In the year 1831, bridges of cast iron existed; but no attempt had
been made to employ wrought iron in girder bridges, although Telford
had employed it in the Menai Suspension Bridge; but in 1881, the
introduction of railways, and the improvement in iron manufactures,
have demanded, and have rendered possible the execution of such
bridges as the tubular one, spanning the Menai Straits, in span of 400
feet, and the Saltash, over the Tamar, with spans of 435 feet; while
recent great improvements in the manufacture of steel have rendered
possible the contemplated construction of the Forth Bridge, where
there are to be spans of 1,700 feet, or one-third of a mile in length.
Mr. Barlow, one of the engineers of this bridge, has told me that
there will be used upwards of 2,000 more tons of material in the Forth
Bridge, to resist the wind pressure, than would have been needed if no
wind had to be taken into account, and if the question of the simple
weight to be carried had alone to be considered. With respect to the
foundation of bridges, that ingenious man, Lord Cochrane, patented a
mode of sinking foundations, even before the first meeting of the
British Association, viz., as far back, I believe, as 1825 or 1826;
and the improvements which he then invented are almost universally in
use in bridge construction at the present day. Cylinders sunk by the
aid of compressed air, airlocks to obtain access to the cylinder, and,
in fact, every means that I know of as having been used in the modern
sinking of cylinder foundations, were described by Lord Cochrane
(afterwards Earl of Dundonald) in that specification.