Produced by Chris Curnow, Martin Mayer and the Online

 [A transcriber's note follows the text.]

  RECORDS
  OF
  STEAM BOILER EXPLOSIONS,

  BY

  EDWARD BINDON MARTEN,

  MEM. INST. OF MECHANICAL ENGINEERS; ASSOCIATE OF INSTITUTION OF
  CIVIL ENGINEERS, AND CHIEF ENGINEER TO THE
  MIDLAND STEAM BOILER INSPECTION AND ASSURANCE CO.

  LONDON;
  E. & F. N. SPON, 48, CHARING CROSS.

  STOURBRIDGE:
  R. BROOMHALL, 148, HIGH STREET.

  1872.

PREFACE.

Accurate information as to Boiler Explosions must always be useful to
those who are interested in the safe working of Steam Boilers.

The following pages contain very brief abstracts of records obtained
for the Midland Steam Boiler Inspection and Assurance Company, by whose
permission they are now republished in a compact and convenient form.

By permission of the Council of the Institution of Mechanical
Engineers, the records are prefaced by a Paper on Steam Boiler
Explosions and their records, and on Inspection as a means of
prevention, read before that Institution at Manchester, August 1st,
1866, and a further Paper on the "Conclusions derived from the
experience of recent Steam Boiler Explosions," read before the same
Institution at Nottingham, August 3rd, 1870.

All names of Works or Firms are omitted from the records as
unnecessary.

ON STEAM BOILER EXPLOSIONS AND THEIR RECORDS, AND ON INSPECTION AS A
MEANS OF PREVENTION, BY EDWARD B. MARTEN, MEM. INST. M.E. A.I.C.E.,
EXCERPT MINUTES OF PROCEEDINGS OF THE MEETING OF THE INSTITUTION
OF MECHANICAL ENGINEERS, AT MANCHESTER, 1ST AUGUST, 1866, JOSEPH
WHITWORTH, ESQ., PRESIDENT, IN THE CHAIR. BY PERMISSION OF THE COUNCIL.

The subject of Steam Boiler Explosions, which was brought before this
Institution in June, 1848, in a paper by the late Mr. William Smith
of Dudley in reference to an explosion near that place, and again in
1859 in a paper by Mr. Longridge on the economy and durability of
stationary boilers, is one of great importance and is now attracting
increased attention. The first public notice of the subject was by a
parliamentary committee in 1817, which was appointed in consequence
of a very fatal boiler explosion in London in 1815; evidence was then
collected as to steamboats, and many boiler explosions were referred
to. That committee recommended among other things that boilers should
be made of wrought iron, instead of cast iron or copper, which had been
the materials mainly used previously; that they should be inspected
and tested; and that there should be two safety valves, each loaded
to one third of the test pressure, under penalties for any excess. A
great part of the information now existing upon the subject, especially
in regard to the earlier explosions, is to be found in the records of
inquests after fatal cases; and some of the careful reports of eminent
engineers on those occasions have materially assisted in the formation
of correct views as to the causes of explosion. Latterly also the
printed reports of the inspectors of mines, and more especially the
reports of the explosions of locomotives, illustrated by diagrams by
the inspectors of railways, have furnished very valuable information.
Since the subject has been taken up by private associations for the
prevention of explosions, many more records have been published,
although their usefulness is much impaired by their not containing the
names of the places whereby the explosions could be identified.

When the writer's attention was first directed to this subject, he
met with great difficulty in obtaining correct records of boiler
explosions, from which to arrive at the results of past experience;
and wishing to base his own opinion on facts, rather than on the
inferences of others however reliable, he followed the example of the
Franklin Institute in their elaborate investigation of the subject, and
collected all the records he could find; and by way of facilitating
reference, arranged an index, a manuscript copy of which is presented
with the present paper to the Library of this Institution. All must
be agreed as to the importance of reliable information on such
accidents as boiler explosions; and the writer would suggest that
this Institution may materially aid in obtaining the desired records
and placing them within easy access, by becoming the depository of
reports on explosions, and by inducing those who have the opportunity
to forward copies of reports, that these may be arranged so as to be
easily found and consulted. It is very desirable that these reports
should as far as possible be illustrated by sketches, as aids to
the description; and also by slight models like those now shown to
the meeting, by which the whole matter may be seen at a glance. So
few persons comparatively have the opportunity of examining boilers
after explosion, that the most erroneous ideas have prevailed, and
theories have been advanced which would soon be dissipated by practical
experience or by reading accurate reports. It would also very much
aid in the understanding of published matter on the subject, if full
descriptions of each case alluded to in illustration could be obtained.
These records are as useful to the engineer as the "precedents" or
"cases" to the lawyer or the surgeon. After any serious explosion,
the newspapers of the neighbourhood in which it has occurred contain
voluminous articles describing the disastrous result and the damage
done, which, although useful as far as they go, do not in the least
assist in arriving at the cause of explosion. The really important
particulars, such as the description and construction of the boiler,
its dimensions, and the pressure at which it worked, are in most cases
omitted altogether.

The record of explosions presented to the Institution contains a list
of the boiler explosions in each year of the present century, as far as
known to the writer, with the names of the places, and the description
and sizes of the boilers, and the supposed cause of explosion, together
with references to the books or papers from which further information
may be obtained. Of course many of the explosions have to be put down
as uncertain in some of the particulars; but every year improves the
record, as fresh information is obtained, and with the assistance of
the members of this Institution it might be made far more perfect and
extensive.

The total number of explosions here recorded is 1046, and they caused
the death of 4076 persons and the injury of 2903. The causes assigned
for the several explosions are very numerous, and are no doubt
incorrect in many cases; but they may be generally stated as follows:

   397 are too uncertain to place under any heading; but of the rest

   145 were from the boilers being worn out, or from corrosion, or
       from deteriorated plates or rivets.

   137 from over pressure, from safety valves being wedged or
       overweighted, in some cases intentionally, or from other
       acts of carelessness.

   125 from faulty construction of boiler or fittings, want of stays, or
       neglect of timely repair.

   119 from collapse of internal tubes, generally from insufficient
       strength.

   114 from shortness of water, or from scurf preventing the proper
       contact of the water with the plates; or from improper
       setting so as to expose the sides of the boiler to the flame
       above the water line.

     9 from extraneous causes, such as effect of lightning striking
       down the stacks upon the boilers, or from fire in the building
       or explosion of gas in the flues.
  ----
  1046 total number of explosions.
  ====

The exploded boilers were of the following descriptions:--

   232 are not sufficiently described to place under any head; but of
       the rest

   320 were Marine boilers of various forms.

   141 were Cornish, Lancashire, or other boilers internally fired.

   120 were Locomotive, or other multitubular boilers.

   116 were plain Cylindrical boilers, externally fired.

    64 were Balloon or haystack, wagon, Butterley, British-tube,
       elephant, or Trevithick boilers.

    29 were Portable, agricultural, upright, or crane boilers.

    14 were Heating apparatus or kitchen boilers.

    10 were Upright boilers attached to puddling or mill furnaces at
       ironworks.
  ----
  1046 total number of explosions.
  ====

[Illustration: _Fig. 1._]

The theories as to the causes of explosion have been numerous. In
the early days of the steam engine, when the steam was used only as
a condensing medium and the pressure in the boiler was frequently
allowed to get below atmospheric pressure, many boilers were destroyed
by the access of the external atmospheric pressure becoming too great,
causing them to be collapsed or crumpled up; and this led to the use
of the atmospheric valve still found on old boilers. Even so lately as
last year, 1865, a boiler in the neighbourhood of Bury, Lancashire,
has suffered in this way by collapse from external pressure; its
appearance after the accident is shown in Fig. 1, which is copied
from a photograph. The early explosions were so palpably due to the
weakness of the boilers, which compared with those of the present day
were most ill constructed, that no one thought of any other cause than
the insufficient strength of the vessel to bear the expansive force
of the steam contained in it. When the advantages of high-pressure
steam became recognized, and the boilers were improved so as to bear
the increased strain, the tremendous havoc caused by an explosion led
many to think that something more must be required than the expansive
force of the steam to produce such an effect; and they appear to have
attributed to steam under certain conditions a detonating force, or
a sudden access of expansive power that overcame all resistance.
To support this somewhat natural supposition, it was asserted that
the steam became partially decomposed into its constituent gases,
forming an explosive mixture within the boiler. That this belief is
still sometimes entertained is seen from the verdict of a jury even
in the present year, 1866, in the case of the explosion of a plain
cylindrical boiler at Leicester, shown in Fig. 2., the real cause of
which appears to have been that the shell of the boiler was weakened
by the manhole. It seems hardly necessary to point out the fallacy of
imagining decomposition and recomposition of the steam to take place
in succession in the same vessel without the introduction of any
new element for causing a change of chemical combination; but it is
necessary to refer to this supposition, as the idea is shown to be not
yet extinct.

[Illustration: _Fig. 2._]

Again it has been asserted that the steam when remaining quite still
in the boiler becomes heated much beyond the temperature due to the
pressure; and that therefore when it is stirred or mixed or brought
more in contact with the water by the opening of a valve or other
cause, the water evaporates so rapidly as to produce an excessive
pressure by accumulation of steam. In support of this view the
frequency of explosions upon the starting of the engine after a short
stand is adduced; but it is very doubtful whether by this means a
sufficient extra pressure could be produced to cause an explosion,
unless the boiler had been previously working up to within a very
small margin of its strength. Explosions are seldom caused by a sudden
increase of pressure, but rather by the pressure gradually mounting to
the bursting point, when of course the effect is sudden enough. Nor
is it necessary in many cases to look for much increase of pressure
as the cause of explosion; for it is far more often the case that the
strength of the boiler has gradually degenerated by wear or corrosion,
until unable to bear even the ordinary working pressure. It is so very
easy, when examining the scene of an explosion, for the first cause of
rupture to be confounded with the causes of the subsequent mischief,
that in many cases erroneous conclusions have been arrived at in this
way.

The most important points to find out in connection with any explosion
are the condition of the boiler and all belonging to it immediately
before the explosion, together with the locality of the first rent, the
direction of the line of rupture, and the nature of the fracture; as
everything occurring after the instant of the first rent is an effect
and not a cause of explosion. As soon as the first rent has taken
place, the balance of strain in the fabric is disturbed, and therefore
the internal pressure has greatly increased power in continuing the
rupture; and also the pressure being then removed from the surface of
the water, which is already heated to the temperature of the steam,
the whole body of the water gives out its heat in the form of steam
at a considerable pressure, and thus supplies the volume of steam for
carrying on the work of destruction. When thus quickly generated, the
steam perhaps carries part of the water with it in the same way that
it does in ordinary priming; and it has been thought by some that the
impact of the water is thus added to that of the steam, to aid in the
shock given to all surrounding obstacles.

It is seldom that one out of a bed of boilers explodes without more
or less injury to the others on either side of it; but sometimes
two boilers in one bed, or three, or even five, have exploded
simultaneously.

The causes of boiler explosions may be considered under the two general
heads of--

Firstly, faults in the fabric of the boiler itself as originally
constructed, such as bad shape, want of stays, bad material, defective
workmanship, or injudicious setting:--and

Secondly, mischief arising during working, either from wear and tear,
or from overheating through shortness of water or accumulation of
scurf; or from corrosion, in its several forms of general thinning,
pitting, furrowing, or channelling of the plates; or from flaws or
fractures in the material, or injury by the effect of repeated strain;
or from undue pressure through want of adequate arrangements for escape
of surplus steam.

[Illustration: _Fig. 3._]

[Illustration: _Fig. 4._]

[Illustration: _Fig. 5._]

There is no doubt that many of the early explosions were from faults
of construction. The stronger materials now used were then found so
difficult to manipulate that others easier to work were chosen, and
often the shape of the boiler was only selected as the one easiest to
make. The early boilers were made of copper or cast iron, with leaden
or even wooden tops, and of the weakest possible shape. Such was the
boiler used by Savery, shown in Fig. 3., and the Tun Boiler and Flange
boiler, Figs. 4 and 5. The very fatal explosion in London in 1815,
referred to by the parliamentary commission previously named, was of a
cast-iron boiler, which failed because one side was too thin to bear
the pressure, as the casting was of irregular thickness. The steam
being at that time used only at or below atmospheric pressure as a
means of obtaining a vacuum by condensation for working by the external
pressure of the atmosphere, so little was pressure of the steam thought
of, that boilers were proposed and it is believed were actually
constructed with hooped wooden shells, like barrels, and internal
fireplaces and flues of copper; and even a stone chamber was named
as being a suitable shell for a boiler, with internal fireplace and
copper flue passing three times the length of the inside and out at the
top, like an ordinary stove and piping. These boilers must have been
something like the sketches given in Figs. 6 and 7, and were intended
to be exposed only to the external pressure of the atmosphere.

