Belmont chronicle. [volume] (St. Clairsville, Ohio) 1855-1973, 1883-12-06
Bridging the Firth of Forth.

London Times.
Considerable progress is now being
made with the works of the great bridge
across the Firth of Forth, which includes
two spans of the unprecedented width OF
one-third Of a mile each, or about four
times that of any existing railway bridge.
Some f30,0OO per month is the present
expenditure in temporary and permanent
works, at which rate the bridge will be
completed in the anticipated period
of five wears from the date of the contract.
Only four classes OF materials will be used
in the permanent works OF the bridge,
and these being steel, granite, whinstone
and Portland cement, will be of the most
durable and trustworthy character. Some
idea NAV be formed of the magnitude of
the undertaking from the statement that
the materials required for the Forth
bridge would fill 1000 goods trains of
average length and capacity. This
enormous weight will, of course, require
to be handled several times, the whin-
stone alone being found on the spot,
the granite being shipped from Aber-
deem. the steel from Glasgow and South
Wales, and the Portland cement from
different places in England. As it is in
tended to manufacture the steel super
structure of the bridge on the spot, very
extensive works have been constructed at
Queensferry, and the plant provided in
cludes about fifty steam engines OF various
classes, and large number of specially
designed hydraulic appliances, drilling
machines, and other tools for dealing with
the $4,000 tons OF steel which will be used
in the bridge.
To fix the position of the piers of a
bridge having two clear spans OF 100 feet
and two of 700 feet across channel 200
feet in depth, is not SO simple a problem
as may appear to some persons. In the
case Of the Forth the banks are more or
less precipitous and the country undulat-
ing, SO that it was difhcult to get base
line for a trigonometrical measurement.
At the request of Mr. Fowler, the Ord
nance department kindly sent party of
sappers to recover the old stations used
in the ordnance survey OF Scotland, and
to take the necessary additional observa-
tions to determine the width across the
Firth of Forth. The calculated result diff
fered about one foot from that obtained
by Messrs. Fowler and Bakers engineers,
which is not much, perhaps, In difficult
measurement OF over mile across deep
water, but enough to make it desirable
that it should be set right. Mr.. Baker
held the view that it was possible to
measure the 100 feet span TO the eighth
of an inch if necessary, and last week the
measurement was made without any dlf-
ficultv whatever. A length of 100 feet
was first measured with what may be
termed absolute exactitude along a
straight piece of the North British rail-
way, and posts of certain height were
G. % A
forte wire was then hung from these
feet at the center, and marks were fixed
to each end of the wire to indicate the
100 feet length. The wire was then
coiled on a roller and taken on board a
steam launch, from which it was laid like
a submarine telegraph cable across the
channel of the Forth. 1t was then hauled
UP above the surface of the water till it
hung with the original droop of twenty
four feet, when, assuming the tempera-
ture to be the same, the marks put on the
pianoforte wire should indicate the re-
quired span of 100 feet. This process
was repeated several times and the wire
taken ashore to the railway to test wheth-
er it had stretched, but all the observa-
tions were in exact accord. The sensi-
tiveness of this simple mode of measuring
the span with wire weighing only eight
pounds in all was quite astonishing. Thus
when Mr.. Baker Stacked out the wire only
one quarter inch in the 100 feet, Sir
Thomas Tancard, one of the contractors,
who was in q boat in the center of the
Forth, at once signaled there was too
much droop in the wire. At another time
he signaled that it was too high, but a
reference to the thermometer showed that
the temperature had fallen two degrees,
so that the wire proved itself most deli
cate indicator of changes of temperature.