382 Hydrodynamics. 



keeps it in that state ; and the force which keeps common air at 

 its ordinary density is the pressure of the atmosphere, and equiv- 

 alent to that of a column of mercury 30 inches in height. If, 

 therefore, we suppose this air, instead of being confined by the 

 top of the vessel, to be pressed down by a rnoveable piston car- 

 rying a cylinder of mercury of the same base and 30 inches 

 high, its elasticity will balance this pressure just as it does the 

 pressure of the atmosphere ; and since from its fluidity the pres- 

 sure received on any one part is propagated through every part 

 and in every direction, it will press on any small portion of the 

 vessel by its elasticity, as when loaded with this column. Hence, 

 if this small portion of the vessel be removed leaving an opening 

 into the void, the air will begin to flow out with the same velo- 

 city as it would flow out when pressed by its own weight only, 

 or with the velocity acquired by falling from the top of a ho- 

 mogeneous atmosphere, or 1338 feet per second. But as soon 

 as a portion of air had passed through the orifice, the density of 

 that remaining in the vessel being reduced, its elasticity, and 

 consequently the expelling force, is diminished. But the matter 

 to be moved is diminished in the same proportion as the density, 

 the capacity of the vessel remaining unchanged ; therefore, since 

 the density and elasticity follow the same law, the mass moved 

 will vary as the moving force, and the velocity will continue the 

 same from the beginning to the end of the efflux. 



495. The velocity with which the air issues out of a vessel 

 under the circumstances above supposed, being constant, we can 

 readily compare the velocity given by the theory with that 

 Fig. 239. found by experiment. Let A be a cask of known capacity in 

 the top of which is an aperture a of a known area. The tube 

 TB, recurved at $, is soldered or screwed into the top of the 

 cask. The aperture a is stopped while water is poured into the 

 tube T till it is full, at which time a quantity of water will have 

 passed out at B condensing the air in the cask till its spring is 

 equal to the weight of the water in the tube. At this time a 

 cock placed over the tube J 1 , sufficiently large to supply water 

 as fast as it can descend into the vessel A, is to be opened to 

 keep the tube constantly filled. For this purpose one person 

 must constantly tend it, while another opens the aperture , 

 which needs only to be closed with the finger, the seconds being 



