ainst these gates when closed depends not only on the form of
the gates, but also upon the projection given to the angle of the sill
in proportion to the width of the lock, which is known as the rise,
and is generally placed at a distance along the centre line of the
lock, from a line joining the centres of the heel-posts, of about
one-fourth the width. With straight gates, the stresses consist, first
of a transverse stress due to the water-pressure against the gate,
which increases with the head of water and length of the gate; and
secondly, of a compressive stress along the gate, resulting from the
pressure of the other gate against its meeting-post, which is equal to
half the water-pressure on the gate multiplied by the tangent of half
the angle between the closed gates, varying inversely with the rise.
Though an increase in the rise reduces this stress, it increases the
length of the gate and the transverse stress, and also the length of
the lock. By curving the gates suitably, the transverse stress is
reduced and the longitudinal compressive stress is augmented, till at
last, when the gates form a horizontal segmental arch, the stresses
become wholly compressive and uniform in each horizontal section,
increasing with the depth; and the total stress is equal to the
pressure on a unit of surface multiplied by the radius of curvature.
Though the water-pressure is most uniformly and economically borne by
cylindrical gates, they are longer, and encroach more upon the lines
of quay with their curved recesses than straighter gates; and,
consequently, Gothic-arched gates are often preferred. Straight gates
afford the greatest simplicity in construction.

[Illustration: FIG. 24.--Sliding Caisson.]

[Illustration: FIG. 25.--Ship Caisson.]

Gates in wide entrances or locks are generally supported towards their
outer end by a roller running along a castiron roller-path on the gate
floor (figs. 19, 21 and 22), as well as by the heelpost, fitted over a
steel pivot at the bottom, and tied back against the hollow quoins at
the top by anchor straps and bolts, on which the gate turns. In some
cases, by placing the water ballast in iron gates close to the
heelpost, a roller has been dispensed with, even, for instance, at the
wide entrance at Havre (fig. 23). The gates are opened and closed,
either by an opening and a closing chain for each gate, fastened on
eit