A double float with two
similar sub-floats at depths of 0.211 and 0.789 of the full depth would
also give this mean with more accuracy and convenience than any
instrument of its class; this instrument is new. Measurement of the
velocity at five eighths depth would also afford a fair approximation.
One hundred and fourteen average transverse velocity curves were
prepared from 714 separate curves. These average curves were all very
flat, and were convex down stream--over a level or concave bed--and
nearly symmetric in a symmetric section. The velocity was greatest near
the center, or deepest channel, decreased very slowly at first toward
both banks, more rapidly with approach to the banks or with shallowing
of the depth, very rapidly close to the banks, and was very small at the
edges, possibly zero. The figure of the curve was found to be determined
by the figure of the bed, a convexity in the bed producing a concavity
in the curve and _vice versa_, and more markedly in shallow than in deep
water. Curves on the same transversal, at the same site, and with
similar conditions, but differing in general velocity, were nearly
parallel projections. At the edges there was a strong transverse surface
flow from the edge toward mid-channel, decreasing rapidly with distance
from the edge. The discussion showed that it was almost hopeless to seek
the geometric figure of the curves from mere experiment.
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