576 Mr. mann’s Treatife 
to the fuperior ones, which in an horizontal bed, with- 
out this caufe, would, have no other motion than that 
which is imprefled upon them by the impulfive force of 
the waters defcending from their elevated fprings. 
From whence it appears, that the fuperior and inferior 
waters in a river communicate reciprocally a part of their 
motion to each other; but this can never go beyond a 
certain point or maximum, which is always, proportion- 
able to the momentum of the river in that place (N° 1 8 .)., 
It follows from hence, that the greateft velocity of a 
river, running in a right-line, is in the center of its fec- 
tion (N° 6.); that is to fay, in that point which is the. 
fartheft poffible from the furface of the water and from 
the bottom and lides of the bed, all taken together. This, 
part has the advantage of one half of the depth of wa- 
ter prefling upon it, and it is exempt from the friction 
of the bottom, and lides of the bed which are there over- 
come and vanifli by the perpendicular compreffion. 
On the contrary, the leaft velocity of the water is at 
the bottom and lides of the bed, becaufe it is there that 
the refinance produced by fribtion is greateft, from 
whence it is communicated to the other parts of the fec- 
tion in an inverfe. duplicate proportion of the diftances from 
the bottom and Jides combined together , until it becomes a 
negative 