[Illustration: _Fig. 6._]

[Illustration: _Fig. 7._]

[Illustration: _Fig. 8._]

[Illustration: _Fig. 9._]

[Illustration: _Fig. 10._]

Cast iron was frequently used for the shell of boilers, with an
internal fireplace and tubes of wrought iron, as shown in Fig. 8, and
boilers of this construction are still to be found in use at some of
the older works at the present day. As the outside shell and front
plate are 1-1/2 inch thick and are not exposed to any wear at all,
these boilers are sufficiently strong. A duplicate front plate with
set of tubes attached is always kept on hand in case of need. Another
form of cast-iron boiler is shown in Fig. 9., made in several parts
put together with flange joints, with an internal fireplace and flue
also made of cast iron. When cast iron was used for the parts exposed
to the fire in boilers intended for high pressure, it was sometimes
employed in the form of tubes of small diameter and proportionately
thinner; as in Woolf's boiler, so much spoken of in the evidence
before the parliamentary committee of 1817. This boiler, shown in Fig.
10., consisted of nine cast-iron pipes, about 1 foot diameter and 9
feet long, set in brickwork so that the flame played all round them.
These small tubes were connected with another of larger size placed
transversely above them, forming a steam receiver, and this again with
a still larger one, which formed a steam chamber. No details of any
explosions of the three last mentioned boilers have been obtained; but
it is known that the cast iron was found a most treacherous material,
especially when exposed to the action of the fire; and that the effect
of explosion was very disastrous, because the boiler burst at once into
many pieces, each of which was driven out with great velocity, and the
danger was not mitigated by the circumstance of large masses holding
together, as is found to be the case with wrought-iron boilers when
exploded.

[Illustration: _Fig. 11._]

[Illustration: _Fig. 12._]

When wrought-iron boilers came into use the shapes were most varied,
and the dimensions much larger than before. One of the earliest was the
Wagon boiler, shown in Fig. 11, with round top and plain flat sides,
which could only be made to bear even the smallest pressure by being
strengthened with numerous stays. In most cases of explosion of this
class of boiler the bottom was torn off, owing to the angle iron round
it being weakened by the alternate bending backwards and forwards under
each variation of pressure, as all the sides and the bottom must be
constantly springing when at work. Such was the explosion at Chester
in 1822, and many others. This shape was soon improved in its steam
generating powers by making the sides concave instead of flat, as shown
in Fig. 12, so that the heating surface was greater and also in a
better position to receive the heat from the flame in the flues. This
shape was further elaborated by rounding the ends as in Fig 13, and
in some cases making the bottom convex to correspond with the top, as
in Fig. 14. All these forms however still required numerous stays to
retain them in shape, the safety of the boiler being dependent upon
the stays; and numerous explosions show the weakness of these boilers.
They generally gave way at the bottom, as in an explosion that occurred
at Manchester in 1842, where the boiler had been weakened by frequent
patching; they also sometimes exploded through the failure of the stays.

[Illustration: _Fig. 13._]

[Illustration: _Fig. 14._]

[Illustration: _Fig. 15._]

[Illustration: _Fig. 16._]

[Illustration: _Fig. 17._]

[Illustration: _Fig. 18._]

A very early improvement in the right direction consisted in making
the shell circular; and some few large boilers still exist that were
made completely spherical, as show in Fig. 15, so that the whole of
the iron was exposed to tension only, and required no assistance from
stays, and the boiler had no tendency to alter its shape under varying
pressure. This shape however had the great disadvantage of possessing
the least amount of heating surface for its size or cubic contents; and
also it was very liable to injury from sediment on the bottom, which
accumulated on the most central spot. The spherical form was therefore
soon modified into the shape shown in Fig. 16, by making the bottom
more shallow, although still convex; and afterwards by putting flat or
concave sides and a flat or concave bottom, with the angle constructed
either of bent plates or angle iron, as in Figs. 17 and 18, which
represent the forms known so well in the Staffordshire district as the
common Balloon or Haystack boiler. Many of these have been made of very
great size, measuring as much as 20 feet diameter, and containing so
much water and steam as to be most formidable magazines for explosion.
Perhaps no form of boiler has exploded more than this, partly because
of the great number that have been used, but chiefly because of the
inherent weakness of the shape. The records however have not been
obtained of the great majority of these explosions, because they seldom
caused sufficient damage or loss of life to attract much attention, as
these boilers generally worked in isolated positions at collieries.
The bottom is only prevented from blowing down into the fireplace by
numerous stays from the top, and the angle iron round the bottom of the
sides is much tried by the constant springing of the plates under every
alternation of pressure; and the weakness thus occasioned is increased
by the angle resting on the brickwork and being exposed to corrosion.
The effect of this continued alternation of strain is well shown by the
elastic model exhibited.

[Illustration: _Fig. 19._]

[Illustration: _Fig. 20._]

[Illustration: _Fig. 21._]

[Illustration: _Fig. 22._]

Notwithstanding the dependence of these boilers upon stays for their
strength, many have been made as large as 12 and 15 feet diameter
without stays; and explosion sooner or later has been the consequence.
Such was an explosion that took place at Smethwick in 1862, which
is shown in Fig. 19. As the force of the explosion was only slight,
the effect of the bottom giving way, and the consequent rolling over
caused by the reaction of the issuing steam and water, is clearly seen.
Another example that occurred at Wednesbury in 1862 is shown in Fig.
20, where the explosion was rather more violent, the bottom of the
boiler being torn off all round and left upon the firegrate, and rent
nearly into two pieces; while the top and sides were thrown some height
in one mass, and were only put out of shape by the fall. The weakness
of this boiler had been further increased by making the bottom angle of
angle iron, as shown enlarged in Fig. 21, with a ring of flat plate
A interposed between the angle-iron ring and the concave bottom of
the boiler; so that all the effect of the springing of the bottom, as
shown by the dotted lines, was thrown upon the angle iron, which was
accordingly found cut off all round. Had the concave bottom been made
to rise direct from the angle iron, as in Fig. 22, the springing could
not have been so great, and the angle iron would only have had to stand
the shearing strain of retaining in its place the rigid bottom; but as
about one foot all round the bottom was flat, and the concavity was
only in the central part, the angle-iron ring had to bear an up and
down strain, as shown by the dotted lines in Fig. 21, and the bending
action was more severe than it would have been if the bottom had even
been made quite flat all over.

[Illustration: _Fig. 23._]

A further form of the Balloon boiler is shown in Fig. 23, where the
heating surface of the bottom is increased by an internal central
dome-shaped fireplace, with an arched and curved flue conducting the
flame through one revolution within the boiler before passing again
round the outside. This construction however must necessarily have
diminished the strength of the boiler greatly. In the drawing the top
of the boiler, as indicated by the dotted lines, is removed to show the
interior.

The desire to add to the strength of boilers by lessening the diameter
of the shell led to the construction of the Plain Cylindrical boilers.
They were made first with flat ends of cast iron, which frequently
cracked and gave way when exposed to the fire, as described in many
of the early American explosions. The flat ends when made of wrought
iron, as shown in Fig. 24, are exposed to the same strain as the bottom
of the balloon and wagon boilers, and are constantly springing with
variation of pressure like drum heads, causing injury to the angle-iron
joint. They also require long stays through them to hold in the ends,
and these are subject to so much vibration that they seldom continue
sound for long together, especially when joined with forked ends and
cotters.

[Illustration: _Fig. 24._]

As the flat ends of such boilers are always being sprung by each
alternation of pressure into a more or less spherical shape, as shown
by the elastic model exhibited, this consideration no doubt led to
the ends being made hemispherical, as shown in Fig. 25; and plain
cylindrical boilers with these hemispherical ends are now so commonly
used that they far outnumber any other form of boiler. Their shape
renders them very strong, as the whole of the iron is in simple
tension, and internal pressure has no tendency to alter the shape, as
is shown by the elastic model exhibited. There is one circumstance very
much in favour of the plain cylindrical boilers, and that is that they
can be so easily cleaned and repaired, as a man can stand properly at
his work at every part and the whole of the interior surface is exposed
equally to view. They are of course exposed to all the evils of boilers
externally fired, the part under greatest strain being weakened by
the action of the fire; and the bottom is also exposed to injury from
accumulation of mud and chips of scurf, which cannot be prevented from
falling there, and lying upon the part exposed to the direct action of
the fire. When made of great length, such as 70 or 80 feet, as is the
practice for applying the waste gas from blast furnaces, these boilers
are also liable to seam-rips or "broken backs," owing to the greater
expansion of the bottom exposed to the fierce flame for its whole
length, than of the top which is kept cooler by exposure to the air;
and it would therefore be better to have a succession of short boilers,
rather than only a single one, where great length is required.

[Illustration: _Fig. 25._]

One boiler has been seen by the writer where extreme length was avoided
by curling the boiler round until the ends met forming a Ring or
Annular boiler. This boiler is shown in Fig. 26, and is 5 feet diameter
with 25 feet external diameter of the ring, or a mean length of about
63 feet; it has been found to work well for some years, although
exposed to the heat of six puddling furnaces.

[Illustration: _Fig. 26._]

[Illustration: _Fig. 27._]

Explosions of plain cylindrical boilers have been very frequent indeed,
although they have not caused a proportionate number of deaths, because
they work usually in isolated positions at colliery and mine engines.
The sketch shown in Fig. 27, represents an explosion that occurred
at Darlaston in 1863, and illustrates the way in which these boilers
usually explode. They generally open first at a longitudinal seam over
the fire, which has become deteriorated by accumulations of scurf
preventing proper contact of the water, so that the plates become
overheated, their quality injured, their edges cracked or burnt, and
the rivets drawn or loosened. The rent generally continues in the
longitudinal direction to the sound seam beyond the bridge at the one
end, and at the other end to the seam joining the front end to the
shell; and then runs up each of the transverse seams, allowing the rent
part of the shell to open out flat on both sides, and liberating both
ends of the boiler, which fly off in opposite directions. Of course
it is seldom that an explosion is quite so simple as this, as the
direction of the flight of the various pieces is so much influenced by
the last part that held in contact with the main body of the boiler.
The want of due observation of this point has often led to erroneous
conclusions.

[Illustration: _Fig. 28._]

In the explosion shown in Fig. 28, and in the model exhibited, which
occurred at Westbromwich in 1864, the lower part of the side of an
upright boiler was blown out; and the liberated part was also divided
into two pieces, each of which fell some distance behind the boiler, in
an opposite direction to the side from which they came. The explanation
of this became obvious on examination, as the cause of the rupture
had been the corrosion of the bottom, and the rent had run up the
seams until it met the angle iron of the side tubes, round which it
ran to the first seam above. This seam acted as a hinge on which the
ruptured pieces turned, and they swung round so violently that they
were wrenched off, but not before they had pulled the boiler over and
received the diverting force which gave them their direction, for they
flew off at a tangent, to the circle in which they had swung round on
the sound upper seam as upon a hinge.

[Illustration: _Fig. 29._]

[Illustration: _Fig. 30._]

In order to avoid having a large diameter for plain cylindrical
boilers, especially where exposed to the fire, boilers have been used
that have supplied the required steam power by a combination of several
cylinders of small diameter. One of these known as the Elephant boiler,
has been so much used in France that it is sometimes called the French
boiler; it is shown in Fig. 29, and consists of two cylinders of small
diameter connected by upright conical tubes to a large cylinder above.
Another form called the Retort Boiler, shown in Fig. 30, has been
described at a previous meeting of this Institution (see Proceedings
Inst. M. E. 1855 page 191). The disadvantages of these two combinations
of plain cylinders are that they are not easy to clean or examine
internally, and also there is not free exit for the steam, which has
to find its way along small channels, and carries the water away with
it, causing priming, and also retarding the generation of steam and
endangering the boiler plates. With a view to strengthen the plain
cylinder made of wrought-iron plates, the seams are sometimes made to
run diagonally, as shown in Fig. 31, on the principle that, as the
longitudinal is the weakest seam and the transverse the strongest, a
diagonal between them gives the greatest amount of strength to the
boiler as a whole.

[Illustration: _Fig. 31._]

[Illustration: _Fig. 32._]

[Illustration: _Fig. 33._]

[Illustration: _Fig. 34._]

[Illustration: _Fig. 35._]

Plain cylindrical and wagon boilers have for many years been made with
internal tubes of various shapes and arrangement, through which the
flame passes to add to the heating surface. These are shown in dotted
lines on the previous drawings of wagon boilers, Figs. 11 and 12. They
are also shown in Fig. 32, where a tube passes from over the fire to
the front of a plain cylindrical boiler; in Fig. 33 two tubes pass
from the sides to the front: in Fig. 34 the tube passes from the back,
but returns over the fire and passes again to the back: and in Fig. 35
a tube from the back passes out through a cross tube in each side. The
boilers in all these cases are fired externally. This addition of tubes
has tended very much to increase the size of these boilers in order to
make room for the tubes. These boilers are now found of 9, 10, and
even 11 feet diameter; and this large shell being fired externally is
exposed to the same dangers as those described in the plain cylindrical
boiler, while it is not so easy to keep clean on account of the
obstruction offered by the internal flues. When the flame has passed
under the whole length of the bottom of these large boilers before
going through any tube, it is doubtful whether the heating surface of
the tube helps much in the generation of steam; but the tube is of
use in reducing the quantity of water in the boiler, as it occupies a
considerable space.

