

THE BAROMETER. 233 



the tube remains filled with mercury, but as soon as 

 the tube is placed in a vertical position the mercury 

 leaves the upper end and forms a column which has 

 a height of somewhat more than 70 cm , as shown in fig. 



161. 



A contrivance for measuring the atmospheric pres- 

 sure is called a barometer. Fig. 160 represents a 

 water-barometer, fig. 161 a mercurial barometer. The 

 former kind is very rarely used, the latter most fre- 

 quently. Observations made at different places prove 

 that the height of the column is not everywhere the 

 same; the pressure of the atmosphere is therefore dif- 

 ferent in different localities. These differences arise 

 from the irregularities in the earth's surface ; the 

 bottom of the aerial ocean is not even, but some 

 regions are elevated, some depressed, and since in the 

 air as in a liquid the pressure increases with the depth, 

 because the upper layers press upon those below, it 

 follows that the pressure of the atmosphere is greater 

 on the plain and in valleys than upon hills and moun- 

 tains. 



The lowest regions on the earth's surface are those 

 situated along the sea-coast; at these localities the 

 mercurial column indicates therefore the greatest pres- 

 sure. The average pressure at the level of the ocean 

 is about 760 mm , and the height of the column decreases 

 by l mm for every ll m which the barometer is carried 

 her; thus at a place which is 330 m above the 

 evel of the sea, the average height of the column is 



OQA 



760- ^ = 730 mm . When the barometer is observed 

 in the same place for some time daily, variations in the 



