38 Mr. Vince's Observations on the Theory 
it, which is to be balanced by a weight W ; E is the cylinder 
suspended to something immoveable at M, having its orifice 
r s as far distant from A B as before it was from the weight in 
the scale ; and let the orifice and scale be equidistant from C D. 
Stop the orifice, and fill the cylinder ; and upon opening the 
orifice, let one person, by means of a cock at v upon a pipe 
which goes into a reservoir xy z, keep the fluid in the cylinder 
exactly at the same altitude, and another put such a weight w 
into the scale L as shall keep A B exactly in the same position ; 
then the weight w is equivalent to the momentum of the fluid 
against A B, together with the momentum of the fluid enter- 
ing the top of the cylinder through the pipe. To determine 
what weight is equivalent to this latter momentum, take away 
the cylinder E and weight w, and bring A B up to the pipe, 
and let the fluid act upon it, and find what weight (v) put into 
the scale will now keep AB horizontal, and this weight (v) 
will be equivalent to the momentum of the fluid flowing into 
the cylinder ; hence w — v is a weight equivalent to the mo- 
mentum of the fluid issuing out of the cylinder at the vena 
contracta , and consequently equivalent to the diminution of 
the pressure upon the bottom after the opening of the orifice. 
In order to keep the fluid accurately at the same altitude, I 
should propose to have a floating gage v (fig. 8.) with a wire 
standing perpendicularly upon it, and entering a cylinder w 
attached to the side of the vessel, and of a bore just large enough 
to give it a free motion ; then the cock must be opened and 
adjusted to give it such an aperture as will keep the top of the 
wire on a level with the top of the cylinder. 
Or we may find the diminution of the pressure upon the 
bottom on opening the orifice in this manner. In fig. 6, take 
