606 Mr. mann’s Treatife 
change the waters at. The Rhine, the Danube, the 
Wolga, the Indus, the Ganges, the Nile, the Miflifippi, 
and many other rivers, are inftances of this. 
4thly, All thefe effects are lefs fenfible at the mouths 
of little rivers, as their currents oppofe no fenfible obfta- 
cle to the flowing of the tides ; fo that the ebb carries off 
again what the flow had brought in. 
45. Whenever the courfe of a river throughout a con- 
derable extent of country approaches towards a right 
line, its current will have a very great rapidity; and the 
velocity wherewith it runs diminifiring the effedt of its 
natural gravitation, the middle of the current will rife 
up, and the furface of the river will form a convex curve 
of fufficient elevation to be perceived by the eye ; the 
higheft point of this curve is always directly above the 
line of great ejl current in the ftream. 
On the contrary, when rivers approach near enough 
to their months for a fenfible effedt to be produced in 
them by the flowing of the tides; and alfo when in 
other parts of their courfe they meet with obftacles at the 
fides of their channel : in both thefe cafes the furface of 
the water at the fides of the current is higher than in the 
middle, even though the ftream be rapid. In this fitua- 
tion of things, the furface of the river forms a concave 
curve, the loweft point of which, or that of inflexion, is 
3 diredtly 
