[ +95 ] 
Therefore the velocity in the open chanel is to 
the velocity in the narrower paffages, as the fec- 
tion of thofe paffages is to the fedtion of the open 
chanel. 
But the heights in both fedtions being equal, 
the fedtions are diredtly as the breadths ; 
Confequently the velocities are reciprocally as 
the breadths. 
VI. In a running ftream , the water above any ob- 
ftacles put therein will rife to fuch a height , that 
by its jail the ftream may be dij charged as faft as 
it comes. 
For the fame body of water, which flowed in 
the open chanel, muft pafs thro’ the paffages made 
by the obftacles : 
And the narrower the paffages, the fwifter will 
be the velocity of the water : 
But the fwifter the velocity of the water, the 
greater is the height, from whence it has de- 
scended : 
Confequently the obftacles, which contract the' 
chanel, caufe the water to rife againft them. 
But the rife will ceafe, when the water can run 
off as faft as it comes : 
And this muft happen, when, by the fall be- 
tween the obftacles, the water will acquire a ve- 
locity in a reciprocal proportion to that in the 
open chanel as the breadth of the open chanel is 
to the breadth of the narrow paffages. 
VII. 'The quantity of the fall caufed by an obft a cle in 
a running ft ream is meaj'ured by the difference be- 
tween 
