The Helena independent. [volume] (Helena, Mont.) 1875-1943, 1890-08-10
Upoaor
E NJ FERRIS, w. A. IMES LINDLEY & GE TCHELL
GARDNER & SMITH, w BOGERT. RAMSEY PERKINS,
THE FORTH BRIDGE.
Most Stupendous Structure in tho History
of Mechanical Engineering.

Most Stupendous Structure in the History
of Mechanical Engineering.
OF all the monuments of this century,
the stupendous bridge across the Forth, in
Scotland, is the grandest and most surpris-
ing construction. The Firth of Forth ia a
wide estuary, so deeply indentinR the east
ern coast of Scotland as to necessitate an
expensive and tedious trans-shipment of
commerce across the gulf, or long detour
around it. For twenty-five years efforts
have been made to obtain permanent way
over the estuary, but the difficulties to be
encountered were so great that it is only
within the last half of the past decade that
engineering genius and mechanical skill!
have been sufficiently developed to rem
der this greatest of metal struc-
t %sss Of pEs SnSa1syh the fix Tth miles
gulf, suddenly contracts leading in ancient
times to the establishment of ferry be-
tween Fife on the north and the town of
Queensferry on the south. The estuary
here is little less than mile and a half
wide, and very OPPORTUNITY small rocky
islet exists almost in the center, which, for
becomes the central pier of the great
bridge.
Curiously, also, that part of the gulf
lying between Queensferry and Fife con-
tains the only rock to be found for miles
about the mouth, thus making the one
appropriate place where adequate founda-
tions for 80 mammoth a structure may be
obtained. Between the little island and
either shore extends space of nearly half
mile, where no foundation was feasible,
and which, therefore, must be crossed with
out single pier, thus outspanning by sev-
eral hundred feet all existing bridges.
Serious gales sweeping through the estuary
ure an enormous force, and to avoid par
allel catastrophe to that ingulhng the I'M
fortunate Tay bridge, the most skillful en-
cheering has been exercised. A the
bridge runs nearly north and south. in the
morning the east side, and in the afternoon
the west side, is exposed directly to the
rays of the sun, while the corresponding
result, temperature stresses are intro
duced, causing the whole mass to wrigRlt
and twist to a considerable degree. In 81
large metal mass effects of temperature
must be carefully provided for. lest its in
sidious operation shall lead to- destruction
Commencing at Queensferry on thu
south, the approaches to the bridge, con
sisting of ten shor irders, of 1913 feet span
extend over the low shores to the first grea
cantileyer. From the main abutment, thu
entire firth is crossed by three lmmens
trusses forming two central spans, eacl
1,7l0 feet in the clear, with two side span
of GOO feet.
The trusses are balanced upon their foun
dations. and the weight of passing trail
transmitted through them is resisted as a
upward pull by the anchorages on eithe
side of the gulf. The cantilever bridg
possesses a very marked advantage in th
ease with which it may be erected; for thi
very property of balancing on its foundu
lions enables the engineer to build out owe
the water simultaneously in both direction
| using the truss-work of the bridge Us tt
) scaffold on which to erect additional piec'
as fast as it is completed. BY this meal
the expense of underneath supports, who
would have been enormous In the case no
3 under consideration. where the depth
Z water in the gulf is over 40 feet, is entire
e avoided.
Q The railway track crosses the bridge
n the height of about $50 feet above Il
a water, giving ample head-Eoom for vesse
q of all descriptions beneath the rails; viii
s above the track the huge trusses tower u
& word more than 200 additional feet:
! Withstand the violent gales of the Fort
e the bridge has beep prop0rtioned to resist
Wlnd-pressure of 5000 tons, and in I
structure itself nearly 10000 tons of ste
have been employed. At any one time I
u exceed 1000 tons. so that, curiOusly en0nG
the weight of the structure itself il thin
v times that of the load which it il intend
to carry. while the wind pressures are
least four times AZ great.-ArthCr Waugh
Q Abbott, in Frank Leslie's Popular Month
Q. for August.