ggo Mr. mann’s freatife 
c derated', fo that their velocity increafes in an inverfe ratio 
of the breadth of the bed , and alfo of the greatnefs of the 
feclion ; from whence are deduced the two following 
univerfal practical rules : 
: ift, To augment the velocity of water in a river or 
canal, without augmenting the declivity of the bed, we 
mull increafe the depth and diminijb the breadth of its 
bed. 
adly, But to diminifh the velocity of water in a river 
or canal, we mull, on the contrary, increafe the breadth 
and diminifh the depth of its bed. 
The above proposition is perfectly conformable to ob- 
servation and experience ; for it is conffcantly feen, that 
the current is the fwifteft where the waters are deepeft 
and the breadth of the bed the lead:; and that they flow 
flowed: where their depth is the lead: and the breadth of 
the bed the greated:. “ The velocity of waters,” fays 
M. de buffon (b> , “ augments in the fame proportion as 
“ the fe<5tion of the channel through which they pafs 
f ‘ diminifhes, the force of impulfion from the back-waters 
“ being fuppofed always the fame. Nothing,” continues 
he, <£ produces So great a diminution in the fwiftnefs of 
M a current as its growing fhallow ; and, on the contrary, 
u the increafe of the volume of water augments its 
(b) Hift. Nat. torn. II. p. 53. 60. edit, in i2mo. 
S u velocity 