[Illustration: _Fig. 36._]

Explosions of these boilers have sometimes taken place by collapse of
the tubes, but much more generally by the failure of the shell over the
fire, as shown in the sketch Fig. 36, representing an explosion that
occurred at Wolverhampton in 1865, in which the first rent took place
in a seam over the fire where frequent repair had led to a considerable
length of longitudinal seam being in one continuous line. The four
plates over the fire parted and opened out until they had ripped two
seams completely round the boiler; and the plates were thrown in one
flat piece, as shown, upon a bank behind. The main body of the boiler
with the tubes was turned over, and the front end blown away.

[Illustration: _Fig. 37._]

A modification or amalgamation of several of the forms of boilers
already mentioned led to the construction known as the Butterley
boiler, shown in Fig. 37, with a wagon-shaped end over the fire,
continued in a single tube within a plain cylindrical shell beyond.
This boiler has been found to generate steam very rapidly; but the
extreme weakness of the construction over the fire and along the tube,
especially at the part where the front end of the tube widens out in
a bell mouth to meet the wagon-topped fireplace, has led to so many
explosions that few boilers are now made of this form. A very early
explosion that occurred at Edinburgh in 1821 was of a boiler somewhat
of this shape, only that the wagon-topped fireplace was much longer.
Other explosions of this form of boiler occurred at Ashton-under-Lyne
in 1845, at Wolverhampton in 1854, and at Tipton in 1856.

[Illustration: _Fig. 38._]

[Illustration: _Fig. 39._]

[Illustration: _Fig. 40._]

[Illustration: _Fig. 41._]

The desire to economise fuel led to placing the fire inside the boiler,
in a tube running from end to end, as shown in Fig. 38, and the great
number of boilers of this form used in Cornwall gave it the name of the
Cornish boiler. The exceedingly good duty performed by these boilers
led many to believe them the most perfect for economy and durability;
but the great number of explosions, or more properly of collapsed
flues, that have happened, have altered this opinion, and led to the
double-flue boiler shown in Fig. 39, in which not only is the heating
surface increased but the strength also, by having two tubes of smaller
diameter in the same shell. There are a great many varieties of the
two-tube boiler, which have been made for the purpose of obtaining
various particular results. In some cases the two tubes have been made
to unite into a single tube immediately behind the fires, forming what
is known as the Breeches-tube boiler, as shown in Fig. 40, and in other
instances the outside shell of the boiler has been made oval, as shown
in Fig. 41, with the two tubes continued through from end to end. The
heating surface has also been increased, and the strength of the main
tubes, by placing smaller transverse tubes across them at right angles;
but these advantages are gained by increased complication, leading of
course to greater difficulty in examination and repair.

The frequent failure of tubes by collapse when used for high pressures,
and also the results of careful experiments, led to the simple addition
of strengthening rings of different makes around the exterior of these
tubes, by which the shell and the tubes are rendered of equal strength.
It has taken considerable time for the belief in the weakness of large
tubes when exposed to external pressure to become general, and a great
many boilers are still made and used having even large tubes without
the strengthening rings; and in some districts such boilers are used
in great numbers and at far higher pressures than can be considered
judicious. In more than one bed of boilers, one boiler after another
has exploded by the collapse of the tube from the want of strengthening
rings, and yet these have still been believed unnecessary; and the
cases of isolated boilers of this construction where the large tubes
have collapsed are extremely numerous, yet any other reason than the
weakness of the tube has been considered more probable as the cause of
explosion. A sketch of a boiler with collapsed flue is given in Fig.
42, which exploded at Burton-on-Trent in 1865; and it is selected from
many others because it was a new boiler, well made and mounted, and was
a good example of the weakness of a large tube to resist high external
pressure when made of great length without the support of strengthening
rings.

[Illustration: _Fig. 42._]

There are a great many advantages in the tubular boiler internally
fired. The shell which is exposed to the greatest tension is not also
exposed to the first action of the fire. The fire is in the midst of
the water, so that the greatest effect is obtained from it; and the
heating surface immediately over the fire, from which most steam is
generated, has not so great a depth of water above it for the steam to
pass through as in the externally fired boilers heated from the bottom.
The tubes also act as stays to the ends; and the mud in the water falls
off the tubes, where it would do mischief, and settles on the bottom,
where it is comparatively harmless.

These tubular boilers are however subject to disadvantages peculiarly
their own. It is not so easy to move about within them for cleaning
and examination as in the plain cylindrical boiler, as the tubes fill
up the space so much. The difference of expansion between the highly
heated tube and the comparatively cool shell produces a strain, which
causes the ends to bulge out; or if the ends are made rigid, the strain
sets up a contortion in the tube, which causes furrowing of the plates
by making the iron softer or more susceptible of corrosion in certain
lines of strain. Notwithstanding these drawbacks however this form of
boiler is an excellent one.

Many modifications in the forms of boilers have been made to enable the
manufacturers to use the waste heat from various processes, especially
from the making of iron. The plain cylindrical boiler has been used
in this way, with sometimes as many as eight puddling furnaces made to
work upon one boiler. One of the earliest special arrangements for this
purpose was the Upright boiler with central tube, shown in Fig. 43,
which was originally made for two furnaces; and about 7 feet diameter
and 16 feet high. The size has since been increased to 10 feet diameter
and 28 feet high, as shown in Fig. 44. These boilers are made for one,
two, three, or four puddling furnaces; and consist of a cylinder with
spherical ends, standing upright, with a central tube from the bottom
to about half the height, into which the side tubes join. The heat from
each furnace plays over a portion of the shell, and then passes through
the side tubes and down the centre tube into the underground flue to
the chimney.

[Illustration: _Fig. 43._]

[Illustration: _Fig. 44._]

These boilers have many good points: there is great heating surface;
and the shell being heated all round does not strain the plates
and seams by unequal expansion so much as in the horizontal plain
cylindrical boiler heated only at the bottom; and as both ends are
spherical there is no alteration of shape under internal pressure.
Moreover in consequence of the upright position of the boiler a safe
depth of water can easily be maintained, and yet the steam is taken off
so high above its surface that there is little priming; and every part
can most easily be cleaned and examined, as a man can stand upright
both in the boiler and in the flues. But the great drawback to this
class of boilers is that they must stand in the midst of the workmen;
so that, although they are not more liable to explode than any other
form of boiler, yet when they do burst they necessarily endanger more
lives than is usually the case with other boilers that can be placed
more away from the men employed at the works. Should anything arise
with the boiler to make it desirable to withdraw the fire, this cannot
be done without much delay, as the furnaces have to be stopped and the
iron run out. Also an explosion can hardly happen without some of the
melted iron being scattered among the men at work.

[Illustration: _Fig. 45._]

[Illustration: _Fig. 46._]

Some of the most fatal explosions of these boilers have arisen from
careless construction. Such was the case in an explosion at Dudley in
1862, shown in Fig. 45, where the crown plate forming the top of the
centre tube was attached to the sides of the tube by so slight an angle
iron, as shown enlarged in Fig. 46, that the pressure of steam on the
flat crown plate fairly sheared the angle iron through, and allowed the
plate to be blown down the centre tube into the chimney flue, whereupon
the boiler was violently thrown off its seating by the reaction of the
issuing steam and water thus liberated.

The double-tube horizontal boiler is also used in connection with
iron-making furnaces in many places, one furnace working into each
tube. Although by this arrangement the boiler can be placed a little
further from the workmen, some very fatal explosions have happened to
such boilers, as at Masborough in 1862.

[Illustration: _Fig. 47._]

Single-furnace boilers have been much used in the form of a single-tube
boiler standing on end, as shown in Fig. 47, with the flame passing up
the tube, which is continued in the form of a chimney on the top of the
boiler. The tube passes through the steam at the top, so that the plate
is not protected from overheating by contact with water; and this has
caused explosion in some instances, although the tube has been lined on
the inside with firebrick to shield the plate from the flame. Another
great disadvantage of this Chimney boiler is that the space between the
tube and the shell is so narrow that it is almost impossible to examine
or clean it internally.

A further arrangement for a single-furnace boiler is the Elbow boiler,
shown in Fig. 48, where the two difficulties mentioned in the previous
boiler are avoided.

Many internally fired upright boilers of various shapes have been
constructed to suit various purposes. One of a large size that has been
at work many years is shown in Fig. 49, with an internal fireplace and
a suspended cone and cross tube for increasing the heating surface.
This boiler is set in brickwork in such a way that the heat passes
through the side tubes and round the exterior shell before going off to
the chimney.

[Illustration: _Fig. 48._]

[Illustration: _Fig. 49._]

A very fatal explosion at Stoke-upon-Trent, in 1863, resulted from an
attempt to work a boiler of somewhat the same general form, but without
the same careful attention to the details of construction. This boiler
is shown in Fig. 50; the internal fireplace is of conical shape, 4
ft. 6 ins. diameter on the top and 6 ft. 10 ins. at bottom, and was
joined to the external shell by a flat annular bottom. Almost the first
time it was worked at high pressure the conical fireplace collapsed,
breaking off at the seam at the top of the cone, and blowing down upon
the grate, as shown in Fig. 51. The flat bottom was then left without
the support of the cone and side tubes, and gave way all round the
outside angle iron; and the top flew up a great height into the air,
and fell a crumpled heap, as shown in the sketch. In this case the only
wonder is that a boiler of such weak construction worked at all without
explosion.

[Illustration: _Fig. 50._]

[Illustration: _Fig. 51._]

There yet remains to be noticed a very large and varied class of
boilers that have been designed with the express object of avoiding
explosion. Some of these, made of cast-iron pipes of small diameter,
have already been referred to. When steam carriages were first
constructed, boilers were tried made of a cluster of small pipes,
set both upright and horizontally, connected with a general receiver
and with each other by still smaller pipes. These were found to
have such small circulation of water that they very soon burnt out,
and also led to much priming. Afterwards, narrow chambers made of
corrugated plates set like the cells of a battery were tried, but
without much success. The multitubular boilers of the locomotive type
soon superseded all others as quick steam generators, and until lately
they have been considered as almost absolutely safe from explosion.
It is found however that the barrel of these boilers is peculiarly
liable to furrowing, owing to the strain weakening the iron in certain
lines. Perhaps no boiler shows more clearly than the locomotive how
necessary it is that every part should be open to examination; and also
how unwise it would be to use for stationary purposes small cramped
up boilers, only intended to meet the necessities of locomotion. Many
explosions of locomotive boilers have taken place; but it is not
necessary to give details in this paper, as they are fully given in the
published official reports of the government inspectors.

Among the form of boilers designed to obtain very rapid generation of
steam, combined with increased safety from explosion, may be specially
named that consisting of a system of small pipes within a shell with
an artificial circulation of water, and also the boiler consisting
of a cluster of cast-iron spheres, both of which have been described
at previous meetings of the Institution (see Proceedings Inst. M. E.
1861 page 30, and 1864 page 61); but neither has been much used in
this country at present. The boilers also which consist chiefly of
small tubes hanging down into the fire, with smaller tubes or other
arrangements within them for securing a natural circulation, deserve
mention, as they appear successfully to accomplish that end.

The principle of all these small boilers appears to be that only a
small quantity of water should be contained in them, so that there
should not be a reservoir of danger in the shape of a mass of highly
heated water ready to be converted into steam if a rupture takes place:
and it cannot be denied that this is an advantage. But on the other
hand these boilers of small capacity, which evaporate their whole
contents in a few minutes, are subject to new dangers from that very
cause; and although admirably adapted for purposes where steam is
wanted quickly on a sudden emergency, as in the case of fire engines,
or where the generating power required varies each moment, as in
the locomotive, they are for the most part ill adapted for ordinary
stationary purposes, such as the mill or the colliery. They require
constant firing and vigilant attention to the feed, and cannot be left
for a time with safety like the ordinary stationary boilers. It has to
be borne in mind also that the very reservoir of danger so much dreaded
is also a reservoir of power, which assists in the steady maintenance
of the machinery in motion. The large mass of water heated to the
evaporating point, the heated brickwork of the flues, and the large
fireplace, are so many assistances to regularity, and enable the man
in charge to attend to his other duties without the risk of spoiling
the boiler or letting down the steam by a few minutes' absence from the
stoke hole. Steam employers are found at present to prefer the known
dangers of the large boilers to the supposed safety of small boilers,
which they fear are troublesome in practice.

Many of the early boilers were rendered weak by the injudicious
manner of arranging the seams. The longitudinal seams were made in a
continuous line from end to end, as shown in Fig. 24, page 20, with
the transverse seams also continued completely round the boiler, so
that at the corner of each plate there were four thicknesses of iron.
The crossing of the seams, as in Fig. 25, page 21, adds much to the
strength, and also often prevents a rent from continuing forward to a
dangerous extent.

