on Rivers and Canals. 633 
fource, being only 250 fathom perpendicular in a courfe 
of 500 leagues, it will be fnfficient to give a notable cur- 
rent in a regular bed throughout all that extent of river, 
as appears from what we have faid above (N° 38.). But 
the depreffion of the curve of the river's bed y below the 
horizontal plane which is a tangent to its fource , in this 
fame extent of courfe, is not lefs than ninety leagues per- 
pendicular, being always the fecant of the arc of the ri- 
ver’s extent minus a radius of the earth in that point. 
70. It follows evidently from the above princples (N° 
62 — 69), that the declivity and velocity of a river are 
lefs in proportion as the bed approaches nearer to being 
concentrical with the curve of the earth’s furface. 
7 x . I fhall now apply the principles laid down to de~ 
termine, as near as poffible,the real quantity of declivity 
in different rivers, making ufe of what is already known 
from experiments and actual menfuration to determine 
the fame in all others by the comparifon of the different 
degrees of velocity in their refpe&ive currents. 
72. It is the general opinion of moft of thofe who 
have examined this fubjecSt (k, y that rivers and canals 
which have lefs than one foot of declivity in x 0,000 feet 
of courfe, will have very little current, unlefs it be by 
means of the great abundance of their upper waters 
(k) Vide deschai.es, de Font, et Fluv. 
a which; ; 
