6oo Mr. mann’s T 'reatlfe 
thefe grounds above the reft of the plain. Again, the 
waters in the bed itfelf depofing inceffantly a part of the 
mud, See. brought down by the ftream, muft continually, 
though infehfibly (for a long fpace of time) raife the 
channel and banks of the river above the reft of the 
plain. Thefe caufes may at laft contribute to the form- 
ing of an intire new bed for the river; for as all rivers 
carry down in their ftreams more or lefs mud and other 
heterogeneous matters, which do not fublkle regularly 
in all parts alike, but muft precipitate fafteft where the 
current is lloweft; there muft accumulate by little and 
little in thefe parts fuch banks of fand and mud, as will 
in time hinder the current of the waters, make them re- 
flow, and at laft totally change their direction. 
Canals are ftill more fubjeft than rivers to have their 
beds raifed and their currents flopped by the fubfiding of 
mud and heterogeneous matter in different places, and 
efpecially juft above their fluices ; becaufe of the fudden 
ftagnation of the water which firft begins there as often 
as the fluices are flint : and as there is a neceflity for 
keeping them for the moft part fliut, the ftagnating wa- 
ters in their beds muft precipitate their mud, See. in a 
much greater proportion than can be done in the cur- 
rents of rivers, which are in a continual motion towards 
the fea, 
42. I 
v 