It is scarcely requisite to mention the necessity of good material
and workmanship to secure strength in a boiler, however perfect the
design. If the plates are of weak and brittle iron, or imperfectly
manufactured, they will never make a good boiler. Apart from the strain
upon the boiler when at work, the iron has to undergo the strain of the
necessary manipulation, shaping, and punching, during the construction
of the boiler. If the plates forming the boiler are not well fitted to
their places before the rivet holes are made, the errors have to be
partially rectified by using the drift in the holes to an unwarrantable
extent, and then using imperfect rivets to fill up the holes that do
not correspond with each other; and the mischief is too frequently
increased afterwards by excessive caulking, in the endeavour to stop
the leaking which is sure to show itself. In this way a boiler is often
exposed to most unequal internal strain between its several parts
before it is set to work at all; and when the heat is applied to it,
the mere expansion causes undue contortion, and leads to seam rips,
and ultimately to disaster. Several specimens of faulty rivetting and
caulking were exhibited to the meeting, and a sketch of one of them is
shown in Fig. 52.

[Illustration: _Fig. 52._]

The strength of a boiler is often very much lessened by the injudicious
manner in which the mountings are fixed upon the boiler, and many
explosions are the consequence of this defect. Not only are a great
many holes for fittings cut out of the boiler in one line, but these
holes are made needlessly large. Steam domes are often so placed as
greatly to weaken the shell of the boiler, the hole cut out of the
plate being made the full diameter of the dome; and in some cases the
domes or steam chests have been made square or rectangular, so as to
weaken the shell still more, as shown in Fig. 53.

Manholes are often a source of danger, if not properly arranged and
duly strengthened. Even in very small boilers they are often placed
with the longest diameter in the longitudinal direction of the boiler,
so that the shell is greatly weakened, as in the sketch, Fig. 54, of
an exploded boiler at Walsall in 1865. This boiler was 5 ft. 3 ins.
long and 2 ft. 6 ins. diameter, and yet the manhole was 18 inches by 13
inches, and placed within a few inches of one end. The end was fastened
in by angle iron, which was not welded, and consequently there was so
little strength at the small portion of the shell between the end and
the manhole that it gave way and liberated the end and the manhole lid,
after which the main body of the boiler was thrown by the reaction
across several streets to a great distance.

[Illustration: _Fig. 53._]

[Illustration: _Fig. 54._]

A somewhat similar injudicious arrangement of the manhole is shown
in Fig. 55, where a manhole 17 inches by 14 inches was cut out of
the flat top of a steam dome only 2 ft. 6 ins. diameter, without
any strengthening ring to compensate for it. The repeated strain of
screwing up the manhole lid, combined with the pressure of the steam,
caused the lid to force its way out through the plate and blow away.
This explosion occurred at Birmingham in 1865.

[Illustration: _Fig. 55._]

The preceding examples have shown how explosions often result from
faults in the construction of boilers; and the following instances
illustrate the explosions caused by mischief arising during working.
A boiler perhaps more than any other structure is subject to wear
and tear; and let it be worked ever so carefully, it will seriously
deteriorate. The wonder is, considering the work they have to perform,
that so many boilers are found which have worked twenty, thirty, or
even fifty years without explosion. The terms wear and tear however
are too vague for this subject, and the mischief met with must be
considered under distinct heads.

There is no doubt that the thing most to be dreaded for boilers is
corrosion; because when the plate is once thinned, it cannot be
strengthened again, but must remain permanently weakened. Corrosion
the more deserves attention because it is easily detected by moderate
vigilance, and can generally be prevented by moderate care, or by the
boilers being so arranged that they can be readily examined in every
part. Corrosion has been the direct and unmistakeable cause of a very
large proportion of the explosions that have happened: it occurs both
inside and outside the boiler, according to circumstances, and attacks
the iron in various ways and in different places.

[Illustration: _Fig. 56._]

[Illustration: _Fig. 57._]

[Illustration: _Fig. 58._]

[Illustration: _Fig. 59._]

_Internal corrosion_ sometimes takes place from bad feed water, and
its effects are different in extent in the different parts of the same
boiler. It very seldom thins the plate over a large surface regularly,
but attacks the iron in spots, pitting it in a number of holes. These
are sometimes large, as if gradually increasing from a centre of
action; and sometimes small, but so close together as to leave very
little more space whole than that which is attacked. A very curious
example of the latter was exhibited to the meeting, and shown in Figs.
56 and 57, cut from the lower part of the shell of a large tubular
boiler externally fired. The corrosion was greatest along that part of
the shell most exposed to heat, and was so extensive that two boilers
exploded simultaneously. The boilers had been at work sixteen years,
but the corrosion commenced about eight years before the explosion,
when the feed water was rendered corrosive by being obtained from
some iron mines. This explosion occurred at Aberaman in 1864. The
corrosion had been seen going on for years, and was not considered
sufficient to cause danger; but the depth to which it extended through
the thickness of the metal is seen in the half size section, Fig.
57. Another sample equally curious was exhibited to the meeting, and
shown in Figs. 58 and 59, taken from the sweep plate over the fire
in a plain cylindrical boiler which had worked about ten years. The
feed water was occasionally bad, and attacked the iron over the area
DDD, where unprotected by scale. The protection afforded by scale
against occasional corrosive feed water is worthy of notice. In the two
specimens exhibited it is seen that the protection has been perfect
where the scale has not been chipped off; and the edge of the sound
part projects over the hollow, as seen in the half size sections,
Figs. 57 and 59, the corrosive water having eaten away a larger area
beneath than that through which it first entered the surface of the
iron.

Internal corrosion is frequently observed where boilers are fed
from canals or streams in the neighbourhood of chemical works from
which corrosive matter is discharged at intervals into the water.
The corrosion takes place in isolated spots, but causes deep holes;
which seems to be accounted for on the supposition that the scale
previously upon the plate cracks during the cooling of the boiler for
cleaning, and forms a blister, so that a piece of about 2 inches area
is raised slightly from the iron. When the boiler is again put to
work, this blister becomes filled with the corrosive water, which is
held there without circulation and causes corrosion. When the boiler
is again emptied these blisters may be seen, and if broken show the
blackened water and the injured surface. In future working each of
these blisters forms a constant unprotected point for attack. It is
frequently seen further that such corrosion is arrested if water be
used which deposits scurf; but fresh blisters and renewed corrosion
will result from a return to the use of the bad water.

The internal corrosion called furrowing has proved a frequent cause
of explosion, especially in locomotive boilers. It differs from other
corrosion by being in deep narrow continuous lines with abrupt edges.
It will sometimes go completely through a plate; and is found where a
sudden change of thickness occurs, either along the lines of the seams,
or opposite the edge of angle-iron attachments. This effect is supposed
to be due to the alternate springing of the plates under each variation
of the pressure or temperature, causing the line of least resistance to
receive a strain somewhat similar to that produced by bending a piece
of iron backwards and forwards for the purpose of breaking it. This
line of injury is exposed to constant attack from corrosion, because
the scurf is always thrown off from it.

_External corrosion_ is a far more frequent cause of explosion in
stationary boilers; and it arises from many causes. The most frequent
cause, although the most easily detected, is leakage from the joints
of the fittings on the top of the boiler, which are too frequently
attached by bolts instead of rivets. This evil is much increased when
the boilers are covered with brickwork, which holds the water against
the plates, and hides the mischief from observation. It is astonishing
to find how much damage is allowed in this way to go on without
attention, until the tops of boilers are corroded so thin that little
holes burst through. These are sometimes found stopped with wooden
pegs or covered by screwed patches of plate, either of which cause
leakage that hastens the mischief, as shown by the sample exhibited.
Boilers exposed to the weather will of course become corroded like
anything else made of iron and not painted; and yet so much mischief
is sometimes caused by leakage beneath improper covering that exposure
may almost be said to be the smaller evil of the two, as it is better
to see what is going on than to rest in false security. No covering
will be found cheaper, or better, in the long run, than a roof, which
prevents the loss of heat by exposure, and yet allows free access to
all the fittings and joints on the top of the boiler.

[Illustration: _Fig. 60._]

Some examples of the evils of covering can be given that have come
under the writer's observation. A set of boilers had been well covered
by arches of brickwork, so built as to keep out all water, and also
set so as to touch the boilers only at intervals, leaving a space
generally of a few inches. After about seven years' working, the whole
of the tops of the boilers were discovered to be dangerously thin, and
had to be renewed. The cause was leakage from the joints of fittings
and seams of the boilers, and the issuing steam had been drawn along
the space between the boilers and the arches, and had escaped at a
place where it had not attracted notice. In another case, a somewhat
similar set of boilers were covered with ashes, to prevent the loss
of heat by radiation; and the rain and the leakage beneath the ashes,
in conjunction with the corrosive matter from the ashes themselves,
thinned the tops of the boilers to a dangerous extent in less than two
years. A sketch of the corrosion caused in this instance by covering
with ashes is shown in Figs. 60 and 61.

[Illustration: _Fig. 61._]

[Illustration: _Fig. 62._]

[Illustration: _Fig. 63._]

Similar mischief has been noticed in boilers covered with sand, as
shown in the sketches Figs. 62 and 63, which represent an instance of
corrosion after eight years' working; although nothing forms a better
covering than sand for preventing loss of heat by radiation. In both
these examples it will be seen that the corrosion has continued until
the thickness of the plate has been so eaten away that a hole has been
burst out at SS. A very good covering is formed by brickwork in cement;
or various cements made for the purpose, which adhere to the surface of
the plate and yet show leakage; or such materials as sacking or felt;
or sheet-iron casing, leaving about 6 inches of air space all round
the boiler. But all these have the great objection that they hide the
boiler from inspection, except by the expensive process of removing
the covering; and in this way dangers that have caused explosion have
remained hidden from observation.

Explosions have also taken place from general corrosion of the surface
of the boilers in the flues. A new boiler which was set on sidewalls
built upon a foundation of porous rock was found to have become
corroded all along the bottom in less than two years, owing to the
dampness which rose from the foundations causing a constant presence
of vapour. The corrosion was peculiar, and more like that found on old
iron left for a long time in a damp place; for the iron plate fell
to pieces when touched, and large flakes could be raised from the
surface, and the greater part of the thickness of the plate could be
removed with the fingers. Somewhat similar corrosion had taken place
in a boiler which exploded at Loughborough in 1863; the bottom of the
shell became rent at the corroded part, and as the fracture continued
spirally round the boiler several times, nearly all the shell was
peeled off in the curious manner shown in Fig. 64. The explosion shown
in Fig. 65, which occurred at Leeds in 1866, also arose from corrosion
of the bottom of the boiler.

[Illustration: _Fig. 64._]

[Illustration: _Fig. 65._]

[Illustration: _Fig. 66._]

The greater part of the corrosion found in the side flues of boilers
is caused by the leakage of seams. Many boilers are emptied for
cleaning as soon as work is over on Saturday night, and long before the
brickwork of the fireplaces and flues has cooled; and consequently,
the boiler, having no water in it, is made much hotter than it ever
is in working, and the seams are injured and sprung and the rivets
loosened by the extra expansion so caused. This is sometimes done
intentionally, in order to loosen the scale by the greater expansion
of the iron than of the scurf. When the boiler is again set to work,
the seams and rivets leak and cause that corrosion which is called
channelling. This has been observed to occur to such an extent that
all the seams in a boiler have been seen thus corroded; and the same
has sometimes been found in all the boilers in a large manufactory.
Specimens of this channelling were exhibited to the meeting. One in
particular, shown in Figs. 66 and 67, deserves attention, as it
shows the effect of a jet of steam and water from the leaking rivet
R, in cutting a series of channels into the plate along the course of
the dotted lines EEE, and producing a hole in the plate at S. This
corrosion had been going on for about four years, but was in a part of
the boiler seldom seen in ordinary examination. Many explosions have
resulted from this form of corrosion; for when a rent is once made, the
fracture continues along the thinned channel of the plate.

[Illustration: _Fig. 67._]

The corrosion most to be dreaded, because most difficult to detect, is
that which takes place where the boiler is in contact with brickwork;
and it is found alike in all forms of boilers set in brickwork. When
found at the part where the side flues are gathered in at the top
against the boiler, it is usually occasioned by the leaking of fittings
or feed pipes, or by rain being allowed to run between the boiler and
the brickwork. More than one explosion has been caused by the droppings
from a roof being allowed to fall upon the tops of the flues. When the
corrosion is found at the point where the bottom flue walls touch the
boiler, it is frequently caused by the leaking of seams that have been
strained by the weight of the boiler; and this often arises from want
of care to replace the brickwork, after repair of the boiler or flues,
in such a position as to take again its proper proportion of the weight
of the boiler. Cases have been met with where the shape of the bottom
of large boilers has been quite altered by such means. The brackets
on the sides of heavy boilers have not only been strained so that the
rivets or bolts have leaked and caused corrosion, but they have also
bent or cracked the side plates of the boiler. The bracket shown at B
in Fig. 53, page 40, made of only an angle iron with a piece of plate
attached, is especially liable to cause injury if the brickwork is not
rebuilt close up to the angle iron, as the leverage is so great. This
is avoided by the better form of bracket shown at C, consisting of an
elbow of flat bar-iron rivetted at top and bottom to the boiler.

