Dock gates.

The gates closing the entrances and locks at docks are made of wood or
of iron. In iron gates, the heelpost, or a vertical closing strip
attached to the outer side of the gate close to the heelpost, the
meeting-post at the end of each gate closing against each other when
the gates are shut, and the sill piece fitting against the sill are
generally made of wood. Wooden gates consist of a series of horizontal
framed beams, made thicker and put closer together towards the bottom
to resist the water-pressure increasing with the depth, fastened to
the heelpost and meeting-post at the two ends and to intermediate
uprights, and supporting water-tight planking on the inner face (fig.
21). Iron gates have generally an outer as well as an inner skin of
iron plates braced vertically and horizontally by plate-iron ribs, the
horizontal ribs being placed nearer together and the plates made
thicker towards the bottom (figs. 22 and 23). Greenheart is the wood
used for gates exposed to salt water, as it resists the attack of the
teredo in temperate climates. As cellular iron gates are made
water-tight, and have to be ballasted with enough water to prevent
their flotation, or are provided with air chambers below and are left
open to the rising tide on the outer side above, the gates are light
in the water and are easily moved; whereas greenheart gates with their
fastenings are considerably heavier than water, so that a considerable
weight has to be moved when the water is somewhat low in the dock and
the gates therefore only partially immersed. On the other hand, wooden
gates are less liable than iron gates to be seriously damaged if run
into by a vessel.

Dock gates are sometimes made straight, closing against a straight
sill (figs. 20 and 23); and occasionally they are made segmental with
the inner faces forming a continuous circular arc and closing against
a sill corresponding to the outer curves of the gates (fig. 22), or by
means of a projecting sill piece against a straight sill (fig. 21).
More frequently the gates, curved on both faces, meet at an angle
forming a Gothic arch in plan, and close by aid of a projecting piece
against a straight sill, which in the Barry entrance gates is modified
by making the outer faces nearly straight (fig. 19), giving an unusual
width to the centre of the gates. The pressures produced by a head of
water ag