Daily evening star. [volume] (Washington [D.C.]) 1852-1854, 1854-05-31
For the Evening Star.
WROUGHT IRON BRIDGES vs.. CAST IRON.
WOODEN, Or STONE Bridges

MEssNs. EDIToRs:-As the subject of
bridges now occupies much of the attep-
tion of Congress, and of this community.
and as my profession of a civil engineer,
and my present occupation have rendered
me somewhat acquainted with the sub
ject, offer you the following suggestions,
which you are at liberty to use as YOU
may think fit.
The properties of wrought iron, which
chiefly contribute to its strength and
utility, are its elasticity and tenacity,
By the first is meant that property, by
virtue of which it returns to its former
figure. state, and relative arrangement
Of its parts; after it has been exposed to
tension, Or the act Of straining. By the
latter is meant that property. by virtue
of which the particles of metal resist
separation or, in other words, it is the
consequence of direct cohesion between
the particles of metal. 1t requires a
much smaller strain to overcome the
former property than that which is neces-
sary to destroy the latter. For instance ::
a weight of 81 to Il tons suspended 10
the end of a bar of wrought iron, of a
square inch section, will overcome its
elasticity; while "a to 26k tons similarly
suspended, are necessary to overcome its
tcnacity, or to produce disruption of the
bar. Hence we see that the elasticity of
the wrought iron may be destroyed, long
before disruption would ensue, and long
before the ordinary observer would dis.
cover that any change had taken place
in the bar, or in any structure of wrought
iron. (Sce Barlow on the Strength OF
Materials
But, if the elasticity of the iron is once
overcome, every additional strain will
cause the iron to become weaker; and it
may finally part, with much less strain,
than it had previously resisted, with i'm.
punity, before its elasticity had been i'm
paired. From this, it would appear, that
in order to secure safety in structures OF
this material, they should not be ex-
| posed to any strain, which was Sufficien,
to overcome the former property, tht
elasticity, of the metal. But experience
has shown that neither the elasticity, MOI
the tenacity of wrought iron, will remain
the same. if the metal is exposed to the
effects of percussion, heat and magnetism
For instance, if you break a wrought
iron furnace bar, after it has been ex-
posed TO the alternate heating and cooling
of furnace for some time, it will be
found to be converted into brittle crystal
lied iron, although it may have been a
tough, fibrous iron when first put iBto
| the furnace.
Again, by rapidly heating and cooling,
by quenching with water few times,
any piece of wrought iron, the same effect
will be produced.
In these experiments at least two OF
the above mentioned causes are in ope-
ration, viz: heat and magnetism! For,
We have the authority of Sir Humphrey
Davy for stating, that all cases of vapori-
zatlon produce negative electricity in the
bodies in contact with the vapor.
We now come to the third cause, which
is, practically, Of far greater importance
in its effects than the other two, they be-
ing more various and extensive. In the
manufacture of some descriptions of ham.
mered iron, the bar is first rolled into
shape, and then one-half the length of Il Z
bar is heated in a furnace, and immedi-
ateiy taken to the tilt hammer, and ham.
mered, and the other end of the bar is
then heated and hammered in the same
manner. And in order ~ avoid any ma
terial difference, in form or color, where
the two distinct operations have termi.
mated, the workmen frequently give the
bar few blows with the hammer on tina,
point which he first operated upon, whicl
part by that time, may have become COM
paratively cold, in which case the effecr
Of the additional hammering is to render