The Sun.
London, England, 1858-10-07 00:00:00

-THE- Vl2

rALTABIL

About eight years ago all-England was talking
of and Europe' wondering at the importance and
magnitude of that Feat engineering work, the
Britannia-bridge. Since then viaducts and bridges
of all kinds and sizes, and each one possessing one
or more features of peculiar difficulty, have been
erected without notice or comment. We have grown
accustomed to the great strides with which our
engineers have advanced, and look with compara-
tive mdifference on what in other countries would
be regarded.justly as national works—the standard
of what their mechanical or scientific skill could
ever accomplish. There needs no greater prof of
this than the case of the Saltash Viaduct, now
nearly built, but with which we have no doubt the
majority of our readers are so little acquainted as,
perhaps, to be even unaware of the precise part of
the United Kingdom in which Saltash is situate.
Nevertheless, in this obscure little village near Ply-
mouth, a bridge is being reared across the Tamar
-which is second to none in the world, and which in
size and in engineering difficulties of construction
far surpasses even that of Anglesey.
The line for which this noble work has been un-
dertaken is the Cornwall Railway, a single pair of
rails running from Plymouth to Falmouth, and
which, with the exception of a mile or so, passes
entirely across the hills and lovely valleys of Corn-
wall. That the whole route presents one continued
series of engineering difficulties can well be under-
stood from the semi-mountainous nature of the"
country which it crosses. In a short distance of
60 miles there are no less than seven tunnels and
43 viaducts—some of them 150 feet high, besides
cuttings almost innumerable. The viaduct of all
others is, of course, that at Saltash, which at pre-
sent is unsurpassed by any in the world, and likely
to be so for years to come. Saltash itself is a poor
insignificant place enough, though not altogether
so much so as one would suppose from its appear-
ance. The mayor of it, we believe, is even a more
important public functionary than the Mayor of
Plymouth, of whom he takes precedence, and by
virtue of his office as Mayor of Saltash is also
coroner for the borough of Plymouth, and takes
cognisance of all violent and sudden deaths which
take place in Plymouth Sound. But though, pro-
bably, our readers may care little and have heard
less about Saltash Proper, it is likely henceforth to
receive a fair share of general attention, and we
can safely say, to those who will journey down to
see the bridge, that the viaduct requires indeed to
be a fine one to attract their attention from the
lovely scenery of the valley of the Tamar, which
it crosses. The banks of this noble river narrow in
considerably as the stream reaches Saltash, and,
hemmed in there to half a mile or so, suddenly
widens out into as fine a sheet of water as any of its
kind in the kingdom, its distant banks covered with
cottages and fringed with undulating woodlands
down to the very edge. Across this narrow part of
the channel, where Saltash in picturesque dirt and
disarray straggles up the banks on one side, and a
steep hill covered with rock and rock-grown under-
woos' forms the other, the viaduct stretches high in
air. The briefest general way of describing it is to
say that it consists of 19 spans or arches, 17 of
which are wider than the widest arches of West-
minster-bridge, and two, resting on a single cast-
iron pier of four columns in the centre of the river,
span the whole stream at one gigantic leap of 910
feet, or a longer distance than the breadth of the
Thames at Westminster. The total length of the
structure from end to end is 2,240 feet, very nearly
half a mile—and 300 feet longer than the entire
stretch of the Britannia-bridge. The greatest width
is only 30 feet at basement, its greatest height from
foundation to summit no less than 260 feet, or 50ft.
higher than the summit of the Monument. The
Britannia-bridge, both in size, purpose, and engi-
neering importance seems to offer the best compari-
son with that at Saltash, but the similarity between
the structures is far from being, as great as might be
at first supposed. The Britannia tube is smaller, and
cost nearly four times the price of the Saltash Via-
duct, though the engineers had natural facilities
which Mr. Brunel for his Cornish bridge certainly
had not. The Menai tube is a pure suspension bridge,
and the main tower had a ready-made foundation in
the Britannia rock, from which the whole structure
now derives its name. To cross the Tamar with one
unsupported span nearly a quarter of a mile in length
was of course impossible, and Mr. Brunel had not
only to make his pier in the centre of the river, but
having no place to which to secure the tension
chains on which the roadway hung, had also to con-
trive to make them in a manner perfectly self-sup-
porting. For this the suspension chains hang down
from the piers in a segment of a circle, and are
bolted to the roadway, while above the road-
way,"so as to form the other segment of the circle,
are two monstrous tubes of arched wrought iron,
connected with the ends of the chains, and which
precisely answer the purpose of metal bows. The
