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 experiments upon
steel; but I speak of it as a matter of congratulation that, in lieu
of such machines being used by the few, and at rare intervals upon
small specimens, for experimental purposes, they are now employed in
daily practice and on a large scale.
In harbor work we have had the principle of construction employed by
Mr. Stoney at Dublin, where cement masonry is moulded into the form
of the wall for its whole height and thickness, and for such a length
forward as can be admitted, having regard to the practical limit of
the weight of the block, and then, the block being carried to its
place, is lowered on to the bottom, which has been prepared to receive
it, and is secured to the work already executed by groove and tongue.
It would not be right, even in this brief notice of such a mode
of construction, to omit mention of the very carefully thought out
apparatus by which the blocks are raised off the seats whereon they
have been made, and are transported to their destination. It is no
simple undertaking (even in these days) to raise (otherwise than
hydraulically) a weight of 350 tons, which is the weight of the
blocks with which Mr. Stoney deals. But he does this by means of
pulley-blocks attached to shears built on the vessel which is to
transport the block, and he contrives to lift the weight without
putting upon his chains the extra strain due to the friction of the
numerous pulleys over which they pass. The height of the lift is only
the few inches needed to raise the block clear of the quay on which it
has been formed, and this is obtained by winding up the chain by steam
gear quite taut, so as to take a considerable strain, but not that
equal to the weight of the block, and then water is pumped into the
opposite end of the vessel to that upon which the shears are carried,
this latter end rises, and the block is raised off the seat on which
it was formed, without the chains being put to work to do the actual
lifting at all. The vessel, with the block suspended to the shear legs
and over the bows, is then ready to be removed to the place where
the block has to be laid. A word must here be said about an extremely
ingenious mode of dealing with the slack chain, to prevent its
becoming fouled, and not paying out properly, when the block is being
lowered. This is accomplished by reeving the slack of each chain over
two fixed sets of multiple sheaves.
A donkey-engine works a little crab having a large drum, the chain
from which is connected with the main chain, and draws it round the
multiple sheaves so as to take up the slack as fast as the main crab
gives it out. The steam is always on the donkey, which is of such
limited dimensions that it can do no injury to the chain even when its
full power is in vain endeavoring to draw it any further; directly,
however, the main crab gives more slack, and the chain between it and
the two sets of sheaves falls into a deeper catenary, and one which
therefore puts less opposition to the motion of the donkey-engine,
that engine goes to work and makes a further haul upon the slack, and
in this way, and automatically, the slack is kept clear.