ights, and without any
diagonals. But, long before 1862, the Warren and other truss-girders
had come into use, and I am inclined to say that, so far as novelty
in the principle of girder-construction is concerned, I must confine
myself to that combination of principles which is represented by the
suspended cantilever, of which the Forth Bridge, only now in course
of construction, affords the most notable instance. It is difficult to
see how a rigid bridge, with 1,700 foot spans, and with the necessity
for so much clear headway below, could have been constructed without
the application of this principle.

BRIDGE CONSTRUCTION.

Pursuing this subject of bridge work, the St. Louis Bridge of Mr. Eads
may, I think, be fairly said to embody a principle of construction
novel since 1862, that of employing for the arch-ribs tubes composed
of steel staves hooped together. Further, in suspension bridges there
has been introduced that which I think is fairly entitled to rank
among principles of construction, the light upper chain, from
which are suspended the linked truss-rods, doing the actual work of
supporting the load, the rods being maintained in straight lines, and
without the flexure at the joints due to their weight. In the East
River Bridge, New York, there was also introduced that which I believe
was a novelty in the mode of applying the wire cables. These were not
made as untwisted cables and then hoisted into place, thereby imposing
severe strains upon many of the wires composing the cable through
their flexure over the saddles and elsewhere, but the individual
wires were led over from side to side, each one having the length
appropriate to its position, and all, therefore, when the bridge was
erected, having the same initial strain and the same fair play. Within
the period we are considering, the employment of testing-machines has
come into the daily practice of the engineer; by the use of these he
is made experimentally acquainted with the various physical properties
of the materials he employs, and is also enabled in the largest of
these machines to test the strength and usefulness of these materials,
when assembled into forms, to resist strains, as columns or
as girders. I of course do not for one moment mean to say that
experimental machines were unknown or unused prior to 1862--chain
cable testing-machines are of old date, and were employed by our past
President, Mr. Barlow, and by others, in their early experi