The tubes were floated to the places whence they were elevated to their
positions on eight huge pontoons, fitted with valves and pumps to
exhaust the water from them, when all was ready to float the prodigious
iron beams. These pontoons or boxes were each 90 feet long, 25 feet
wide, and 15 feet deep. The pontoons having been placed under one of the
tubes (sections), the floating was easily effected, and the operation is
thus described by the "Assistant Engineer."
"The operation of floating the tubes (the four sections, and one only at
a time), will be commenced by closing the valves in the pontoons at low
water; as the tide rises, the pontoons will begin to float, and shortly
afterwards to bear the weight of the tube, which will at last be raised
by them entirely off its temporary supporting piers; about an hour and a
half before high water, the current running about four miles an hour, it
will be dragged out into the middle of the stream, by powerful capstans
and hawsers, reaching from the pontoons at each end, to the opposite
shore. In order to guide it into its place with the greatest possible
certainty, three large hawsers will be laid down the stream, one end of
two of them being made fast to the towers (piers) between which the tube
is intended to rest, and the other to strong fixed points on the two
shores, near to and opposite the further end of the tube platforms; in
their course, they will pass over and rest upon the pontoons, being
taken through 'cable-stoppers' which are contrivances for embracing and
gripping the hawser extended across the stream, and thereby retarding,
or if necessary entirely destroying, the speed induced by the current."