Vermont watchman and State journal. [volume] (Montpelier, Vt.) 1836-1883, 1849-12-13
A Railway Bridge OF Iron.
From tho n. Y. Courier unl Enluitcr.

The London Quarterly Review has a vc-
ry interesting description of the 7HJulay
Bridges recently erected by the Chester
and Holyhead Railway Company across the
Menai Straits, which lead from St. Georges
Channel into the Irish Sea and divide Car
narvon from the Island of Anglescy This
is undoubtedly the most stupendous work of
Modern Mechanism and as its consiruc-
ton has established principles which must
prove of the greatest importance TO Bridge
building i'll all time to come, ii condensed
description of it cannot be without value
and interest for our readers. The article
in the quarterly is evidently from the pen of
Sir Francis Bond Head, whose papers Civ
ing detailed accounts Of the practical work
ing Of the English Railway system, have
been widely read. In mnuteness of de-
scription, combined with the clearness and
picturesqueness of the general impression
which his sketches CONVEY ~ has few supc-
riorS.
In building their road, the directors of
the Chester aud Holyhead Company found
serious obstacle in the necessity Of eros
sing the Menai Straits. Their length is a.
bout twelve mles,-the shores are precipi-
the tide varies every day the height of the
imprisoned water, from twenty to twenty
five feet The Government required that
in constructing bridge across these straits,
the Company. should build it fully 100 feet
throughout its whole length, above high wa
ter mark, and that neither scaliolding nor
centreing should be used in its construction,
as these would obstruct the navigation Of
the Straits. Mr.. Robert Stephenson, the
Engineer OF the Company to whose ingenu-
ity the whole matter was entrusted, devoted
himself with unflnchlng courage TO the So
luton of what seemed, under these hard
conditions, almost an impossibilty and
after protractod and painful investigation,
he announced to the company his readiness
to build the Bridge. The outline of his
plan was this he proposed to throw a.
cross the Straits, two long, low hotYow tubes,
-one for up-trans and the other for down
trains,-to be made Of wrought-iron boiler
plates, firmly rlvetted together, and to rest
for support upon three intermediate towers.
Each of these tubes, at each end, was to
rest upon a solid abutment of masonry, ter
mnatiug the railway Embankment the in
termediate portions, were to rest upon three
LOFTY and massive towers, -one being built
on each side of the Straits at high water
mark, and the third, 2l0 feet in height, to
be erected, as nearly as possible, in the mid
die OF the stream upon a tiny rock which,
covered with about ten feet of water at high
tide, was exposed at low water and had long
been deemed a serious obstruction to navi-
CATION. The length OF each portion Of the
two tubes was to be as lelows
From embankment to the first tower PA #
From first tower to central tower 472
From central tower to second tower 472
From 2nd tower to 2nd embankment pa
Total length of each tube 1892
The design thus involved the construc-
ton of hollow tube, four hundred and
seventy-two feet in length resting each end
upon solid basis without any support
IN The middle, strong enough to sustain,
without the slightest yielding, the weight of
an enormous railway train, going at undi-
mlnlshed speed And when four tubes of
this kind, and four PA feet IN length had
been constructed, they were to be laid a.
cross the rapid stream, upon the top of
the towers destined for their reception and
wv ~" lScl ABOVE ul5h wafer marx.
Mr. Stephenson at once instituted Sc
rles OF experiments, conducted by skllul
aud experienced men, to determine the best
form, the best materials, &c. &c, of the
structure he had undertaken to build lie
started upon principle which is clearly H.
lustrated by familiar example. In break
ing by pressure "a stick of wood with the
bark on, it will be noted that on the upper
side, the bark is crushed together, while
that on the under side is torn apart. In a
beam under heavy pressure, also, the upper
side is compressed, while the lower is dis
tended. The former, therefore, must resist
compression, and the latter must resist ex-
tension. AS they approach each other, how
ever, these opposite results diminish in de-
gree, until in the middle of the wood the
two forces neutralize each other--and the
fbres there, are of course useless The
pressure, therefore, upon the middle Of a