about a third of a mile long, divided into two parts by a ridge of hills
which runs through the centre of the valley--each part forming a separate
viaduct, the one of seven equal spans of 150 feet, the other of three
spans of the same diameter. The bridge has been very skilfully designed
and constructed, and, by reason of its great dimensions and novel
arrangements, is entitled to be regarded as one of the most remarkable
engineering works of the day.

"In calculating the strength of these different classes of girders," Mr.
Stephenson observed, "one ruling principle appertains, and is common to
all of them. Primarily and essentially, the ultimate strength is
considered to exist in the top and bottom,--the former being exposed to a
compression force by the action of the load, and the latter to a force of
tension; therefore, whatever be the class or denomination of girders,
they must all be alike in amount of effective material in these members,
if their spans and depths are the same, and they have to sustain the same
amount of load. Hence, the question of comparative merit amongst the
different classes of construction of beams or girders is really narrowed
to the method of connecting the top and bottom _webs_, so called." In
the tubular system the connexion is effected by continuous boiler plates
riveted together; and in the lattice and trellis bridges by flat iron
bars, more or less numerous, forming a series of struts and ties. Those
engineers who advocate the employment of the latter form of construction,
set forth as its principal advantage the saving of material which is
effected by employing bars instead of iron plates; whereas Mr. Stephenson
and his followers urge, that in point of economy the boiler plate side is
equal to the bars, whilst in point of effective strength and rigidity it
is decidedly superior. To show the comparative economy of material, he
contrasted the lattice girder bridge over the river Trent, on the Great
Northern Railway near Newark, with the tubes of the Victoria Bridge. In
the former case, where the span is 240.5 feet, and the bridge 13 feet
wide, the weight including bearings is 292 tons; in the latter, where the
span is 242 feet, the width of the tube 16 feet, the weight including
bearings is 275 tons, showing a balance in favour of the Victoria Tube of
17 tons. The comparison between the Newark Dyke Bridge and the Tubular
Bridge over the river Aire is equally favourable to the la