BAROMETER. 351 



the frequent changes in the density of the atmos- 

 phere, and consequent fluctuations of the baro- 

 meter. 



5. Corresponding with the foregoing facts, the 

 barometer rises to the greatest height during cold 

 northerly winds in the middle latitudes, and falls 

 during warm southerly winds. That is, a mass 

 of air from the torrid zone is specifically lighter 

 than one from the polar regions, and must cause 

 a proportional fall of the mercurial column, until 

 the equilibrium is restored. 



But in addition to the immediate agency of 

 temperature in modifying the specific gravity of 

 the atmosphere, the amount of aqueous vapour 

 diffused through it exerts a material influence. 

 It has been already shewn, that the quantity of 

 vapour in the atmosphere, c&teris paribus, is 

 proportional to temperature, the greater part of 

 which is confined to its lower strata, (within 

 two or three miles of the earth's surface) ; and 

 that at the temperature of 80 F., it is capable of 

 containing an amount of vapour equal to the 

 pressure of an inch of mercury, or ^th of its 

 whole weight. If, then, we take the estimate of 

 Gay Lussac, that the specific gravity of aqueous 

 vapour is less than that of air at the same tem- 

 perature, in the ratio of six to ten, it is evident, 

 that the specific gravity of the atmosphere must 

 be diminished in proportion to the quantity of 

 vapour diffused through it. Hence it is, that 



