that is, the normal velocities are inversely as the areas of the cross
sections. This is true of the mean velocities, if at each section the
velocity of the stream varies. In a river of varying slope the
velocity varies with the slope. It is easy therefore to see that in
parts of large cross section the slope is smaller than in parts of
small cross section.
If we conceive a space in a liquid bounded by normal sections at A1,
A2 and between A1, A2 by stream lines (fig. 8), then, as there is no
flow across the stream lines,