[Illustration: _Fig. 68._]

In the old balloon and wagon boilers, the angle where the bottom joined
the sides scarcely ever remained sound for long when in contact with
the brickwork, and many of those that exploded have been found almost
corroded through where they stood upon the brickwork. The explosion
before alluded to and shown in Fig. 7, was caused by corrosion of the
bottom of the boiler where it was set on the brickwork. Many boilers
are so set that the brickwork of the flues is made to follow the shape
of the boiler, with as little space between as possible; but the slight
advantage gained in increased heating effect is far outweighed by the
impossibility of getting into the flues for examination. It is only
by having the flues sufficiently roomy that proper examination can be
made, and that the indications on the brickwork of leaking can be seen
and remedied, and corrosion arrested. A remarkable case of corrosion
occurred in a boiler with an oval shell, set upon a middle wall. The
flues were too narrow for a man to enter, and a leak in the bottom was
only discovered by the boiler nearly running empty while the engine
pumps were standing for a short time. It was subsequently found that
the whole bottom where it rested on the wall was extensively corroded
in a continuous line, and that explosion was only prevented by the
numerous stays across the bottom to compensate for the oval shape. Fig.
68, shows the position and extent of the corrosion, and the plate was
completely in holes at the parts indicated by the black marks. This
corrosion was supposed to have been going on for about three years.

[Illustration: _Fig. 69._]

[Illustration: _Fig. 70._]

It is sometimes asserted that corrosion cannot be the cause of an
explosion, because the corroded place would simply give way and let off
the steam harmlessly, or at least the boiler would not be displaced
from its seating. When the corrosion is only local, and surrounded by
sound plates of sufficient strength to arrest the extension of the
fracture, this may be the case, as in an explosion at Sheffield in
1865, shown in Fig. 69, where a piece of plate was blown out on one
side of the boiler, allowing the steam and water to escape without
displacing the boiler; the thickness of the plate at that part had
been reduced to 1/8th inch by corrosion in about 1-1/2 years, which
had been caused by leakage at the seams from inefficient repair with
bolts instead of rivets, and also from the moisture having been allowed
to be kept against the plate by the brickwork. But even under such
circumstances, if the piece blown out should be from the bottom, the
whole boiler may be thrown a great distance by the reaction of the
issuing steam, as in an explosion at Leeds in 1865, shown in Fig. 70.
If the corrosion extends for any length, the first rent is almost
sure to continue until a complete explosion is the result. Several of
the small models exhibited to the meeting showed the line of fracture
in various cases of explosion. One showed the appearance of a plain
cylindrical boiler after explosion caused by corrosion along the whole
length where it rested on brickwork; this explosion occurred at Wigan
in 1865, and a sketch of it is given in Fig. 71.

[Illustration: _Fig. 71._]

Many explosions of boilers have been caused by accumulation of scurf.
The mischief is not so much from scurf being gradually deposited all
over the interior of the boiler to a dangerous thickness as from the
chips off the sides falling in heaps on the bottom. The plate beneath
this accumulation becomes overheated, because not in contact with the
water, and softens and sinks down into a "pocket," which if unnoticed
will soon burn quite through. If the scurf that has caused the mischief
is thick and hard enough to resist the pressure for a little time,
the hole enlarges, until the scurf suddenly gives way and allows the
contents to issue so violently as to disturb the boiler, or at least
to blow the fire out of the grate. Such was an explosion at Bilston
in 1863, where a large plain cylindrical boiler, 9 ft. diameter, was
heated by three large fires placed side by side along the bottom;
and a large "pocket" burst out over the third grate, and scalded the
attendant to death. A similar pocket in a boiler, 4 ft. 6 ins. diameter,
which exploded at Dudley in 1864, after having been at work six weeks
without cleaning, is shown in the transverse section, Fig. 72. In this
case the scurf had filled up the circle of the boiler to a depth of 3
inches at the bottom, as shown in the drawing, and was of a very hard
description; and the boiler plate was bent out in a gradual curve, and
thinned to about 1/16 inch, the original thickness being 1/2 inch.

[Illustration: _Fig. 72._]

[Illustration: _Fig. 73._]

The whole bottom of a boiler is sometimes injured, and the plates
buckled and the seams sprung, from the accumulation of mud. One case
may be mentioned where the water was very full of mud, and the boilers
were worked day and night during the week but stopped for several hours
on Sunday, during which time the deposit of mud was so thick that it
did not get thoroughly disengaged again from the bottom when the boiler
was set to work, but hardened into a mass. Although many of these
pockets and injuries to the plates may occur without serious damage,
they sometimes cause that first rent which destroys the equilibrium of
the structure and leads to explosion. Some of the specimens of scurf
exhibited to the Meeting show that their thickness is made up of small
chips, carelessly left after cleaning or fallen from the sides of the
boiler, as seen in Fig. 73, or from cotton waste or other matter left
in the boiler and forming a nucleus for the scurf to accumulate upon.
Other specimens show that foreign matter must have been put into the
boiler to stop leaking.

Accumulations of scurf in the feed pipes at the point of entrance into
the boiler have also caused explosion by stopping the supply of water.
The same result is caused by the freezing of the water in the pipes
which are exposed, and each winter one or two boilers are injured or
exploded from this cause, especially small household boilers placed
behind kitchen grates. Scurf cannot be considered so great an evil as
corrosion, since it can be removed, and if this is done in time, the
boiler is restored to its original condition.

The advantage of a pure water, which does not deposit scurf, is so
great for the supply of boilers that it is always worth while to
go to considerable expense for obtaining it; or to take some steps
for purifying the feed water as much as possible. If it is only mud
mechanically suspended, which would deposit by gravity on the bottom
of the boiler, frequent use should be made of the blow-off apparatus.
If the impurity is light enough to be carried to the surface in the
form of scum, the blow-off apparatus should discharge from the surface
of the water as well as from the bottom. If the impurity is chemically
suspended in the water, some one of the many substances which form
the refuse from various manufactures, and which may contain suitable
ingredients, should be used to counteract the effect of the impurity.
Common soda will answer the purpose perhaps better than anything else.
It must not be forgotten however that the blow-off apparatus must
afterwards be used more frequently, to rid the boiler of the foreign
matter, or the mischief will be increased. In marine boilers, constant
attention is necessary to get rid of the saline deposit; and in
stationary boilers using impure water an equally systematic attention
is needed to get rid of the earthy deposit.

Perhaps no cause of explosion is oftener mentioned than shortness
of water, and this is not unfrequently coupled with turning on the
feed suddenly into an overheated boiler. Many explosions have been
attributed to this cause, when closer investigation would have
revealed some far more probable reason. For instance, shortness of
water was stated as the cause of the explosion, at Abercarn in 1865,
of a single-tube boiler with a very large flue tube, which collapsed
upwards from the bottom. The top of the tube and the sides of the shell
had not the slightest mark of overheating, although exposed to the
flame of three furnaces, one of which worked through the tube, and the
others on each side of the shell. In this case the cause of explosion
was clearly the weakness of the tube, and not shortness of water. It
is erroneous to suppose that if a boiler runs dry, or if the feed is
turned into a red-hot boiler, there must necessarily be an explosion.
If a boiler unconnected with any other runs rapidly empty, from the
breaking of the blow-off pipe or any such cause, it will simply get
red-hot and sink out of shape upon the fire, as may often be seen, but
no explosion would happen. If the water only falls gradually, as it
would if the feed were turned off and evaporation continued, the parts
exposed to the fire would get overheated as the water left them. If the
subsidence of the water were very slow, those parts might get red-hot,
and so much softened and weakened as to be incapable of bearing the
pressure, when an explosion would take place, as at Smethwick, in the
present year, where the flues were set above the water line, as shown
in Fig. 74.

[Illustration: _Fig. 74._]

If however the water were turned on again before the overheating had
gone so far, and the feed pipe were, as usual, carried down to nearly
the bottom of the boiler, the water would gradually creep up the heated
sides and cool the plates, the heat of which would not be sufficient
to cause greater evaporation than the ordinary safety valves would
carry off. The danger would not arise so much from the excess of steam
generated by the heat accumulated in the heated plates of the boiler,
as from the injury and strain that would be caused to the plates by
the undue expansion and sudden contraction, especially as this action
would take place on only a portion of the boiler. A singular case,
bearing on this point, may be mentioned. A four-furnace upright boiler,
like that shown in Fig. 44, happened to run so nearly empty, through
the accidental sticking of the self-acting feed apparatus, that the
level of the water sank to the top of the hemispherical end forming
the bottom of the boiler. The feed apparatus then became released of
itself, and, the feed being turned full on, the water gradually rose
until the whole occurrence was only discovered by the leaking at the
seams that had been sprung, which caused so much steam in the flues
as to stop the working of the furnaces. The overheating had been
sufficient to buckle the plates, and in one place a rupture had almost
commenced; but there was no explosion. By way of direct experiment upon
this point, boilers have been purposely made red-hot and then filled
with cold water, without causing explosion.

It has been supposed that boilers sometimes explode from overheating
without the water level being below the usual point, or without
the accumulation of scurf previously alluded to, but simply by the
rapidity of the evaporation from an intensely heated surface causing
such a continuous current of steam as to prevent the proper contact
of the water with the heated plate. Such has been the cause assigned
for the explosion of a three-furnace upright boiler at Birmingham in
1865, shown in Fig. 75. A piece of plate about 3 ft. by 1-1/2 ft. was
blown out of the side, at a place where an enormous flame impinged
continually. The plates had first bulged out, and then given way in the
centre of the bulge, each edge being doubled back and broken off. There
was no positive evidence as to the water supply; but the crown of the
centre tube, which was much above the bottom of the part blown out,
remained uninjured.

A somewhat similar case was that of a large horizontal boiler at
Kidderminster, the tube of which collapsed in 1865, as shown in Fig.
76. It was heated by four furnaces, one of which worked into the tube,
one under the bottom, and one on each side; and all the furnaces worked
into the same end of the boiler. The tube was found to have partially
collapsed at that end, and the top had dropped 11 inches. This was
repaired in the first instance, but was afterwards again found injured
by overheating, although not so seriously. It is very probable that the
extremely rapid ebullition from the sides and bottom, from which the
steam had to pass up the narrow space between the tube and the shell,
produced such a foaming that very little solid water could reach the
top of the tube where it was exposed to extreme heat.

[Illustration: _Fig. 75._]

[Illustration: _Fig. 76._]

Many explosions have been attributed to deterioration of the iron
through long use, as in an explosion at Durham in 1864, and another at
Haswell, near Sunderland, 1865, where the boilers had worked constantly
for 25 and 30 years respectively. When an explosion arises from the
failure of a plate which has not been properly welded in rolling,
there is no question that it was unsound when put in, and escaped
notice; but when the plate that fails is found to be brittle and of
bad iron, the fault is rather attributed to the effect of working than
to original bad quality. Of course this is not always the case, as
the injury done to plates by overheating has been already explained.
Pieces of plate have in some cases been erroneously pronounced to be
deteriorated by work, which have been taken from situations in the
boilers where they were not exposed to any action of fire that could
cause overheating; and therefore in reality the injury could only have
taken place when the boiler was being made, by burning the iron in
bending it to the required shape. A frequent cause of fatal injury to
boilers is injudicious repair, whereby the crossing of the seams is
destroyed, as in the explosion at Wolverhampton in 1865, previously
referred to and shown in Fig. 36. Moreover the edges of the old plates,
already tried by the first rivetting and the subsequent cutting out
of the rivets, are frequently strained again by the use of the drift
to draw them up to the strong new plates; and many a seam rip is thus
started which ultimately causes explosion.

Many explosions have been caused by the want of proper apparatus for
enabling the attendant to tell the height of the water and the pressure
of the steam, and also by the want of sufficient apparatus for supply
of feed water and escape of steam, or by the failure of one or other
of these; but such explosions can only be referred to generally in
the present paper. The mountings on a boiler are usually so open to
observation, and the importance of having them good and efficient is
so universally acknowledged, that much remark is not needed. Mention
has already been made of the sticking of self-acting feed apparatus as
a cause of mischief, and similar failures of floats and gauges have
constantly happened; but this should by no means be considered to
condemn self-acting apparatus, either for assisting in the steadiness
of working, or for giving warning of danger. The apparatus however
should be relied on for assistance only; and an attendant cannot be
called careful who leaves a boiler dependent on such apparatus without
watching. The self-acting principle has been seen by the writer applied
in a novel and useful way in a recording pressure gauge, which proved
the more interesting as it had shown the actual pressure of steam
at the time of the explosion of one of the boilers with which it was
connected.

