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52
HARBOUR ENGINEERING.
body of water in motion as
to make it move directly counter to the under
Fio. 49. — Float.
Sheet lead about
Hb. we/ght ; exact
Weight required
determined by
experiment.
portion. Then again, there are cross channel
currents producing a spiral action, in such
a manner that the particles of water move
across the bottom in a totally different direc-
tion to that which they take along the
surface.
And not only is direction affected, but
velocity is very much more involved. The
velocity of a moving stream varies very
materially throughout its depth. The
maximum lies about midway between the
surface and the bottom, at both of which
points there are sources of retardation, due,
in the former case, to aerial movement,
and, in the latter, to friction with the
ground.
This being so, floats for determining
the flow of currents should extend some
depth into the water so as to partake of
the influence of as many layers as possible.
Furthermore, for purposes of observation,
they must project sufficiently above the surface level to be accurately
observed, without exposing too great a surface to wind pressure.
A circular or square pole with a wooden
cylinder or prism at its lower end, weighted
so as to float vertically, affords a suitable
form of instrument. Such an indicator is
shown in fig. 49.
There are two ways of taking the neces-
sary observations, which fix the location
of the pole at any desired point of time.
Fio. 51.
In the first method, two operators, each with theodolite or sextant, are
stationed at a fixed distance apart along a base line on the shore (fig. 50). At
concerted signals each operator
measures the angle subtended
by the line joining the other
operatorand the float. The in-
tersection of the lines forming
these two angles respectively
determines the position of the
float at the time of observation.
For the second method (fig.
51) one operator will suffice, but
he must generally be provided with two sextants. Following closely in a