242 EXPERIMENTS ON MAHIOTTE'S LAW. 



atmosphere. The tube b is now made to slide upwards, 

 and clamped so that the top of the mercury in it is oppo- 

 site to the division 169 cm of the scale, as in fig. 166 B. 

 The air above the mercury in a is now not only under 

 the pressure of the atmosphere, but it has to bear the 

 additional pressure of the raised column of mercury, 

 and is therefore compressed. It will be found that the 

 column on the left side, in a, rises to 95 cm , when that at 

 the right side, in $, stands :at 169 cm . It follows that 

 the volume of air in a is diminished by <one half, and 

 supports an additional pressure of 169 ;95 = 74 cm , 

 or, since the pressure of the external atmosphere 

 is also 74 cm , the air in a is now under a pressure 

 wliicli is double of what it was originally. If b 

 is now made to slide downwards, and the top of 

 the column reaches again 90 cm , the column in a is 

 .also at 90 cm . and the volume of the air above the 

 mercury in it has again increased. When b slides 

 lower down than 90 cm , the mercury in & also falls 

 below 90 cm but not so far as that in b is lowered ; 

 for example, if b be now clamped, so that the mer- 

 cury in it -stands at 43 cm , as shown in fig. 166 C, the 

 mercury in a will stand at 80 cm . Evidently in this 

 position of the two columns, the pressure of the air 

 above the mercury in a must be less than the pres- 

 sure of the external air upon the column in b ; thef 

 ^difference in the height of the two columns is, 

 ;80 4.3 = 37 cm , the pressure therefore in a must bei 

 less by 37 em than the atmospheric pressure which acts 

 in 6, that is, the air in <a is now under a pressure of 

 ;74 3(7 = 37 cm , or the -pressure is half of what it was 

 originally. These two experiments demonstrate, that 



