56 



pressure or relieved tlierefrom. In the former case it must 

 be alternately raised and depressed from 80 inches below h 

 up to that level. In the latter it must be raised and 

 depressed from the level of b to 30 inches above it. Castor 

 oil is a useful medium to prevent the passage of air between 

 mercury and the glass vessels. 



It is important to add a little sulphuric acid to the mer- 

 cury, in order to remove the film of water which adheres to 

 the inside of the globe h. On this account it would, perhaps, 

 be desirable to substitute a i)lug of glass for the indiarubber 

 one between a and h. 



