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BREAKWATER CONSTRUCTION.
177
30 inches only above the surface. When about to be moved, they were
emptied by a centrifugal pump, which was inclosed in sheet iron casing to
protect it from the water, for it had to be sunk below the surface to avoid the
necessity of charging it. As the process of evacuation proceeded, two rows of
props or stays were inserted in the caisson at right angles to the sides at
each counterfort. These stays were destined to resist the external pressure
of the water, the side walls of the caisson being insufficiently strong in
themselves.
Next was placed on top of the
caisson a stout log of tiniber 60 feet
long and 15J inches square, and it was
firmly secured to anchorages in the
concrete. The object of this was to
assist the alignment of the caisson.
Two longitudinal timber walings
were placed along the sides of the
caisson and connected by through
bolts, forming transverse ties. This
was necessary to strengthen the side
walls in case of an excess of internal
pressure, and to prevent them from
yielding outwards in case of invasion
by waves in a rough sea.
Finally, a huge steel cable en-
circled the caisson and formed a
means of attachment to the tug.
The block was thus in every
««, «quipped »d ready for tarage. F». 1M--Sg^
It fioated easily. The hollow spaces
in its interior were adequate to counter-balance the weight of the concrete
and framing, and to permit it to emerge from 2 feet to 2 feet 6 inches out
of the water.
It was towed through the lock by a tug of 300 H.-P. At the outer gates
a second tug of the same power placed itself in tandem. In the open water a
third tug joined the rear of the procession so as to prevent the block from
yawing. Departure took place about the time of high water. At this time
the flood current is still very strong, and the caisson presenting its broadside
thereto tended to be drawn in a direction away from the jetty. About an
hour and a half was necessary for each journey.
It was found that on the way the caisson often shipped quantities of
water, and so wood coamings were provided to inerease the freeboard and
prevent the incursion of waves. This was the more necessary during that
period of the work when the roadstead was very exposed. As the work pro-
gressed, more and more shelter was obtained, and the blocks were finally able
to travel in perfect tranquillity.