22 HYDRAULICS AND ITS APPLICATIONS 



in feet of water. 1 An air cock is provided at C, by which air may be 

 pumped into or withdrawn from the tubes; so that whatever the mean 

 pressure in the pipe, the free surfaces of the columns may be brought 

 to a convenient height on the gauge scales. For very small pressure 

 differences the upper part of the U is filled with some liquid whose specific 

 -mvity is slightly less than that of water. Gasoline (specific gravity 

 approx. -71); kerosene (specific gravity approx. '8); toluene (methyl- 



Air Cock 



.C 



.0. 



Km. 10. 



benzene, specific gravity approx. '87); or sperm oil, are suitable liquids 

 for this purpose. If S is the specific gravity of this liquid, and h is the 

 difference of level of the surfaces of separation in the two tubes, the 

 difference of pressure is that due to the difference in the weights of two 

 columns, one of water and the other of this secondary liquid, and is 



1 Actually, the difference between the pressures produced by a column of water and one of 

 air, each of the same height h. Owing to the small specific gravity of air the weight of the 

 latter may, however, bo safely neglected, except where the pressure in the pipe is very great. 



