Weekly expositor. (Brockway Centre, Mich.) 1882-1894, 1888-04-19
THE FIRTH OF FORTH BRIDGE.

work will be completed. and the time
given by the resident engineer is toward
the end of next year. Describing the
progress of the work, The Scotsman
says that the huge 'straddle legged
structures which are reared upon three
large piers were pract ically completed
before the end of the year, These rise
to the great height of three hundred
feet above high water mark, and form
the points Fappai structural basis from
which the "cantaevers" are being built
out on either side. The term cantlev-
ors Is applied to the wing-shaped
structures which shoot out from the
large pier-towers and reach toward one
another across the great spaces that
have to be spanned. The building of
these cantalevers has at present reach
ed most interesting stage. From the
summit of the towering pier structure
arms are being stretched out on each
side into m d-air. These are each COM
posed of two pairs of huge booms, con-
structed on the glider or lattice work
principle, and project sheer above the
waters, which are full three hundred
feet below. There is not the slightest
under support: the whole fabric relies
upon the strength of the ties which
bind it to the top of the cantalever
towers. Incredible rs it may appear,
these top members have built out the
to the distance of l25 feet, and seem
just now to hang in a precarious posi-
ton like the ash upon n half smoked
cigar. 1t Is N this part of their work
| that the designers and contractors claim
that they have vindcatcd the soundness
OF their calculations. IL was in the
! springing of these top members or
arms. and carrying them out un5upport-
ell the necessary distance, that cavaliers
and detractors of the scheme have
chiefly prophesied failure. Yet from
both the Queensberry and Fife pier
structures these serial platforms have
been built out, two from each. without
n hitch, to the distance of 125 feet
! They now await the rearing of a
! temporary column which is running up
| from below, and which will form a sup
port. enabling them TO be carried still
| further out till they meet the first per-
manent suppOrts-the c'oss struts or
tubes which spring from the bases of
! the per-towerS, the lower arms,
meantime have been proceeding even
more rap. dly. These are immense tubes
which protude outward and upward,
and along with the pressure eXerted by
the weight of the spans is carried back
TO the stone piers. They have been
carried out TO the distance OF fully 100
feet. 1t may be explained here that
the top arms, which are constructed on
the order system. are the tension MEM
5ers-that is TO say, are subjected to
pull by the weight of the structure
which they support, while the lower
arms. which are of tubular cOnstruc-
ton, are the compression members. and
bear the down push. as L were. These
! lower tubes have been built out in
( exactly the same way rs the upper ones
that is, without any support from
underneath The workers to-day are
practically standing upon their labors
of YESTERDAY. AS soon as fresh round
of steel plates is added TO the tubes or
en- additional order section riveted to
the top arms, the platforms, with their
freight of men, cranes. and other
mechan cal appliances are slid out cor
respondently. and new piece of work
is begun, which again when completed,
will give the necessary standing sup
port for further extension. Indeed,
this is characteristic of the whole of the
work at the bridge Every piece of
work done becomes the basis of another
advance.
The point. however, of this great
work of of engineering which is of pre-
eminentinterest. and about which spec.
ulat on and prophecy are most busted,
is the problem of bridging over the two
great spans of II. 10 feet each. which ex-
tended north and south of Inch Garvie.
The largest Tav bridge spans are just
245 feet. so that the proposed spans at
Queensberry will be seven times us long
The largest span of any bridge in this
country is that of the Britannia bridge,
over the Menia strait, which is 400 feet
in length, or somewhat over quarter
of the SIZE of the Forth bridge spans.
to throw single span II cross such q
! space as LILO feet would, even under
ordinary conditions, be n tremendous
undertaking But in the present case
the difficulties are intellectually enhanc-
ed, because the whole distance has to
be bridged over without any support
from below. The water is two humid.
red feet deep on each side of Inch Gar
vie, so that it is impossible to found
temporary scaflolding or piers upon the
sea bottom. nor can any device of pom
toons be of practicable service. The
problem. then, comes simply this-to
join the Inch Garvic pier with the north
and south piers leach LILO feet apart
by simply building straight out across
the waters at height sufficient TO al
low the largest ships to pass freely UP
and down the Firth, 1t is this that ex.
plains WHY the immense "straddle-leg-
ged" towers which are the prominent
features of the structure just now, have
been reared rs high as $50 sect. These
great spans will never be entire-tha
is, the Two halves will never be joined
Each half of each great span will hang
entirely by its own supports on the
main piers. Owing to the large expan'
sion and contraction of so immense 6
structure of steel under the charges o.
temperature. it would endanger the