on Rivers and Canals. 
59 S 
charge their waters, being fixed according to what we 
have faid in N° 37. the depth of the reft of the beds, and 
the quantity of declivity therein, muft be regulated in 
confequence thereof, fo as to increafe regularly the quan- 
tity of declivity in equal fpaces the farther we recede 
from their mouths, and proceed towards their fources or 
to the part where the regular current is to take place. 
If the depth and volume of water in a river or canal 
is confiderable, it will fuffice, in the part next the mouth, 
to allow one foot perpendicular of declivity through fix,, 
eight, or even, according to deschales (d) , ten thoufand 
feet in horizontal extent ; at mo ft it muft not be abqve 
one in fix or feven thoufand. From hence the quantity 
of declivity in equal fpaces muft flowly and gradually in- 
creafe as far as the current is to be made fit for naviga- 
tion ; but in fuch a manner, as that at this upper end 
there may not be above one foot of perpendicular decli vity 
in four thoufand feet of horizontal extent. If it be made, 
greater than that in a regular bed containing a confider- 
able volume of water, the current will be fo ftrong as to 
be found very unfit for the purpofes of navigation, as 
will appear hereafter, when 1 come to inveftigate the 
quantity of declivity in feveral rivers, the degree of 
fwiftnefs of whofe currents is well known. 
(d) De Fontibus ct Fluviis> prop. 4.9, 
39. £ 
