Dr. Grangier, surgeon in the French army, writes from Algeria: "A few
days after the occupation of Brizerte, when the military authorities had
forbidden, under the severest penalties, the discharge of firearms within
the town, the whole garrison was awakened at three o'clock one morning by
the tremendous explosion of a heavily loaded gun in the neighborhood of
the ramparts; a guard of soldiers rushed into the house from whence the
sound had come, and found a woman lying on the floor with a newly born
babe between her thighs. The father of the child stood over his wife with
the smoking musket still in his hand, but his intentions in firing the
gun had been wholly medical, and not hostile to the French troops. The
husband discovered that his wife had been in labor for thirty-six hours.
Labor was slow and the contractions weak and far apart. He had thought it
advisable to provoke speedy contraction, and, following the Algerian
custom to _scare the baby_ out, he had fired the musket near his wife's
ear; instantanously the accouchement was terminated. After being
imprisoned twenty-four hours, the Arab was released."--_Cincinnati
Lancet._
* * * * *
"ELASTIC LIMIT" IN METAL.
The _Engineering and Mining Journal_ raises the question whether steel,
which is becoming so popular a substitute for wrought iron, will, when it
is subjected to continuous strain in suspension bridges and other similar
structures, do as well as iron has proved that it can. Recent tests of
sections from the cables at Fairmount Park, Philadelphia, and at Niagara
Falls show that long use has not materially changed the structure. The
_Journal_ says: "It is a serious question, and one which time only can
completely answer, whether steel structures will prove as uniformly and
permanently reliable as wrought iron has proved itself to be. In other
words, whether the fibrous texture of wrought iron can be equaled in this
respect by the granulated texture of steel or ingot iron. In this
connection it is interesting to note that the fibrous texture referred to
is imparted to wrought iron by the presence in it of a small proportion
of slag from the puddling furnace, and that this can be secured in the
Bessemer converter also if desired. The so-called _Klein-Bessemerei,_
carried on at Avesta in Sweden for several years past, produces an
exclusively soft, fibrous iron by the simple device of pouring slag and
iron together into the ingot mould. This requires however a very small
charge (usually not more than half a ton), and a direct pouring from the
converter, without the intervention of a ladle, which would chill the
slag."
The effect of the introduction of slag would seem to be to retrace the
steps usually taken in producing steel, viz., to separate the iron from
its impurities, and then to add definite quantities of carbon and such
other ingredients as are found to neutralize the effects of certain
impurities not fully removed.
The most intelligent engineers, after ascertaining by exhaustive physical
tests what they need, present their "requirements" to the iron and steel
makers, whose practical experience and science guide them in the
protracted metallurgical experiments necessary to find the exact process
required. The engineer verifies the product by further tests, and by
practical use may find that his "requirement" needs further
modifications. As a result of all this care, some degree of certainty is
secured as to what the material may be expected to do.
No doubt the chemical composition of the slag used at Avesta was known
and met some equally well known want in the iron, and thus the result
arrived at was one which had been definitely and intelligently sought.
An important factor in selecting material for the cables of suspension
bridges is its _true elastic limit_. By this term we mean the percentage
of the total strength of the material which it can exert continuously
without losing its resilience, i.e., its power to resume its former shape
and position when stress is removed. Now, in the case particularly of
steel wire as commonly furnished in spiral coils, the curve put into the
wire in the process of manufacture seriously diminishes this available
sustaining power.
For it is evident that it would be unsafe to subject these cables at any
time to a stress beyond their elastic limit. If, e.g., a snowstorm or a
great crowd of people should load a bridge beyond this limit, when the
extra weight was removed the cables could not bring the bridge back to
its normal place, and the result would be a permanent flattening and
weakening of the arch.
By a process invented and patented by Col. Paine, the wire in the New
York and Brooklyn bridge was furnished _straight_ instead of curved. Now,
if a short piece of common steel wire is taken from the coil, and pulled
toward a straight position, and then released, it springs back into its
former curve; but if a short piece of the straight-furnished wire that
was put into this bridge is bent, and then released, it springs back
toward its straight position.
It is easy to see that if a curved wire is pulled straight, there must
occur a distention of the particles on the inside of the curve and a
compression of those on the outside. The inside is in fact strained past
its elastic limit before _any_ stress comes upon the outside. Hence,
after the wire has been pulled straight, the elastic limit of only a
portion of it can be taken into the account in calculating the load that
can safely be put upon it. In the case of curved steel wire pulled
straight, its ultimate strength was found to be only about 90 per cent.
that of similar wire furnished straight by this process. The superior
ductility of iron wire in some measure compensates for the distention of
the particles on the inside of the curve, and that is a reason why it has
heretofore been used for suspension bridges. But with straight steel wire
there is no such distention, and its _entire elastic limit_ is available.
This elastic limit is 66 per cent. of the ultimate strength, and,
besides, that ultimate strength is 10 per cent. greater than that of
similar curved wire. Thus if we have a curved steel wire large enough to
sustain 1,000 lb. without breaking, a similar straight wire, such as
those in this bridge, will hold up 1,100 lb., and 66 per cent. of this
1,100 lb = 720 lb.
The elastic limit of curved wire has never been determined, since any
stress that will cause it to reach a straight line is beyond the elastic
limit of the inside of its sectional area. That of curved iron wire has
been estimated at 40 per cent. of its ultimate strength, which is about
half the ultimate strength of curved steel wire; that is, it would be
unsafe to put more than 40 per cent. of 500 lb.--or 200 lb.--upon a
curved iron wire when a _straight_ steel one can sustain 720 lb. without
injury. In the New York and Brooklyn bridge the cost of a sufficient
amount of such iron wire as is used in all other suspension bridges would
have been some $200,000 greater than that of the straight steel wire
which was used. At five per cent., this effects an annual saving in
interest of $10,000.
There must, too, be a considerable saving in the current expense for
painting and care, to say nothing of the more neat and elegant appearance
of the less bulky steel. And as the whole area of the section of these
wires is subjected to an even strain that is always far within the
elastic limit, there is no danger of a change of structure under that
stress.
It is highly probable--although Col. Paine has been too busy to work up
the matter--that piano wire made in this straight method could be drawn
up to and kept at pitch, without approaching very near the elastic limit.
In that case not only would they seldom if ever require tuning, but
probably all along the tone would be more satisfactory. And there would
not be those exasperating periods when the pitch is not quite perfect,
but yet is not far enough out to make it seem worth while to send for a
tuner.
* * * * *
A catalogue, containing brief notices of many important scientific papers
heretofore published in the SUPPLEMENT, may be had gratis at this office.
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