, and
also for the side walls and invert commonly enclosing the lock-chamber
at the sides and bottom, are generally constructed simultaneously with
the dock works, under shelter of a cofferdam across the entrance
channel, and in the excavations kept dry by means of pumps. The
foundations under the sills and adjacent side walls are carried down
to a lower level than the rest, and if possible to a water-tight
stratum, to prevent infiltration of water under them owing to the
water-pressure on the upper side of the gates; or sometimes one or two
rows of sheet piling have been driven across the lock under the sills
to an impermeable stratum, to stop any flow. The foundations for the
sills consist usually of concrete deposited in a trench extended out
under the adjoining side walls. The sill, projecting generally about 2
ft. above the adjacent gate floor over which the gates turn, is built
of granite; and the same material is also used for the hollow quoins
in which the heelpost, or pivot, of the dock gates turns, and which,
together with the sills, are exposed to considerable wear. The side
walls of the lock-chamber are very similar in construction to the dock
walls; but they are strengthened against the loss of water-pressure in
front of them when the water is lowered in the chamber by an inverted
arch of masonry, brickwork or concrete, termed an "invert," laid
across the bottom of the chamber along its whole length, against which
the toe of each side wall abuts and effectually prevents any forward
movement. The side walls also, alongside the gates at each end, abut
against a thick level gate floor and apron, and, moreover, are
considerably widened to provide space for the sluiceways and gate
machinery.

The new Florida lock (fig. 20), forming the main entrance through the
new approach harbour and tidal harbour to the Eure dock and other
docks of the port of Havre, is the largest lock hitherto constructed.
It has an available length of chamber between the gates of 805 ft., a
width of 98-1/2 ft., and depths over the sills of 15-3/4 ft. at the
lowest low water of spring tides, 23-1/2 ft. at low water of neap
tides, 35 ft. at high water of neap tides, and 40-1/2 ft. at high
water of spring tides. Owing to the alluvial stratum at the site of
the lock close to the Seine estuary, of which it doubtless at one time
formed part, the foundations for the sill and