If, by chance, the sea should break through, the peril to life and
property would be great.
He therefore caused to be constructed and fitted inside each tunnel, at
the point where it branched from its main gallery, a stout iron door,
roughly hinged at the top and falling, in case of need, into the flange
of a thick wooden frame. The framework was fitted to the opening on the
seaward side, in a groove cut deep into the rock round each side and
top and bottom. The heavy iron door, when open, lay up against the roof
of the tunnel and was supported by two wooden legs. If the sea should
break through, the first rush of the water would sweep away the
supporting legs, the iron door would fall with a crash into the flange
of the wooden frame, and the greater the pressure the tighter it would
fit.
So the weight of the sea would seal the iron door against the wooden
casement, which would swell and press always tighter against the rock,
and that boring would be closed for ever. And if any man should be
inside the tunnel when the sea broke through, there he must stop,
drowned like a rat in its hole, unless by a miracle he could make his
way along the tunnel before the trap-door fell.
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