[ 70 ] 



demanded, and thus rarefied the air in contad with them ; or 

 on windy days, which did not permit the air to Magnate ; or 

 when a foutherly wind prevailed above and not below. It was 

 from a condenfation of this fort, apparent in the menfuration of 

 Hackluyt-hill in Spitfbergen, that General Roy inferred that the 

 polar air, though afl'eded with the fame temperature and pref- 

 furc, was fpecifically heavier than in the middle zones, which 

 cannot be flrid:ly true, as certainly the polar air muft by the 

 many ftorms that have blown to and from it, be long ago 

 mixed with the common mafs of the atmofphere. 



I MUST admit that the juftiy celebrated Bouguer was of a 

 contrary opinion ; for, from fome experiments made with a 

 pendulum, he concluded that air on the higheft mountains was 

 proportionably more elaftic and lefs denfe than that on the fur- 

 face of the earth ; but one of his comparative experiments on 

 which he refted this conclufion, was made in the moift air of 

 Popayan, with a pendulum made of the fibres of the aloe leaf ; 

 and I have been affured by a very competent judsje both of 

 hygrometers and pendulums*, that this vegetable fenfibly attrads 

 moiflure ; fo that its weight being encreafcd, it is not furprifing 

 that its vibrations were retarded in greater proportion than they 

 otherwife would be. 



As I have all along fuppofed tlie rarefailion of the atmo- 

 fphere in the polar regions to proceed from the auroras borealis 

 and auftralis, which I take to be a combuftion of inflammable 



* Mr. Whitehurft. See alfo Herbert de Igne, p. 1 8. 



air 



