structural purposes is not older than the time of Smeaton, who in 1755
employed it for mill construction, and about the same time the great
Coalbrookdale Viaduct was erected across the Severn near Broseley, which
gave an impetus to the use of cast iron for bridge construction. The
viaduct had a span of 100 feet, and was composed of ribs cast in two
pieces; it was erected from castings designed by Mr. Pritchard, of
Shrewsbury, an architect, and this circumstance is worthy of note as
showing that an architect really was the first to employ this material
for important structural work, and that the same profession was the
first to reject it upon traditional grounds. It is quite certain,
however, the bridge-builder lost no time in trying his hand upon so
tractable a material; for not long after Telford erected a bridge at
Buildwas of even a greater span, and the famous cast-iron bridge over
the river Wear at Sunderland was erected from the designs of Thomas
Paine, the author of the "Age of Reason." Iron bridges quickly followed
upon these early experiments, for we hear of several being built on
the arched system, and large cotton-mills being erected upon fireproof
principles at the commencement of the present century, the iron girders
and columns of one mill being designed by Boulton and Watt. A little
later, Eaton Hodgkinson proved by experiments the uncertainty of cast
iron with regard to tensile strength, which he showed to be much less
than had been stated by Tredgold. Cast iron was afterwards largely
adopted by engineers. The experiments of Hodgkinson supplied a safe
foundation of facts to work upon, and cast iron has ever since retained
its hold. Thomas Paine's celebrated bridge at Sunderland had a span of
236 feet and a rise of 34 feet, and was constructed of six ribs, and is
remarkable from the fact that the arched girder principle used in the
Coalbrookdale and Buildwas bridges was rejected, that the ribs were
composed of segments or voussoirs, each made up of 125 parts, thus
treating the material in the manner of stone. Each voussoir was a