ly 100 feet above the cross.

The Conway Bridge is, in most respects, similar to the Britannia,
consisting of two tubes, of 400 feet span, placed side by side, each
weighing 1180 tons. The principle adopted in the construction of the
tubes, and the mode of floating and raising them, were nearly the same as
at the Britannia Bridge, though the general arrangement of the plates is
in many respects different.

It was determined to construct the shorter outer tubes of the Britannia
Bridge on scaffoldings in the positions in which they were permanently to
remain, and to erect the larger tubes upon wooden platforms at
high-water-mark on the Caernarvon shore, from whence they were to be
floated in pontoons.

The floating of the tubes on pontoons, from the places where they had
been constructed, to the recesses in the masonry of the towers, up which
they were to be hoisted to the positions they were permanently to occupy,
was an anxious and exciting operation. The first part of this process
was performed at Conway, where Mr. Stephenson directed it in person,
assisted by Captain Claxton, Mr. Brunel, and other engineering friends.
On the 6th March, 1848, the pontoons bearing the first great tube of the
up-line were floated round quietly and majestically into their place
between the towers in about twenty minutes. Unfortunately, one of the
sets of pontoons had become slightly slued by the stream, by which the
Conway end of the tube was prevented from being brought home; and five
anxious days to all concerned intervened before it could be set in its
place. In the mean time, the presses and raising machinery had been
fitted in the towers above, and the lifting process was begun on the 8th
April, when the immense mass was raised 8 feet, at the rate of about 2
inches a minute. On the 16th, the tube had been raised and finally
lowered into its permanent bed; the rails were laid along it; and, on the
18th, Mr. Stephenson passed through with the first locomotive. The
second tube was proceeded with on the removal of the first from the
platform, and was completed and floated in seven months. The rapidity
with which this second tube was constructed was in no small degree owing
to the Jacquard punching-machine, contrived for the purpose by Mr.
Roberts of Manchester. This tube was finally fixed in its permanent bed
on the 2nd of January, 1849.

[Picture: Conway Tubular Bridge]

The floating and fixing of the g