Year: 1879

by seven inches. noteworthy feature of the bridge
was that on the large spans of 245 feet the engines
and trains ran between the girders the rail platform
resting on the bottom booms. but on all the other
spans the trains platforms ixed on the upper
booms. This plan aimed to q oven
gradient and cheaper construction.


OBSTACLES OVERCOME.


1t would occupy far too much space to describe in
detail the succession of apparently nsurmountable
obstacles which the ingenuity skill Of the build
overcsme during the progress of the work. or
the novel engineering feats which were accom-
piished The huge grders for the 245 foot span al
though Weighing 200 tons each put together on
shore. hoated into the SWIFT stream pontoons and


lifted bodily into place by the rising of the tide and
by new arrangements OF powerful hydraulic rams.
The placing OF these grders and the erection Of the
iron piers carried sim ultsneously the pier
itself beng consolidated after tho girders had
been placed in position. In laying tho foun-
dations in the cylinders tho workmon labored
In compressed air. and new methods OF protecting
them from its efocts. had to be devised. New ma
chnery constructed for handling the huge cyln-
ders. and the electric light pressed into
service. as on this work. in 1871 It found Of its
first and most extensive utilizatons in tho iumina-
ton Of the shops and yards ut night while the
building of the superstructuro being hurried
completion.


MAsTEnpIEcn COMPLETED.


In its design, construction and its successful &C.
complishmont England regarded the Tay Bridge as q
masterpiece. 1t something which American
bridge builders-wonderfiifly rapid and daring
their advancement had been-had not yet achieved.
"Even after our American cousins have got all due
credit for tho big things they have done in this
direction. crowed the Newcastle Chronicle in 1874"
the fact will still remain that the Yay Bridge
Is in its way perhaps tho most remarkable structure
in the world" That the bridge was amply strong
to sustain any wind pressure to which 1t might be
subjected believed its engineers to be beyond
doubt It in answer to other engineers, who
hatly asserted that tho structure would be blown
over. that one of its promoters said in Q published
paper in Engineering, 1.-THE exposed surface Of
large pier about eight hundred square feet and of
the superstructure. which depends upon 1t, about
eight hundred feet more. and 80, giving eight hum
dred feet for train above have 400 square
Twenty-one pounds per square foot is the force Of
q very strong gale. but It would take less than
ninety-six pounds per square foot the surface
given to overturn the pier. Even the most severe
hurricane on record would equal only onehalf this
resistant power"


THE OFFICIAL TEST


It is also Of especial interest now to turn to the
results Of the official test Of the bridge the latter
being conducted soon after the completion Of the
Geuora1
Autchson. Inspector Of Railways for Scotland
Five locomotive engines with their tenders aggre-
eating weight Of $50 tops distributed over
Of the largest spans OF 2A5 feet In length This
load first placed the girder as . dead load and
then as 9 moving quantity-the utmost detection
or bending being only l4 inches. It deter
mined that this weight was fully double that
Of all the coal wagons and three times
that Of all the Allied passenger carriages which could
be got the span. Speaking Of this test Engineer
ing says:-The result is the complete establishment
Of this fact $50 important to the pubhc) that the
bridge is strong out OF all proportion to its possible
necessities. As matter of fact. the load which the
structure is calculated to carry is six times greater
| than that to which it was subjected by Geperal
Hutchison."


INQUIRY INTO THE CAUSES


Further detnils OF tho falling blowing down Of
the bridge must be had before an opinion as to the
true cause Of the disaster be reached. That An
accurate recognition Of this is Of immense i'm
portance to the public goes without saying Abridge
the construction Of which has taken nearly seven
years. built close to region which produces some
OF the finest iron In tho world. embodying in itself
| the most recent and novel improvements in en-
gneering practice; and more than all this. proved
| apparently. as conclusively figures and actual
tests be made to show to possess twice the re-
StSt0pce necessary 10 wltnetaud the blast OF tho
fercest hurricane. and six times the strength need
full hold up the heaviest railway load which can
DO hesped on 1t. does not tumble down
In 4 strong gale without very palpable cause.
| II the material was at fault. then.
| as exporters OF large amounts OF such
| metal. should know the facts II this form Of
| bridge. which may constantly be chosen for its
| cheapness. apparent strength and economy Of ma
terial Is bad 1t is important that this should be
clearly dehned IL the designers and builders of the
| bridge have been misled by erroneous theories in
engineering science it Is still important that
these should be exposed and refuted. Two such dis.
which has just occurred in Scotland. both charge
able to English speaking engineers and both tho
| modern system of iron bridge construction, may
well disrust of accepted engineering practice
in this speciality and invite closer Investigation Of
| the principles and data on which 1t depends.
DESCRIPTION OF THE ILLUsTRArIoNs
The accompanying cuts. for which we are in.
debted to Messrs. Mum Co.. will give the reader
good idea Of the plan which tho bridge across
tho Firth of lay was constructed. The first shows
between the PIERS It will be noticed that the depth
| of the trnsswork Is very great, amount
| ing TO one-cighth tho length Of the span,
| and, consequently. the superstructure was
capable of standing Q tremendous vertical strain.
Tho surface exposed by each span to the lateral
ton Of the wind is about two hundred feet but tho
latticework boing very open tho effects OF even q
strong gale would hardly be apparent unless tho