Another method for constructinfr a foundation under water was suc-
cessfully practised during the building of the light-house at Haut
Banc du Nord. This is a limestone plateau about two miles off the
northeastern extremity of the Isle of R<5. It is nearly horizontal
and about fifteen hundred feet long by five hundred broad. It is cut
up by numerous channels of various depths, and is only uncovered
at low spring tides, when the wind happens at the same time to be
off shore. The rocky shoals surrounding it permit only crafts of
small tonnage to approach, and at low tide the winding channels
render this approach difficult even for the smallest boats. It was
therefore necessary to anchor the transports at some distance from
the rock, and to convey the material to it in barges — a difficult and
risky business, even in a moderately rough sea.
To establish the foundation there was first constructed a large iron
bottomless caisson, in the form of a twelve-sided prism, which was
suitably braced on the inside. It was nearly forty-seven feet in
diameter, four feet high, and weii»hed about five tons. Sustained by
means of floats, it was towed into place at high tide by a small
steamer and then jjroundcd on the rock. After tho tide had fallen
sufficiently, and while it was not too rough, a movable flooring was
placed on the upper frame of the interior bracing, and on it the work-
men made the concrete for filling the caisson.
Tho lower part of the foundation was made of concrete poured
through copper tubes one foot in diameter, and funnel-shaped at top.
