on Rivers and Canals. 607 
directly over tbe line of greatejl current. The reafon 
thereof is, that there are in this cafe two different and 
oppofite currents in the river; that whereby the waters 
flow towards the fea, and preferve their motion therein 
even to a conflderable diftance; and that of the waters 
which remount, either by the flowing of the tide, or by 
their meeting with local obftacles, which form a counter 
current , fo much the more fenfible as the flowing of the 
tide is ftronger, or as the percuflion of the water is made 
again ft greater obftacles, and in a direction nearer to a 
perpendicular to them. From both thefe caufes, the 
greater of which by far is that of the tides, the water near 
the ftdes of the channel, where the velocity of the de- 
fending ftream is naturally the leaft (N° 25), takes a 
contrary direction, and runs back in the river, while that 
in the middle continues to flow on towards the fea. 
This counter current is what the French call a remous. 
An ifland in the middle of a river produces the fame 
effect as obftacles at the fides, regard being had to the 
difference of fituation of each. 
Eddies and whirlpools in rivers, in the center of which 
there appears a conical or fpiral cavity, and about which 
the water turns with great rapidity and fucks in whatever 
approaches it, proceed in general from the mutual per- 
cuflion of thefe two counter currents and the vacuity in 
4 K 2 the 