Among the numerous boiler explosions that have been attributed to
over-pressure through deficient arrangements for escape of steam, in
many cases the safety valves have been placed on the steam pipes in
such a manner that the communication with them was cut off whenever
the steam stop-valve was shut, which is just the time when the safety
valves are most wanted. Safety valves are too often found needlessly
overweighted; and it is believed that many boilers are constantly
worked with safety valves so imprudently arranged and weighted, that
they could not carry off all the steam the boilers would generate
without a very great increase of pressure.

It is concluded that enough has now been said to show that boiler
explosions do not arise from mysterious causes, but generally from some
defect which could have been remedied if it had been known to exist. It
only remains therefore to consider what is the most ready and efficient
way to discover the true condition of a boiler. It has been maintained
that this end is best accomplished by what is called the hydraulic
test, in which a pressure of water is maintained in the boiler for a
given time at a certain excess above the working pressure. This test
is undoubtedly useful so far as it goes, and is perhaps the only one
that can be applied to boilers with small internal spaces, such as
locomotive boilers, not admitting of personal inspection over the whole
of the interior; and it is also admirable for testing the workmanship
of a new boiler. But on the other hand the conditions of a boiler at
work are so different from those which exist during the hydraulic test,
that this alone cannot be depended on; for old boilers have been known
to stand this test to double their working pressure without apparent
injury, although known to be dangerously corroded. The difficulty also
of seeing or measuring the effect of the hydraulic test upon large
boilers set in elaborate brickwork is so great that little practical
benefit has resulted in many cases.

It is believed by the writer that the surest way to ascertain the true
condition of a boiler is to examine it at frequent intervals in every
part, both inside and outside; and as this can only be done when both
the boilers and the flues can be readily entered, it is specially
important that facility for examination should be made a consideration
in selecting a construction of boiler. Permanent safety should be
considered as an element of economy, in addition to its still higher
importance in reference to the preservation of life.

ON THE CONCLUSIONS DERIVED FROM THE EXPERIENCE OF RECENT STEAM BOILER
EXPLOSIONS, BY EDWARD B. MARTEN, MEM. INST. M.E., EXCERPT MINUTES OF
PROCEEDINGS OF THE MEETING OF THE INSTITUTION OF MECHANICAL ENGINEERS,
AT NOTTINGHAM, 3RD AUGUST, 1870, THOMAS HAWKSLEY, ESQ., VICE-PRESIDENT,
IN THE CHAIR. BY PERMISSION OF THE COUNCIL.

The records of Steam Boiler Explosions in recent years are very
numerous, as the increased attention drawn to the subject in this
and other countries has placed far more information at disposal; and
the experience of the last four years, since a former paper was read
by the writer on the subject of boiler explosions, has confirmed the
opinion then expressed, that all boilers, however good in original
construction, are liable in the course of time to get into bad order
and explode. The particulars of the explosions during this period
are given in the Tables appended to the present paper, which show
the number of explosions due to each cause in each class of boiler,
distinguishing those of the United Kingdom from those in foreign
countries. An analysis is also given of the explosions in the last four
years, showing the causes of explosion of each form of boiler; and also
a summary of the causes of explosion under the three general heads
of--(1) faults in construction or repair: (2) faults in working which
creep on insidiously and unnoticed: (3) faults which might be seen and
guarded against by careful attendants. Nearly all of the faults in
these three classes would have been detected by periodical examination.

In the case of Cornish, Lancashire, and other boilers with internal
flues, the faults of construction which have caused explosions have
been weakness in the tubes, combustion chambers, ends, domes, or
manholes; and explosions in these, as in other classes of boilers,
have also resulted from external or internal corrosion, shortness of
water, undue pressure of steam, and scale or mud on the boiler plates.

In plain cylindrical boilers, and others without internal flues,
explosions have resulted from the boiler ends being made flat, and also
from frequent repairs producing seam rips, especially in boilers having
the plates arranged lengthways instead of in rings.

In marine boilers, weak flues and weak ends have also led to explosion,
in addition to the other causes mentioned above.

Locomotive boilers have in two cases exploded in consequence of the
strains thrown upon them by their being used as a frame for the engine.

Other explosions have resulted from want of stays, and from too much
heat impinging on some particular part; and in domestic boilers from
freezing of pipes under pressure.

Altogether the total number of explosions in this country that have
been recorded during the past four years has amounted to 219, which may
be classed under the following heads:--

  Faults of construction or repair                            95
  Faults to be detected only by periodical examination        62
  Faults which should be prevented by careful attendants      54
  Extraneous or uncertain causes                               8
                                                             ---
            Total                                            219
                                                             ---

By these 219 explosions 315 persons were killed and 450 injured.

The following are the particulars of the construction of the 219
exploded boilers:--

  Cornish, Lancashire, or other boilers with internal flues       84
  Plain Cylindrical boilers or others without internal flues      54
  Marine boilers                                                  12
  Agricultural boilers                                            11
  Locomotive boilers                                              10
  Furnace-upright boilers                                          8
  Crane boilers                                                    6
  Rag steamers, &c.                                                6
  Balloon and Elephant boilers                                     5
  Domestic boilers, &c.                                           16
  Not sufficiently described                                       7
                                                                 ---
            Total                                                219
                                                                 ---

The causes of these 219 explosions may also be classed as follows:--

  Worn out, corroded, or burnt plates                                 89
  Undue pressure, overloaded valves, intentional or from carelessness 25
  Bad construction, defective fittings or stays, or want of repair    69
  Shortness of water, formation of scale or mud, or external flues
    set too high                                                      28
  Extraneous or uncertain causes                                       8
                                                                     ---
            Total                                                    219
                                                                     ---

Sketches are given of the most instructive examples of boiler
explosions during the last four years, which are sufficient to explain
themselves, with a brief reference to their special features.

[Illustration: _Fig. 1._]

Although the importance of periodical examination as the best safeguard
against explosion is generally admitted, a great number of those who
make or use boilers have not at present sufficient belief in its
importance to adopt this course. Boilers are still constructed or set
in such a manner as to render examination next to impossible; and are
continued to be worked without making it the duty of those who mind
them, or of any one else, to examine every part at frequent intervals;
and hence such explosions have occurred as shown in Fig. 1, No. 12,
1870, in which the original position of the boiler before explosion is
indicated by the dotted lines. It is thought by many steam users that
all has been done which is possible, if their boilers are the best that
can be procured, and are set in the most approved way; and it is taken
for granted that such boilers should last for many years, under the
idea that a good boiler can never explode unless the feed is neglected.
Similar boilers are often referred to as having worked safely for ten
or twenty years, but it is forgotten that they may be exposed to the
insidious action of furrowing on the inside or channelling on the
outside, such as caused the explosions of the originally good boilers
shown in Figs. 2, 3, 4, and 5, Nos. 35, 1870; 50, 1866; 46, 1869; and
25, 1870.

[Illustration: _Fig. 2._]

[Illustration: _Fig. 3._]

Much mischief arises from special classes of boilers, fittings, or
apparatus, being looked upon as promising permanent safety from
explosion; while the inevitable circumstance is overlooked that it is
only so long as everything is maintained in good condition that safety
is insured. An apparatus, for instance, for preventing explosion from
shortness of water or over-pressure, however perfect for any such
object, would be quite inefficient as a safeguard against explosion
from corrosion, furrowing, channelling, or weak construction. It is
curious to note how often it is the case that every other part of an
establishment is subject to severe and perpetual scrutiny, the engines
especially being overhauled with the most scrupulous regularity; while
the boilers, the very source of the power and the heart of the whole
business, are left to themselves for long periods, even for years,
without examination; and it is too often only after bitter experience
that owners have understood the need of this examination. In this, as
in many other matters, experience has shown that there is no royal road
to safety, and that immunity is only secured by unremitting care and
constant watchfulness. It should never be forgotten that even a good
boiler can explode; for however good at the outset, sooner or later
the time must eventually arrive, when such wear and tear will have
taken place as will result in dangerous weakness, unless the boiler is
carefully and systematically attended to. Although a boiler may even
last safely for ten to thirty or more years if worked slowly and with
care, no confidence can be placed in a boiler which has worked so long,
unless it is examined in every part.

[Illustration: _Fig. 4._]

[Illustration: _Fig. 5._]

[Illustration: _Fig. 6._]

[Illustration: _Fig. 7._]

The opinion is more general than many are aware of, that explosions
as a rule are caused by shortness of water and the sudden turning on
of the feed water upon red-hot plates; and the appearances of injury
in the plates from fire, arising in the ordinary course of working,
have been frequently mistaken for signs of overheating from shortness
of water at the time of explosion, as illustrated in Figs. 6 and 7,
No. 24, 1867, and No. 59, 1866. Although boilers do explode from the
softening of the plates by overheating in consequence of shortness
of water, yet it is very doubtful whether the turning on of the cold
water at such a time is ever the cause of explosion. The feed water
being always introduced at the bottom of the boiler, as in Figs. 8, 9,
and 10, cannot be scattered suddenly near the overheated parts, but
must rise gradually up the sides; and the boiler would have gone to
pieces from the giving way of the softened parts long before the water
reached them, as was the case in the explosions shown in Figs. 11 and
12, end of 1868. The experiment of injecting cold water into red-hot
boilers has been carefully tried more than once, without producing any
explosion.

[Illustration: _Fig. 8._]

[Illustration: _Fig. 9._]

[Illustration: _Fig. 10._]

Although it may be too much to suppose that boiler explosions will ever
be entirely prevented, it is important that those who have the care of
boilers should understand better what are the true causes of explosion,
in order that they may know what to guard against in addition to
shortness of water. This better understanding of the subject has been
much retarded by the supposition that the causes of boiler explosions
are beyond the comprehension of the boiler minders; and still more by
the important differences of opinion among those under whom they work.
Much evil has resulted from the promulgation of strongly expressed
views, which have been founded upon facts but of too limited extent,
and such as must become modified by consideration of the facts of
a large number and variety of explosions. Mysterious theories to
account for explosions have been resorted to only from want of clearer
explanations.

[Illustration: _Fig. 11._]

[Illustration: _Fig. 12._]

[Illustration: _Fig. 13._]

[Illustration: _Fig. 14._]

Before considering in detail the causes of explosion, it is necessary
to recall to mind that beyond question there is sufficient accumulated
force in any working boiler to cause all the violent effects of an
explosion, if this force be suddenly liberated. In Figs. 13 and 14,
No. 18, 1869, and No. 63, 1866, are shown the violent effects of the
rupture of vessels employed for steaming rags, which were filled with
steam only. In ordinary boilers however there is present, besides the
steam, a quantity of water heated much beyond the atmospheric boiling
point; and when rupture takes place and the pressure is suddenly
relieved, part of this water evaporates, and keeps up the supply of
steam to continue the rupture and destruction. The explosion of a
boiler differs from the discharge of electricity or lightning, which
cleaves the air and instantly leaves a vacuum; it also differs from
the discharge of detonating compounds which act suddenly and leave a
vacuum; but it more nearly resembles the discharge of gunpowder, which
burns sufficiently slowly to keep up a continuous pressure behind a
projectile until it leaves a gun; and each cubic foot of water in a
boiler working at 60 lbs. pressure has been shown to produce in steam
an explosive effect equal to one pound of gunpowder. None of the
elaborate but unlikely theories of decomposed steam, or of electric
accumulations, suppose a force so fitted to cause destruction as that
contained in the highly heated water existing in all working boilers.

The following appear to be the general results to be derived from the
experience of the explosions in this country during the last four years.

[Illustration: _Fig. 15._]

[Illustration: _Fig. 16._]

First as to faults of construction which fall under the department
of the boiler maker or repairer. One of the most apparent causes of
explosion in stationery boilers is the loss of strength occasioned by
frequent repair, not only from the injury done to the old plates by
removing rivets, but from the want of bond in the new work. This has
lead to many of the explosions of the Plain Cylindrical boilers, such
as are shown in Figs. 15, 16, and 17, No. 45, 1869, No. 32, 1870, and
No. 20, 1870. Where the plates are arranged longitudinally instead
of in rings, the danger is increased, as there is less chance of a
dangerous rip being arrested by a crossed joint. So great a number of
boilers with continuous longitudinal seams, especially in the North,
have worked for twenty or thirty years, that it can hardly be supposed
they are any weaker than the boilers made in rings; but they are more
liable to explode, for if a seam rip occurs, it more easily extends
along the seam, and leads to the general break up of the boiler, shown
in Fig. 18, No. 59, 1869.

[Illustration: _Fig. 17._]

[Illustration: _Fig. 18._]

Perhaps no boilers have worked for a greater number of years than
the Plain Cylindrical boilers, many specimens being in existence and
apparently in good order which were put to work fifty or sixty years
ago. When such boilers have been too much or injudiciously repaired,
they are treacherous and uncertain; but their rupture and explosion
occur not so much on account of fault of shape, as from the simple
reason that like willing horses they are easily overworked. The grates
are usually twice as large as the fair proportion to the heating
surface, producing the double evil of forcing more heat through the
iron plates over the fire than they can transmit without injury, and
allowing a great amount of heat to pass away to the chimney without
useful effect. Careful experiment shows that nearly as good duty can
be obtained with the plain cylindrical boiler as with any other form,
provided the rate of combustion is in fair proportion to the extent
of heating surface in the boiler. The circumstance that many plain
cylindrical boilers have exploded is not sufficient to condemn this
make of boiler, which is the cheapest, simplest, and most easily set.
If the number of explosions alone were to be taken as the guide, it
would lead to the condemnation of the Cornish and Lancashire boilers,
from the experience of the past four years. But in case of both plain
cylindrical and other forms of boilers, most of the dangers admit of
remedy, and can be guarded against by frequent examination.

