on Rivers and Canals. 603 
made under a greater angle of incidence ; from whence 
it follows, that the force of percuffion, and the quantity 
of corrolion and of detriment done to the banks and 
weirs of rivers, and to the walls of buildings made therein, 
and which are expofed to that percuffion, are always in a 
direct compound proportion of the angle of incidence , of the 
greatnefs and depth of the feciion together , and of the 
quantity of velocity of the current. 
5thly, It may happen in time, that the excavation of 
the bottom, and the corrolion of the lides, will have fo 
changed the form of the bed as to bring the force of per- 
cuffion into equilibrium with the velocity and direction 
of the current; in that cafe, all farther corrolion and ex- 
cavation of the bed ceafes (N° 31.) 
6thly, This gives the reafon why when one river 
falls into another almoft in a perpendicular direction, and 
makes with it too great an angle of incidence, this di- 
rection is changed in time, by corrolions and alluvions, 
into an angle much more acute, till the whole comes 
into equilibrium. 
7thly, So great and fuch continued irregularities, from 
local caufes, may happen in the motion of a river, as will 
intirely change its ancient bed, corrode through the 
banks, where they are expofed to the greateft violence of 
percuffion 
