152 EQUILIBRIUM OF FLUIDS. 



EQUILIBRIUM OF FLUIDS. 



ALTHOUGH the actions of various machines already spoken of depend in a 

 great measure upon the laws of hydrostatics and pneumatics, they may be 

 perfectly well understood, without particular reference being made to the 

 theory of the equilibrium and motion of elastic and non-elastic fluids, and 

 without having recourse to the various formulae by which their effects are 

 calculated. 



For instance, in describing the action of a gasometer, it would be only 

 necessary, in a practical point of view, to know that the pressure of the gas 

 upon the surface of the water contained in the tank displaces a column of 

 that water equal to the weight or gravity of the vessel, and causes it to float 

 upwards until the gas ceases to enter, and which, if both ingress and exit are 

 prevented, will remain at the height it has attained as long as the temperature 

 remains the same. The action of all such vessels will depend upon the fol- 

 lowing laws : 



1 . The upper surface of any fluid in any vessel, or in a number of com- 

 municating vessels, is horizontal. 



2. Pressure is distributed equally in all directions, and acts perpendicularly 

 upon every point of the surface of the vessel which contains that fluid. 



3. The pressure of a fluid on the horizontal base of a vessel in which it is 

 contained is as the base and perpendicular altitude, whatever be the shape of 

 the vessel which contains it. 



4. When the heights of the same fluid are equal, the pressures are as the 

 bases ; when the bases are equal, the pressures are as the heights ; when both 

 heights and bases are equal, the pressures on the horizontal bottoms are equal 

 in all, however irregular the shape and capacities of the' vessels may be. 



5. In different vessels containing different fluids, the pressures are as the 

 areas of the bottoms multiplied by their depths, multiplied by their specific 

 gravities. 