[Illustration: _Fig. 19._]

[Illustration: _Fig. 20._]

Five very fatal explosions have occurred of boilers heated by Puddling
and Mill Furnaces, leading in some cases to the supposition that this
form of boiler is more liable to explosion than others. They were not
adopted however in the iron-making districts without great care and
consideration, and there does not seem ground for attributing special
danger to them. The causes of the five explosions referred to of these
boilers were manifest, and would have led to the explosion of any form
of boiler; the loss of life however was great, because the situation
of the boilers was among a large number of workmen. The steam power
required in ironworks so far exceeds that in any other trade, that
an ironwork is half composed of boilers; the workmen are necessarily
within the range of explosion of many boilers, and hence the great loss
of life when such an accident occurs. The explosions of such boilers
shown in Figs. 19 and 20, No. 24, 1868, and No. 31, 1868, were from
external and internal corrosion respectively of the bottoms, rendering
them too weak to bear the ordinary pressure.

[Illustration: _Fig. 21._]

[Illustration: _Fig. 22._]

[Illustration: _Fig. 24._]

Those shown in Figs. 21 and 22, No. 23, 1870, and No. 53, 1869, were
from the collapse of the central tubes, which were weakened by external
and internal corrosion respectively. In Fig. 24, No. 35, 1868, the
shell was in bad order from over work and receiving too much heat from
four large furnaces, one of these especially causing a constant mass of
flame to impinge upon a single plate, which resulted in a seam rip.

[Illustration: _Fig. 23._]

[Illustration: _Fig. 25._]

[Illustration: _Fig. 26._]

[Illustration: _Fig. 27._]

The greatest number of explosions and the greatest loss of life and
personal injury have been in the case of Cornish and Lancashire
boilers, or others with internal flues. In the county of Cornwall
itself there have been many explosions, as often from the rupture of
the shell, Fig. 23, No. 58, 1869, as from the collapse of the tube,
Fig. 25, No. 35, 1869. The temporary patching on some of these old
boilers was most extensive, Fig. 26, No. 52, 1869, and the only wonder
really was that they held together as long as they did. The belief that
shortness of water is the only cause which can lead to the collapse of
tubes is so strong, that the boiler minders have often been condemned
almost unheard in cases of explosion, as if there were no room for
doubt that their neglect was the cause. Explosions from weakness of
tubes are not however confined to Cornwall, as for example in Fig.
27, No. 42, 1868, where the flue was oval and very weak: although it
was supposed that shortness of water caused the accident, from the
idea that nothing else could account for it. The strain caused by the
varying temperature of the internal tubes in Cornish or Lancashire
boilers, and the difficulty of staying their flat ends so as to make
them sufficiently secure without being too rigid to allow for the
expansion of the tubes, render them liable to corrosion or "furrowing"
in particular lines of strain, the destructive action of which is
very rapid; while the large quantity of brickwork around the outside,
necessary to form the external flues, also renders them liable to
corrosion in the parts most difficult of access. In this favourite
form of boiler therefore careful and frequent examination in every part
is more needed than in boilers of simpler form and setting; and the
increasing number of explosions among these boilers seems to establish
that they are only trustworthy if frequently examined and kept in
perfect order.

[Illustration: _Fig. 28._]

[Illustration: _Fig. 29._]

[Illustration: _Fig. 30._]

Several instances have occurred of explosion of Portable Crane Boilers.
Their small size has led to their condition being disregarded, under
the idea that scarcely any pressure could burst them. In practice it
is found however that they are often exposed to greater pressure than
other boilers, because the fire is large and quick in proportion to
their size; and they often have to stand for a considerable time with
the steam up, and their exposed position and long intervals of rest add
to the chances of corrosion, as shown by the example in Fig. 28, No.
14, 1869. The large manholes without strengthening rings, that are so
often put in these boilers, have been the cause of explosions such as
that shown in Fig 29, No. 57, 1866.

[Illustration: _Fig. 31._]

The same remark applies to some of the portable or agricultural
boilers which have exploded, such as those shown in Figs. 30 and 31,
No. 43, 1868, and No. 12, 1869.

Much mischief is often caused by bad imitation of well planned boilers.
Thus in boilers of the Cornish form, the ends are made sometimes so
rigid as to give no allowance for the expansion of the tube, and the
result is such continued strain as to cause constant leaking and the
consequent risk of fracture. In furnace boilers the tops of the crowns
of the inside tubes are often made flat, as in Fig. 37, instead of
being domed; or the inside tube is of undue size, as in Fig. 21, No.
23, 1870, see page 73. Furnace boilers have been made with the omission
of the stays that are so peculiarly necessary in that form, whereby
both ends have been left free to bulge outwards with the pressure, as
in Fig. 32.

[Illustration: _Fig. 37._]

[Illustration: _Fig. 32._]

[Illustration: _Fig. 33._]

[Illustration: _Fig. 34._]

[Illustration: _Fig. 35._]

Cornish boilers are often altered to the plain cylindrical form,
without compensation being made for the loss of strength caused by
the removal of the tube; this has led to such explosions as shown in
Fig. 33, No. 47, 1869, where two tubes where taken out, and Fig. 34,
No. 42, 1867, where one tube was taken out. One of the most frequent
and serious causes of loss of strength is the repairing of externally
fired boilers. Not only are the patches sometimes only bolted on in a
temporary manner, as in Fig. 35, No. 29, 1869, but even where they are
rivetted on there is an entire want of bond or crossed joint, as in the
case of the exploded boilers shown in Figs. 15, 16, and 17, see page
70 and 71, No. 10, 1869, and Fig. 36.

[Illustration: _Fig. 36._]

[Illustration: _Fig. 38._]

An attempt is made in Fig. 38 to show the effect of wear and tear
of boiler plate in an ordinary upright furnace boiler, such as is
shown in Fig. 37. The external surface is exposed to intense heat
and consequently expands, while the internal surface is kept cool by
contact with the water and expands to a much less degree. The continued
repetition of this process produces the same effect of cracking the
surface as that seen in the anvil blocks of steam hammers; and the
strength of the plate is reduced in proportion to the destruction of
the continuity of its surface. The deleterious effect of this process
is much increased if the boiler is subject to alternate heating
and streams of cold air on opening the fire-doors. To avoid it the
flame should have room to spread over as large a surface as possible,
without impinging on one particular point, and the firing should be
as regular as possible; and hence the greater freedom from injury in
boilers mechanically fired or heated by gas. The above action is quite
distinct from the overheating of the plates that occurs when no water
is in contact with them, which simply softens them and reduces their
strength, as in Fig. 39. It is believed that many boilers suffer from
overheating without being short of water: and an attempt is made to
show this action in such an upright boiler as is represented in Fig.
37, page 78, by the enlarged section of the side shown in Fig. 40.
The flame is shown impinging on a limited surface, as before, and
the steam rises so rapidly from the inner surface as to maintain a
continuous stratum of steam between the iron and the water, and the
plate consequently becomes overheated at that part. When the intense
flame subsides by an alteration of the working of the furnace, the
stream of steam diminishes, and the water returns and suddenly cools
and contracts the plate, but often not before it has commenced to get
out of shape. This has perhaps led to the explosion shown in Fig. 59,
No. 37, 1868, page 82. The same thing may happen to the crowns of tubes
of internally fired boilers when over fired, as in Fig. 41. Success
has attended the use of internal linings to boilers, arranged so as
to ensure a rapid circulation over the most heated parts, and also to
catch all the mud and loose scale.

[Illustration: _Fig. 39._]

[Illustration: _Fig. 40._]

[Illustration: _Fig. 59._]

[Illustration: _Fig. 41._]

[Illustration: _Fig. 42._]

[Illustration: _Fig. 43._]

[Illustration: _Fig. 44._]

In order to enable boiler minders to make proper periodical
examinations, it is necessary that care should be taken to arrange
both the boilers and the flues with that view; and this can be done
without materially injuring the efficiency of the boiler. Ordinary
plain cylindrical boilers can be entered easily, as in Fig. 42; and
although the small spaces between the tubes and the shells of Cornish
and Lancashire Boilers, as shown in Fig. 43, render the complete
examination troublesome, there is no difficulty in seeing those parts
most likely to need examination, such as the crowns of the tubes and
the end plates and angle iron. It is in the external flues that greater
accommodation is needed, as in many cases these are so narrow that the
boiler is quite inaccessible without pulling down the brickwork, as in
Figs. 44 and 45. The loss of heating effect caused by the use of wider
flues is so little, that it is far outweighed by the greater security
obtained from the more efficient examination that is thereby rendered
practicable. The flues of the plain cylindrical boiler are easily
made wide enough for a man to pass through them. The flues of Cornish
and Lancashire boilers should be made as shown in Figs. 46 and 47, so
that a man can enter them without such inconvenience as in Fig. 48.
One point of danger being the use of wide mid-feather walls, on which
corrosion is apt to take place, these should be narrowed and the weight
of the boiler supported on side brackets; the top of the mid-feather
and side walls can then be constructed with sight holes as at A A in
Figs. 49 and 50, so as to give the means of examining the plates near
each seam by simply removing loose bricks.

[Illustration: _Fig. 45._]

[Illustration: _Fig. 46._]

[Illustration: _Fig. 47._]

[Illustration: _Fig. 48._]

[Illustration: _Fig. 49._]

[Illustration: _Fig. 50._]

[Illustration: _Fig. 51._]

[Illustration: _Fig. 52._]

[Illustration: _Fig. 53._]

[Illustration: _Fig. 54._]

[Illustration: _Fig. 55._]

The explosions of fourteen Domestic or Heating-Apparatus Boilers are
included in the list of explosions, Table III; and some notice is
required to be taken of these, because they have led to the loss of the
lives of those who could not be expected to know their construction or
how to guard against accident; and as these boilers are seldom seen or
examined after they are once set, they should be the more carefully
constructed. In one or two cases these boilers were of a rectangular
shape, as in Fig. 51, No. 41, 1868, ill adapted to bear internal
pressure, and yet placed in connection with cisterns in the roofs of
lofty houses, so as to expose them to a hydrostatic pressure almost
up to their bursting strength without any addition of steam pressure.
The most usual cause of explosion is the lighting of the fire during
frosty weather in a house that has been left vacant, so that steam
pressure accumulates in the boiler whilst the exit is frozen up, as was
the case in Fig. 52, No. 6, 1870. The cast-iron boilers commonly used,
Fig. 53, end of 1869, are capable of bearing but little pressure; and
the wrought iron boilers, as in Fig. 54, No. 7, 1870, are found often
so badly welded as to be but little stronger; but even if they were as
strong as they could be made, the stoppage of the pipes by ice would
lead to explosion. Steam pressure may be guarded against by a safety
valve; but as this may become set fast in a little time, it would
be far better to avoid all chance of steam accumulation by such an
arrangement as that shown in Fig. 55, where the circulating boiler is
placed within an open-topped boiler behind the kitchen fire, and only
receives its heat through the hot water surrounding it, and therefore
cannot itself become sufficiently hot to generate steam.

[Illustration: _Fig. 56._]

A few remarks may be useful as to those faults arising in working
which fall under the department of the boiler minders. Not a few of
the explosions during the last four years have occurred from acts of
simple carelessness, such as where a blow-off pipe was left open,
so that the boiler was nearly emptied of water while at work; or in
another case where two boilers were fed at the same time through a
common pipe without a back valve, and the water from one "kicked" over
into the other. Undue pressure has been allowed to accumulate by safety
valves being tied down, as in the agricultural boiler, Fig. 56, No. 16,
1867; or by an extra weight being put upon the safety valve, as in an
instance where three bricks were fastened to the lever and the fires
were lighted earlier than usual, under the idea that an accumulation
of steam could be raised during the night to make a good start in the
morning. Another explosion was caused by working a boiler at more than
three times its proper pressure to meet a temporary emergency. In not a
few cases of explosion there was no pressure gauge on the boiler, or
the gauge was out of repair in consequence of being placed on the steam
pipe, so that it vibrated with every stroke of the engine; as in the
examples shown in Figs. 24 and 16, No. 35, 1868, No. 32, 1870, pages 70
and 74.

[Illustration: _Fig. 57._]

Corrosion has been the direct cause of many of the explosions. In one
or two cases the corrosion was known to exist, but the renewal of the
boiler was too long delayed, as in Fig. 57, No. 8, 1869, in others it
took both owners and minders by surprise, as in Fig. 1, No. 12, 1870,
page 63. It is said that to produce rapid rusting of iron there must be
present oxygen, water, and carbonic acid; and as all these are present
in a boiler flue when there are leaks, it is not surprising that so
many cases occur of explosions from corrosion.

