[Illustration: Hours before turn of tide. FIG 10]

It will be noticed that certain of the points thus obtained can
be joined up by a regular curve which can be utilised for
ascertaining the probable time at which the current will turn
on tides of height intermediate to those at which observations
were actually taken. For instance, from the diagram given it
can be seen that on a 20 ft tide the current will turn thirty
minutes before the tide, or on a 15 ft tide the current will
turn one hour before the tide. Some of the points lie at a
considerable distance from the regular curve, showing that the
currents on those occasions were affected by some disturbing
influence which the observer will probably be able to explain
by a reference to his notes, and therefore those particular
observations must be used with caution.

The rate of travel of the currents varies in accordance with
the time they have been running. Directly after the turn there
is scarcely any movement, but the speed increases until it
reaches a maximum about three hours later and then it decreases
until the next turn, when dead water occurs again.

Those observations which were started at the turn of the
current and continued through the whole tide should be plotted
as shown in Fig. 11, which gives the curves relating to three
different tides, but, provided a sufficiently large scale is
adopted, there is no reason why curves relating to the whole
range of the tides should not be plotted on one diagram. This
chart shows the total distance that would be covered by a float