184 THE CHEMISTRY OF CREATION. 



therefore, we state that the pressure of the 

 atmosphere is 151bs. on a square inch, we must 

 not forget that a part of this is due to the pres- 

 sure of the watery vapour in the air, though only 

 a small part in comparison with that caused by 

 the gaseous constituents of the air. 



Proportionally to the decrease of tempera- 

 ture, the elastic force of the vapour, or, in other 

 words, its pressure on the barometer, will be 

 diminished. In the late Antarctic Expedition, 

 the effect of a decrease of temperature on the 

 barometral pressure was particularly striking. 

 From the tropic of Capricorn, southward, a gra- 

 dual diminution in the height of the mercurial 

 column was observed as the latitude increased. 

 At Kerguelen's Island the mean height of the 

 barometer was 29.497. In S. lat. 66, 29.078 ; 

 and in lat. 74, it was only 28.928. Thus the ' 

 mean position of the mercury in the higher lati- 

 tudes of the antarctic regions, was nearly an inch 

 lower than in other parts of the world. Similar 

 results have been observed in Siberia. 



The manner of estimating the pressure of the 

 watery vapour, independently of the gaseous 

 atmosphere, is by calculations based upon an 

 ingenious little instrument called Daniell's Dew- 

 Point Hygrometer, which will be -noticed on 

 another occasion. 



Although insensible to ourselves, the pressure 

 of the air is of great consequence to our well- 



