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SURVEYING, MARINE AND SUBMARINE. 51
with a movable pencil suitably situated so as to mark the surface of a paper-
covered cylinder, which is rotated by clockwork.
In taking soundings, it is necessary to have a tide-gauge immediately
adjacent to the scene of operations, and when the scope of these is extensive,
several gauges at varions points will be required, because fluctuations in the
water level are frequently local, and they are by no means uniform.
Examples of permanent tide-gauge stations are shown in figs. 47 and 48:
one situated at the edge of a quay wall, and the other on a river bank. In
Tank for Flushing
IPurposes
Fig. 48.—Tide-gauge House on River Bank.
124 inch FarthepwarePipa
muddy places a flushing bank is provided for cleansing the gauge well and
maintaining free access of water to it.
The Determination of Currents.—It is important to the engineer
to know the directions taken by tidal currents at various times during the
day, and to observe their relation in regard to the configuration of the coast-
line and the maintenance of navigable inlets. To acquire this knowledge,
he has in many instances to fall back upon personal observation, and one of
the earliest steps in connection with the laying out of harbour work will be
to acquire the requisite data in regard to current flow.
Floats.—The most obvions method of observing the set or direction of
a current is by means of some floating object. Any substance drifting upon
the surface of the water affords a means of recognising the trend of tide or
stream. Yet it must be pointed out that so simple an expédient—despite
its apparent reliability—is not without very serious drawbacks, and that its
universal efficacy is by no means to be taken for granted.
In the first place, paradoxical as it may seem, the topmost layer of the
water may flow in a different direction to the lower layers or main body.
Fresh water has a less specific gravity than salt water, and does not readily
mix with it. A fresh-water stream, encountering a tidal inset, will therefore
flow over it for some distance before becoming incorporated therein. The
wind also is capable of exerting so powerful an influence on the surface of a