tubes and tension chains being connected with iron
trusses and both fastened to the roadway, the whole
acts as a double bow. As the tension chains give
under a strain they straighten down the bow above
them, and so the ends of the chains are supported
and kept in place, while the deflections of the tubes
themselves in turn relieve the roadway. It is very
difficult to convey to the general reader an accurate
and at the same time an intelligible description of
such complicated engineering works as these, and
perhaps the best and shortest way of showing the
nature of the difficulties that existed is to explain
how they were overcome, and in the actual progress
of the structure.
The 17 smaller spans, or arches if we may so call
them, which lead from the hills at either side to the
edge of the Tamar in the centre of the valley, were
easily, made, since they only consisted of double
columns of masonry, with wrought-iron longitudinal
beams of boiler-plate between them, for each side
of the roadway. The two main stone piers, which
at the water's edge on either side support the ends
of the great spans crossing the river, are of course
of a more solid kind, and well calculated to resist
either thrust or tension. Each of these is of masonry
29 feet wide by 47 feet thick, about 190 feet from
foundation to summit. It is, however, on the great
main pier, in the centre of the river, that all the
strain and pressure comes, and nothing short of the
solid rock would suffice for its foundation. To
reach this, however, was a matter of no ordinary
difficulty, as some 70 feet of water, with. 20 feet of
mud and concrete gravel, lay between Mr. Brunel
and the stone on which he wished to build. A com-
mon cofferdam would be impossible in such a case,
but by a novel application of the cofferdam prin-
ciple the difficulty was overcome at last. An im-
mense wrought iron cylinder 37 feet in diameter,
100 fept high, and weighing 390 tons, was made
and exactly in the spot where the masonry was
to rise. From this the water was pumped
out and air forced in ; the men descended, and,
working as in a huge diving-bell at the bottom of
the river, cleared away the mud and gravel till the
rock was reached. Steam airpumps were necessary
to keep the men supplied, and, as a matter of course,
they worked at a pressure of upwards of 381 b. to
the inch. At first this affected them severely ; many
were seized with cramps, faintness, and insensi-
bility, and one even died in a few minutes. But this
was only at the beginning, and after a time 40
labourers could remain at work at once in the huge
diving apparatus with apparently very little incon-
venience to themselves. It was, however, always
unpleasant labour, and all were glad when the first
great difficulty was overcome, and the solid column
of granite built inside the cylinder rose at last
above the water's edge.
On this noble pile of stone, springing many feet
below the river's bed, the iron columns for the
centre pier were raised. Until these ponderous
masses were cast metal columns of such gigantic
dimensions were never dreamt of. There are four of
them, octagon in shape, 10 feet in diameter, and 100
feet high. The four stand each about 10 feet apart
from the other in the centre of the granite column,
so as to form a square of about 30 feet, binding the
four together in a handsome and massive lattice
work of wrought iron to prevent any lateral move-
ment. The weight of these columns is about 150
tons each. The -way they were made was in 6-feet
joints, each joint being cast in two separate pieces.
The thickness of the metal is two inches, and inside
each column is stayed and supported with massive
ribs. 'When all the pieces of the four columns
had been cast, Km+ was planed down and fitted
together with tine neatness of joiner's work. Thus
finished, all were tent off piecemeal to the centre
pier, though not erected, as they could only be
built up under the centre spans as the latter were
gradually lifted to their places by hydraulic pres-
sure in one gigantic piece weighing some 1,200 tons.
These two main spans or trusses consist each of a
wrought iron arched tube with two double sets of
tension chains attached to either end, which carry
the road for the railway. These, as we have said,
were put together at the river's bank, floated out
to their place, said then raised in one mass. The
process of raising them, for reasons we will men-
tion, occupies some five or six months. The western
span has been lined to its place some time, the east-
ern is now in course of being raised, at the rate of
six feet per week. Each wrought iron arched tube
is elliptical in form, being 12 feet high by 17 feet
wide, and is ;made throughout of inch boiler
plate. Inside, at intervals of every 20 feet, is a
wrought iron diaphragm, and tie rods and angle
iron still further add to its strength. The curve of
the arch is 28 feet, and the tension chains at the
ends have, of course, the same. These chains are
similar in princ-i,ple to those of an ordinary suspen-
sion bridge, exc apt that, instead of being made with
links of seven b ars, each link consists of 14 bars of
iron an inch thick and six inches wide. Each of

fiese chains has: been tested with a tension train of
between three And four tons to the inch. Both