THE BAROMETER. 243 



the heights of mountains, hills, and the leading points of 

 an extensive district of country. In rising above the level 

 of the sea, the weight of the air above us becomes less ; 

 that is, the pressure of the air upon the barometer de- 

 creases, and the column of mercury gradually falls as ^\e 

 ascend. To determine, therefore, the height of a mount- 

 ain, we have only to place one barometer at its foot while 

 another stands at the top, and then, by observing the 

 difference in the height of the mercury, we are enabled to 

 calculate the height of the mountain. The following ta- 

 ble shows how much the barometer falls at different alti- 

 tudes, thirty inches being taken for the sea-level :* 



At 1,000 feet above the sea, the column flills to 28.91 inches. 



2,000 " " " " " 27.86 " 



3,000 " " " '' " 26.85 " 



4,000 ' " '' " " 25.87 " 



5,000 " '' " " " 24.93 " 



Iniile " '^ '^ " 24.67 



2 " " " " " 20.20 " 



3 " '' " '^ " 16.68 '' 



4 ^' '' " " " 13.72 " 



5 " '' '^ " " 11.28 " 

 10 " " *' " '* 4.24 " 

 15 '' " " " " 1.60 " 

 20 " " " " " 0.95 " 



At the level of the sea, the barometer falls about one 

 hundredth of an inch for a rise of nine feet, or a little 

 more than the tenth of an inch for a rise of one hundred 

 feet. At a height of one mile it requires about eleven 

 feet rise to sink the mercury a hundredth of an inch. 



In selecting land in mountainous districts of the coun- 

 .try, where degrees of frost increase with increased alti- 

 tudes, and* where the height of one portion above another 

 has an important relation to the cost of drawing loads up 



* The mercury rarely stands as high as 30 inches at the level of the 

 sea, the mean height being about 29.5 inches. But this does not affect 

 the measurement of heights, which is determined, not by the actual 

 height, but by the difference in heights. 



