, it again gives out the heat it has acquired to the bodies it becomes in 

 contact with. The same thing must happen in respect to the higher regions of 

 the atmosphere, which are regions of perpetual frost, as was always suspected, 

 and has of late been demonstrated by the aerial navigators. When large dis- 

 tricts of air from the lower parts of the atmosphere are raised 2 or 3 miles high, 

 they become so much expanded by the great diminution of the pressure over 

 them, and thence become so cold, that hail or snow is produced from the pre- 

 cipitated vapour, if they contain any : and as there is, in these high provinces 

 of the atmosphere, nothing else for the expanded air to acquire heat from, after 

 the precipitation of its vapour, the same degree of cold continues till the air, on 

 descending to the earth, acquires again its former state of condensation and of 

 warmth. The Andes, almost under the line, rests its base on burning sands ; 

 about its middle height is a most pleasant and temperate climate, covering an 

 extensive plain, on which is built the city of Quito ; while its forehead is encir- 

 cled with eternal snow, coeval perhaps with the elevation of the mountain : yet, 

 according to the accounts of Ulloa, these 3 discordant climates seldom entrench 

 much on each other's territories. The hot winds below, if they ascend, become 

 cooled by their expansion, and hence cannot affect the snow on the summit ; 

 and the cold winds, that sweep the summit, become condensed as they descend, 

 and of temperate warmth, before they reach the fertile plains of Quito. 



Correspondence of the heat of the atmosphere with the height of the baro- 

 meter. — From this principle some of the sudden changes of our atmosphere 

 from hot to cold, and from dry to moist, may also be accounted for. During 

 the last year I frequently observed, that when the barometer rose (the wind cout 

 tinuing in the same quarter, viz. n. e. or s. w.) the air became many degrees 

 warmer. A similar fact is related from Musschenbroek, in Mr. Kirwan's inge- 



