of the Motion and Resistance of Fluids . 37 
theory, the pressure on the bottom of the vessel is wholly 
taken off at the instant of time at which the water begins to 
flow ; and as this conclusion cannot be admitted, we may from 
hence learn, says the author, that this theory is not to be con- 
sidered as perfectly exact. It appears therefore to be an im- 
portant point to determine, what is the pressure of the fluid 
upon the bottom of a vessel compared with its whole weight at 
the time the fluid is running out. This may be determined to 
a great degree of accuracy by experiments constructed in the 
following manner. 
Let A B C D (Tab. III. fig. 6.) be a pair of scales, and O 
the fulcrum ; at the end of the arm C suspend a cylinder E, 
having an orifice r s , immediately under which place a weight 
w, so that the upper surface may be in the vena contracta, or at 
so small a distance below it that gravity can have produced 
no sensible effect upon the effluent fluid. Stop the orifice r s, 
and fill the cylinder with a fluid, and balance it by a weight W 
in the other scale. Then open the orifice, and the fluid will run 
out and strike w, and then be caught in the scale D. Now when 
the orifice is opened and the fluid flows out, the pressure upon 
the bottom of the cylinder is diminished, part of the fluid now 
not being supported, notwithstanding which the equilibrium is 
still continued ; which shows that the action of the fluid against 
w is exactly equal to the loss of weight in the cylinder by the 
motion of the fluid through the orifice. In order therefore to 
find the diminution of the weight upon the bottom of the cy- 
linder, we have only to find a weight equivalent to the mo- 
mentum of the fluid against w. 
Let AB (fig. 7 .) be a lever flat on the upper side, sus- 
pended by an horizontal axis C D ; L a scale hanging from 
