New-York daily tribune. [volume] (New-York [N.Y.]) 1842-1866, 1854-06-19
IRON BRiDGEs- The Niagara Falls Gazette replies

IRON BRlDGEs.-The Niagara Falls Gazette replies
at length to THE Democracy Buffalo as 10 the alleged
deterioration Of Suspension Chain Bridges from the
effect of mere vibration use. Says Tie Gazette
The marching Of troops over any bridge, whether sus-
pension arch produces effect entirely different from
that OccasOned by the passage OF railroad trains Inc
latter produce irregular succession Of short CONCESSIONS
e severity Of which depends upon the uneyenness OF the
track and which may therefore be avoided by perfect
track always properly adjusted On the other hand, the
marching OF troops produces q measured uniform sue
ceson Of oscillations in the flooring Now if these ocil-
latons continued and are not restrained IN their am
pltude by the inherent stiffness OF the structure or by
stays, the momentum resulting from nc great weight OF
the bridge itself may become so orcbe as to cause It's
destruction.
The remark Of Tile Democracy, that vibrations the
principal of danger TO suspension bridges We uy
indorseh,nce the urgent necessity OF guarding against
them from the outset But vibrations are fruitful VI dan
GER not only TO suspension bridges but 10 an Others IN
deed we must regard vibration ole OF the principal
agents of destruction IN all human fabrics especially those
into whose combination metal enters largely as the ma
teral. Our contemporary has raised questions of
great interest-he has ventured upon speculative field
which offers wide scope for the research OF the philosopher
and of the practical worker IN metals And here we must
observe in relation 10 the idea that iron uadergoes
structural change by vibrations that is hitherto only
idea not established fact and bids fair speedily to
become an obsolete idea. Those who have paid
most attention 10 this matter, and experimental largely
upon utterly deny deny that the structure OF IRON changes
in consequence of vibrations ordinary Numerous
perments made with Railroad axes, in this country
well Europe have established e act that no granulaon
takes place where well developed fiber existed before.
Fiber is produced in no by compression and elongatiOn
under the hammer the rolls hi sh temperature The
notion that the strength of iron depends upon fibre is
trey erroneous Fibre renders iron duce and will enable
il bend short without breaking but dsAes lis strength.
The ibrous irons the weakest The strength Of
iron resist lateral well "a tensile force, depends alto
eeher upon lis ecity that quality which enables its
fbers after being bent elongated certain extent to
regain their former position without having suffered injury
or change. For instance bar of iron inch square,
when subjected to tension of one un weight will stretch
thOusandth part of its whole length if 10 weight
of 10 uns il viii stretch ten times much and after the
10 tun removed the bar viii regain It's former length
If however the weight be increased 10 than To- say
20 tuns there viii be found its removal that per-
eongation stretch has taken place in the bar;
and if this strain Of than ten tuns be often repeated
a rupture of the fbers viii at length be effected. The
pont where this permanent elongation takes place
called the limit of elasticity and in good bar iron
ponds one-iLrd Of its ultimate strength; in wire, 10
onehalf and ia steel TO than oneha according 10
quality. Experience however has taught that for prac
teal purposes, mach larger allowance Of strength is
cessaTy.
1.-THE Suspension Bridge over the Niagara River,
meeting the Great Western and New FORK Central Railways,
viii possess strength equal 10 six times the tension that can
be produced by maximum strain the upper floor and
full load the lower floor AT the time. The per-
maHent tension of tho cables from their weight and
that of the supersructGr0 will be only one-teh of their
ultimate strength There is, therefore, ample provision
for safety Moreover, the whole structure viii be
arranged and stayed, that oscillations very pcrcep
tble movement can take place. hurricane which would
PROSTATE the whole City of Buffalo and every wooden
bridge in the country would be harmless TO this bridge
SO well will it be guarded by stays"