on Rivers and Canals. 587 
portion thereto (N° 23.)* Now as the waters muft con- 
tinue to come down from their lources, if their free run- 
ning off is hindered by any obftacles whatever, their re- 
lative height back from them muft neceffarily be increafed 
until their elevation, combined with the velocity of their 
current proceeding from it, be arrived to fuch a pitch at 
the point where the obftacles exift, as to counterbalance 
the quantity of oppolition or impediment proceeding 
from thence, which frequently does not happen until 
all the lower parts of the country round about are laid 
under water. 
34. Now it is certain from all experience, that the 
beds of rivers and canals in general are fubjecl to feme 
or others of the obftacles above mentioned. If rocks or 
trees do not bar their channels, at leaft the quantity of 
fand, earth, and mud, which their ftreams never fail to 
bring down, particularly in floods, and which are un- 
equally depofed according to the various windings and 
degrees of fwiftnefs in the current, muft unavoidably, in 
courfe of time, fill up, in part, different places in the 
channel, and thereby hinder the free running off of the 
back waters. This is certainly the cafe, more or lefs, in 
all rivers, and in all canals of long, ftanding, as is 
notorious to all thofe well acquainted with them. 
Hence, if thefe accidents are not carefully, and with a 
conftant 
