New Orleans daily crescent. [volume] ([New Orleans, La.]) 1851-1866, 1866-06-02

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every time it occurred, may have left no traces
When ordinary scrutiny could detect Not bolt
Or Eyet has been started, nor a fracture made
and the commissioner has pronounced it safe. and
yet such a disarrangement Of particles and weak
ealng OF structure may have occurred az to render
us use hereafter, under ordinary pTegsure ex-
tremey hazardous Repeated strains beyond the
nlnts Of elasticity steam boiler must inevfta-
ply result in an explosion, nRless the laws of co.
,,sion d9d elasticity are suspended to suit the
/ PIICe OF ignorance of man.
:. LIMITS OF SAFETY.-What are the limits Of
aaalety u any special case is at times, very diff
~ a,ulG l question The conditions involved may be
~ SO numerous and some Of them so obscure, that
d az e"Hact soluton becomes impossible. IT would
pc an easy task to determine how many tons of
Dfl2k ana mortar could be safely buHt upon cast
~ plt.at me strength Of the pillar is known,
gravity are known No change ia these conditions.
can occur, and the architect who ahouId faii IN
ga s9ch 3 case would justly sacrifice his reputation,
The fall OF the Pemberton Mills with ali ia har-
rowing details. shows how easy it ia to over
step the limits of safety in the most srnuls cases
and what are the consequences Of a disregard or
ignorance Of these limits, The fall of a binding
without some extraordinary cause is eeHfessedIy q
reprOaeh to any architect; while the expIosinn of
a steam boiler seems to infiiet reproach upon no.
body or nothing, unless it be upon the fractions
and capTicious disposition Of that scapegoat--
steam. But there must be responsibility some
where--divide it as you will-for such chapter
OF accidents as daily meets the eye-fayed bodies
and mangled forms. There are serious difficulties
IN the way of securing safety in the of steam.
There are many things to be discovered, many to
pc learned more fully, more care in applying the
hhoWledge already possessed, and less eagerness
70 fealze high dividends at any risk. We're the
limits OF elasticity in every specimen of iron ex-
acty known; were the engineer absolutely sure
that there was no defect In the boiler plates none
IN the rlFets, and none in the workmanship were
all the conditions in the generation of steam en-
tIrely understood, were the terrific forces which
lie hid IN the Vapor so subdued to our bidding as
never 70 pass subscribed boundaries; were engi-
neers never incompetent, never forgetfu or care
ieFsf could no contingencies arise which were
! not anticipated and provided for, then might the
pressure q steam be kept close mp TO the limits
OF elasticity, and the confines Of danger be an
preached nearly, but not passed. The eye that
cannot be turned aside from its proper object the
ieet that never slide, and the body that cannot
lose i's balance, may tread as safely along the
verge OF the precipice as in the valley below.
But, unfortunately none of these conditions exist;
hence, it is wise to keep at respectful distance
ron ne preelpiee and in the use Of steam it is
gfeauy wise 10 leave margin for safety 80 broad
as to include all the contingencies named above
and any others which may arise.
The working pressure should be so far from the
line Of danger as TO meet all the ordinary, and
most, at least, Of the extraordinary conditions
which can occur. In structures liable to sudden
and, it may be, an unexpected increase Of the
strain, as in bridges and boilers, the margin for
safety should be broader than in other cases.
An intelligent engineer, who valued his reputa-
ton would not subject the cables of bridge to a.
tension greater than one-sxth of that required to
break them. Even this is carrying the strain up
to the limits of elasticity. The usual rule is them
plainly this: first, to determine the tension to
which the structure will in any case be exposed
and second, to make it strong that this tension
shall never exceed onehalf the limits of elasticity,
OF one-sixth OF the breaking strength. The intel
gent and skillful architect Of the great bridge
soon to spanthe Ohio informed the writer that the
weight on its twelve-inch cables is not to exceed
one-si1th of that which would tear them aslnder.
The largest tube in the Victoria bridge across the
st. Lawrence, at Montreal, is three hundred and
thirty feet, with breadth of sixteen feet and q
hight of twenty-one feet and eight inches. This
would sustain before breaking nearly one million
five hundred thousand pounds placed on its centre,
excluding the weight Of the tube No train of cars
while crossing could exert strain equal to one
sixteenth the breaking strength.
The stones at the base of the great dome of st.
Peters at Rome, and of st. Paul's in London
tain the first one-sixteenth, and the second one
fourteenth of the pressure required to crush them.
These facts reveal clearly the opinions Of the cele-
brated architects Michael Angelo and Sir Christo-
pher Wren, as to the limits of safety in such
cases. This degree of earefulness is
not extravagant even in bridges and
buildings where the conditions of thefoEGe applied
SO easily ascertaned. But if is proper to con-
| fine the strain on the cables of q bridge within
such narrow limits, what should be the strain a
boiler which is more liable originally to imperfec-
tions; which grows weaker daily by oxidation and
the action Of intense heat and where the eondi-
tons under which the force is generated are un-
certain or entirely unknown; and where sudden
increase of that force will occasionally take
place beyond any ability We at present possess TO
prevent?
Would it not excite surprise and alarm, ig it
should be found that the tensien on boilers is
frequently greater than that which is considered
safe on the cables of the bridge We shall see.
MIAMI UNIVERSITY, Nav 1886, -