on Rivers and Canats. 581 
<l velocity more than any other caufe whatever.” The 
celebrated Wolf, in his Hydraulics^, aftiires us, that 
“ it is a conftant and univerfal pradtice, for accelerating 
“ the current of waters, to deepen the bed, and at the 
“ fame time to render it narrower.” 
2 8 . When the velocity which a river has acquired by 
the elevation of its fprings and the impulfe of the back- 
water, is at laft totally deftroyed by the different caufes 
of refiftance which we have enumerated above (N° 21.) 
becoming equal or greater than the firft, the bed and 
current at the fame time being exadtly horizontal, no- 
thing elfe remains to propagate the motion, except the 
foie perpendicular comprejfon of the upper waters upon 
the lower , which is always in a direSl ratio of their depth. 
But this neceffary refource, this remaining caufe of mo- 
tion in rivers, augments in proportion as all the other di- 
minifh, and as the want of it increafes : for as the waters 
of rivers in extenfive plains lofe the acceleration of mo- 
tion acquired in their defcent from their fprings, their 
quantity accumulates in the fame bed by the junction of 
feveral ftreams together, and their depth increafes in 
confequence thereof. This jundtion and fucceffive accu- 
mulation of many ftreams in the fame bed, which we 
fee univerfally in a greater or lefs degree in all rivers 
throughout the known world, and which is fo abfolutely 
(c) N. 224* 
Vol. LXIX. 4 G neceffary 
