eg ® 8 Mr. -mann’s T'reatife 
con ft ant attention prevented, come inundations, which 
fometimes lay wafte whole diftridls, and ruin the fineft 
trails of ground, by covering them with fand : hence 
rivers become unnavigable, and canals ufelefs, for 
the purpofes for which they were con ftr oiled. Canals, 
in particular, by reafon that their waters for the moft 
part remain ftagnant in them, are ft ill more liable than 
rivers to have their beds fill up by the fubfidingof mud, 
andfthat efpecially for fome diftance above each of their 
Unices; infomuch, that if continual care is not taken to 
prevent it, or remedy it as often as it happens, they will 
boon become incapable of receiving and palling the fame 
veil els as formerly. Nay, the very fluices themfelves, if 
the floors of their bottoms are not of a depth conform- 
able to the bed of the canal, will produce the fame acci- 
dents as thofe we have been fpeaking of; for if they are 
placed too low, they will be continually filling up with 
fand or mud; if too high, they have the fame effedl as 
banks or bars in the bed of a river, that is, they hinder 
all the back-waters under their level from running off, 
and foon fill up the bed to that height by the fubfiding 
of mud. This effedt is much accelerated by the fhutting 
of the lower fluices, which makes a great volume of 
water reflow -back to thofe next above them, till the 
whole is filled and becomes ftagnant. Now it is evident, 
that 
