PROPERTIES OF GASES 



from the surface of the mercury in the basin to the top of the mercury 

 column. 



Slant the barometer and measure the vertical height of the mercury 

 by means of a plumb-line. Compare it with the previous reading. 



The mercurial barometer. The apparatus represented in Fig. 50 

 calls to mind the U-tube method of determining densities by balancing 



__ columns of liquid (p. 61). Here, 



however, the columns balancing the 

 mercury in the tube A is not 

 apparent. The experimental fact 

 that the column A does not rush up 

 through to adjust the liquid levela 

 to equality demands explanation. 

 Remember that the top of the 

 column A is closed and protected 



31 

 301 



FIG. 50. The pressure of the air on the 

 mercury in the small tube i? able to keep 

 up the column of mercury in the long tube. 



FIG. 51. Construction of a barometer. 



while is open. Something acts at O and is capable of sustaining 

 the column A. This something is the weight of the air. In fact, a 



