National opinion. [volume] (Bradford, Vt.) 1865-1874, 1871-08-18
Unequal Expansion OF Boilers.

The opinion Of Norman Wizard, nm en-
gineer, who examined the exploded boil
er Of the New York Ferry boat, IN that
the cause Of the explosion was unequal
expulsion" This is also the published
opinion of James A. Whitney, practi-
cal engineer, the result of whose inyesti-
CATION appears to tally throughout with
that of Mr.. Wizard The examination
by Mr. Wizard was continued for several
hours, and was based upon 10 years
study Of the subject Of boiler explosions.
His theories are as follows
Boilers most commonly are fractnr-
ed longiiudinally, the line Of the fracture
being about that of the water level with
in. Water receives heat slowly from
above for that which il the warmest
remains at the top. Steam, on the other
hand, absorbs heat more rapidly than
any known substance. The consequence
is that there is almost always an immense
difference between the heat of the steam
and the water below it, The boiler iron
also partakes of these differences of tenn
pcrature. The result is unequal expan-
sion, and, in case of oscillation or other
cause of change of relative positions,
fracture of fbcrs. The water in the boil
ers, beneath the lower range of flues, is
comparatively cool, or, say, Mt 800, while
above the temperature of the water may
be 260l and Of the steam much greater.
Hence the boiler is shorter at the bot
tom than at the top, The tlues, being
heated first, produce strong pressure
against the ends of the boiler. BY this
means transverse rupture is made on
the under side OF the shell, where the
cool water hes These ruptures are, how
ever, not regarded by engineers as dan
gerous, as they simply produce slight
leaks. The boiler of the Westfield shows
a transverse rupture of this kind which
had evidently existed for some time.
This flaw extends over four feet, along
the line of the rivets on the under side
of the middle section. II does not ap-
pear to have extended quite to the point
on the lowest west side Of the shell where
the first break of the explosion occurred.
A zigzag line of partial fracture may
have reached ont toward that point, and
probably did, thus making that the weak
est part of the boiler. The cool water
rising just to the line of the lower range
of fues these being intensely heated
from the furnace the engineer having
been absent minutes 10y his own con-
fession-but much longer, others say,
and having declared that the pressure of
steam was then u pounds, leaving a
clear presumption that, as they were
firing up for departure, the pressure of
steam rapidly increased, it seems per
fectly plain that any slight oscillation of
the vessel, from whatt yer cause, which
lifted the cool water against a surface of
higher temperature so as to produce a
change of position of particles of iron
from unequal expansion, must have re-
duced the cohesion of the sheet enough
to make explosion almost inevitable In
a long list of boiler explosions upon
steamers, it is shown that by far the
greater part of them occurred at the
moment Of starting, proving that oscilln-
ton of the water within the shells was
the cause. The existence of the flaw In
the boiler of the Westfield was very like
ly known to the engineer and tiremen,
from the leaking that must sometimes
have occurred at that point. But the
break at the riveting, while it shows that
the latter was thin and weak, has not
yet been found to show any trace of the
extension of the flaw.