584 -Mh mann’s Treat if* 
31. N ot wit hftanding all we have faid concerning the 
neceffity of augmenting the depth of a river in a greater 
proportion than its breadth, if we would accelerate its 
current; yet it is certain, that this can only be done to a 
certain point, without deftroying that equilibrium which 
ought to reign between the depth and the breadth of the 
fedtion of the ftream, and thereby putting the river into 
a ftate of continual violence, which will incefiantly exert 
itfelf to the deftrudtion of the banks and wiers made to 
keep it in, and that adtion will always exert itfelf in a 
diredt ratio of the greater or lefs want of equilibrium, as 
it would be eafy to demonftrate by the principles of hy- 
draulics. Thefe fame principles give likewife the juft pro- 
portions of this equilibrium between the perpendicular 
and lateral compreffion of the water in any river or canal 
whatfoever, which vary in an inverfe proportion, accord- 
ing to the different degrees of the declivity and ■velocity 
of the current; and in a diredt one of the greater or lefs 
coherence and hardnefs of the fubftances which com- 
pofe the bed. Rivers which flow in beds compofed of 
homogeneous matter of little confiftency, fuch as fand, 
See. are always more broad than deep, when compared to 
thofe which run in beds of matter of greater tenacity. It 
is manjfeft, that the equilibrium here fpoken of is real, 
4 becaufe 