Much mischief is often done by the injudicious use of compositions in
the boiler that are designed to prevent incrustation, especially where
there is no blow-off cock or where its use is neglected. A hard deposit
on the boiler plates is, in the writer's opinion, not so injurious as
the soft and muddy deposit produced by the use of such compositions.
A hard scale is equivalent to thickening the plate; and although this
is sufficiently mischievous, the injury to the plates is much more
rapid when a thicker but spongy deposit entirely prevents contact of
the water and impedes the transmission of the heat. An attempt to
illustrate this is given in Fig. 58, which is an enlarged view of a
portion of such a boiler as is shown in Fig. 37. The money spent in
boiler compositions would be better applied in securing a supply of
proper water, or in filtering and purifying the water before it enters
the boiler.

[Illustration: _Fig. 58._]

The writer has had to mention only faults in boilers; but it is not
to be inferred that all boilers are working in actual danger. A very
small percentage perhaps are so; but without periodical examination
no one can feel sure of the condition of any boiler. It is not likely
that explosions in future will be from exactly the same causes as those
now described, because the known faults will be avoided. For instance
no new Balloon, Wagon, or Butterley boilers are now made; and the
peculiar faults and the weakness of the tubes in Cornish and others of
the better classes of boilers are now so well known as to be generally
avoided; and as information spreads, many evils will become things of
the past.

As periodical examination has been so strongly advocated, it might seem
natural to desire that it should be enforced by government authority;
but this is by no means recommended. A select parliamentary committee
has been recently investigating the subject, with a view to ascertain
whether that would be desirable, but has adjourned for the session
without coming to any decision on this point. Even if a perfect
system of government inspection could be contrived and perfectly
administered, it would have the effect of taking the responsibility
from the owners, who are the natural guardians of the safety of their
boilers. Although the loss of 70 lives per annum by boiler explosions
is sufficiently deplorable, the deaths by railway accidents are more
than three times that number; yet very little inspection of railways
is held to be necessary, and that inspection takes place chiefly before
the commencement of working or after accidents. A coercive system
may introduce more evils than it cures, especially as at present so
much difference of opinion exists respecting the causes of boiler
explosions. In the opinion of the writer, far more real good arises
from the calm discussion of the facts and from the spread of correct
information by such societies as this Institution, than from enforcing
by law any action which is not perhaps believed by the majority of
steam users to be at all necessary or useful. It has been at times
suggested to increase the power and responsibility of coroners in
holding inquests upon those killed by boiler explosions, by requiring
them to obtain scientific evidence and to insist that the causes of
the explosions shall be added to the verdicts of juries. But it is
believed that this would only encumber an important institution,
because a jury who might well decide whether a person had been killed
by any criminal carelessness would not be a suitable tribunal to
decide between possibly conflicting scientific evidence; and also,
as an inquest may result in a verdict of manslaughter, the eliciting
of information on such an occasion is checked by the natural fear of
inadvertently involving some one in so serious a charge. The public
at large, and steam users generally, would gain more information and
guidance from the scientific evidence itself than from the verdict
of a coroner's jury; and it is believed much good has resulted in
preventing locomotive boiler explosions by publishing the reports of
the government inspecting engineers, who have gained their knowledge
of the facts in conversation with all those concerned, and have added
recommendations which have been promptly acted upon.

The writer's object has been that the boilers found most convenient and
best suited for the different purposes for which they are used should
be made to work with safety, rather than that reliance should be placed
upon the qualities of any particular kind of boiler or fittings. No
form of boiler at present admits of absolute reliance upon its freedom
from risk.

The following general conclusions appear to arise from the
consideration of the records of boiler explosions.

1. That the force accumulated in an ordinary boiler is enough to
account for the violence of an explosion.

2. That no form of boiler, however well constructed and fitted, is free
from the liability to explosion, if allowed to get out of order; and
that boilers which bear the hydraulic test may still be dangerous.

3. That the condition of a boiler can be satisfactorily ascertained
only by periodical examinations, and that no boiler should work without
being thoroughly examined at short intervals.

4. That the cost of periodical examination is so little as to be far
outweighed by the greater security obtained; and that the settings of
all boilers should be constructed with a view to facilitate examination.

5. That the surest way to make systematic examination general is to
spread as widely as possible correct information as to the facts and
ascertained causes of boiler explosions, and to inform boiler owners
and minders what dangers to guard against; and that this is preferable,
and more likely to lessen explosions than enforcing any system of
inspection by legal enactment.

                                 TABLE I.

           _Summary of Records of Steam Boiler Explosions_
                       _up to 30th June, 1870,_
              _showing Description of Exploded Boilers._

  +------------------++-----------------------++-----------------------+
  |  Description of  || Explosions up to June ||  Explosions in the 4  |
  |     Boiler.      ||         1866.         || years from June 1866  |
  |                  ||                       ||     to June 1870.     |
  +                  |+--------+--------+-----++--------+--------+-----+
  |                  ||English.|Foreign.|Total||English.|Foreign.|Total|
  +==================++========+========+=====++========+========+=====+
  |Marine            ||     57 |    203 | 320 ||     12 |     64 |  76 |
  +------------------++--------+--------+-----++--------+--------+-----+
  |Cornish,          ||    140 |      1 | 141 ||     84 |      3 |  87 |
  |Lancashire, or    ||        |        |     ||        |        |     |
  |others with       ||        |        |     ||        |        |     |
  |internal flues    ||        |        |     ||        |        |     |
  +------------------++--------+--------+-----++--------+--------+-----+
  |Locomotive        ||     91 |     29 | 120 ||     10 |     68 |  78 |
  +------------------++--------+--------+-----++--------+--------+-----+
  |Plain Cylindrical ||    114 |      2 | 116 ||     54 |      3 |  57 |
  |externally fired  ||        |        |     ||        |        |     |
  +------------------++--------+--------+-----++--------+--------+-----+
  |Balloon,          ||        |        |     ||        |        |     |
  |Haystack, Wagon,  ||     62 |      2 |  64 ||      5 |      2 |   7 |
  |Butterley,        ||        |        |     ||        |        |     |
  |British-Tube      ||        |        |     ||        |        |     |
  |Elephant, or      ||        |        |     ||        |        |     |
  |Trevithick        ||        |        |     ||        |        |     |
  +------------------++--------+--------+-----++--------+--------+-----+
  |Portable,         ||        |        |     ||        |        |     |
  |Agricultural,     ||     28 |      1 |  29 ||     17 |     17 |  34 |
  |Upright, Crane,   ||        |        |     ||        |        |     |
  |or very small     ||        |        |     ||        |        |     |
  +------------------++--------+--------+-----++--------+--------+-----+
  |Heating, Kitchen, ||     14 |     .. |  14 ||     22 |     14 |  36 |
  |Domestic, Rag     ||        |        |     ||        |        |     |
  |Steamers, &c.     ||        |        |     ||        |        |     |
  +------------------++--------+--------+-----++--------+--------+-----+
  |Furnace-upright   ||     10 |     .. |  10 ||      8 |     .. |   8 |
  +------------------++--------+--------+-----++--------+--------+-----+
  |Not sufficiently  ||    203 |     29 | 232 ||      7 |    175 | 182 |
  |described to be   ||        |        |     ||        |        |     |
  |classified        ||        |        |     ||        |        |     |
  +==================++========+========+=====++========+========+=====+
  |Totals            ||    719 |    327 |1046 ||    219 |    346 | 565 |
  +------------------++--------+--------+-----++--------+--------+-----+

                          TABLE I (continued)

  +------------------++--------------------------+
  |  Description of  ||     Total Explosions     |
  |      Boiler.     ||     up to June, 1870.    |
  |                  |+--------+--------+--------+
  |                  ||English.|Foreign.| Total. |
  +==================++========+========+========+
  |Marine            ||   69   |   327  |  396   |
  +------------------++--------+--------+--------|
  |Cornish,          ||  224   |     4  |  228   |
  |Lancashire, or    ||        |        |        |
  |others with       ||        |        |        |
  |internal flues    ||        |        |        |
  +------------------++--------+--------+--------+
  |Locomotive        ||  101   |    97  |  198   |
  +------------------++--------+--------+--------+
  |Plain Cylindrical ||  168   |     5  |  173   |
  |externally fired  ||        |        |        |
  +------------------++--------+--------+--------|
  |Balloon,          ||        |        |        |
  |Haystack, Wagon,  ||   67   |     4  |   71   |
  |Butterley,        ||        |        |        |
  |British-Tube      ||        |        |        |
  |Elephant, or      ||        |        |        |
  |Trevithick        ||        |        |        |
  +------------------++--------+--------+--------+
  |Portable,         ||        |        |        |
  |Agricultural,     ||   45   |    18  |   63   |
  |Upright, Crane,   ||        |        |        |
  |or very small     ||        |        |        |
  +------------------++--------+--------+--------+
  |Heating, Kitchen, ||   36   |    14  |   50   |
  |Domestic, Rag     ||        |        |        |
  |Steamers, &c.     ||        |        |        |
  +------------------++--------+--------+--------+
  |Furnace-upright   ||   18   |    ..  |   18   |
  +------------------++--------+--------+--------+
  |Not sufficiently  ||  210   |   204  |  414   |
  |described to be   ||        |        |        |
  |classified        ||        |        |        |
  +==================++========+========+========+
  |Totals            ||  938   |   673  | 1611   |
  +------------------++--------+--------+--------+

                                 TABLE II.

           _Summary of Records of Steam Boiler Explosions_
                       _up to 30th June, 1870,_
                   _showing Causes of Explosions._

  +-----------------++------------------------++------------------------+
  |Cause of         || Explosions up to June  ||  Explosions in the 4   |
  |explosion.       ||         1866.          ||years from June 1866 to |
  |                 ||                        ||       June 1870.       |
  |                 |+--------+--------+------++--------+--------+------+
  |                 ||English.|Foreign.|Total.||English.|Foreign.|Total.|
  +=================++========+========+======++========+========+======+
  |Worn out,        ||     92 |     53 |  145 ||     89 |      5 |   94 |
  |corroded, or     ||        |        |      ||        |        |      |
  |burnt plates     ||        |        |      ||        |        |      |
  +-----------------++--------+--------+------++--------+--------+------+
  |Undue pressure,  ||    132 |      5 |  137 ||     25 |      6 |   31 |
  |overloaded       ||        |        |      ||        |        |      |
  |valves,          ||        |        |      ||        |        |      |
  |intentional or   ||        |        |      ||        |        |      |
  |from carelessness||        |        |      ||        |        |      |
  +-----------------++--------+--------+------++--------+--------+------+
  |Bad              ||    136 |    108 |  244 ||     69 |      8 |   77 |
  |construction,    ||        |        |      ||        |        |      |
  |weak tubes,      ||        |        |      ||        |        |      |
  |defective        ||        |        |      ||        |        |      |
  |fittings or      ||        |        |      ||        |        |      |
  |stays, or want   ||        |        |      ||        |        |      |
  |of repair        ||        |        |      ||        |        |      |
  +-----------------++--------+--------+------++--------+--------+------+
  |Shortness of     ||    106 |      8 |  114 ||     28 |      2 |   30 |
  |water, formation ||        |        |      ||        |        |      |
  |of scale or mud, ||        |        |      ||        |        |      |
  |or external      ||        |        |      ||        |        |      |
  |flues set too    ||        |        |      ||        |        |      |
  |high             ||        |        |      ||        |        |      |
  +-----------------++--------+--------+------++--------+--------+------+
  |Extraneous       ||      6 |      3 |    9 ||      2 |     .. |    2 |
  |causes,          ||        |        |      ||        |        |      |
  |lightning, fire, ||        |        |      ||        |        |      |
  |gas, &c.         ||        |        |      ||        |        |      |
  |-----------------++--------+--------+------++--------+--------+------+
  |Too uncertain to ||    247 |    150 |  397 ||      6 |    325 |  331 |
  |be classified    ||        |        |      ||        |        |      |
  |=================++========+========+======++========+========+======+
  |Totals           ||    719 |    327 | 1046 ||    219 |    346 |  565 |
  +-----------------++--------+--------+------++--------+--------+------+

                         Table II (continued)

  +-----------------++------------------------+
  |Cause of         || Total Explosions up to |
  |explosion.       ||       June 1870.       |
  |                 ||                        |
  |                 |+--------+--------+------+
  |                 ||English.|Foreign.|Total |
  +=================++========+========+======+
  |Worn out,        ||    181 |     58 |  239 |
  |corroded, or     ||        |        |      |
  |burnt plates     ||        |        |      |
  +-----------------++--------+--------+------+
  |Undue pressure,  ||    157 |     11 |  168 |
  |overloaded       ||        |        |      |
  |valves,          ||        |        |      |
  |intentional or   ||        |        |      |
  |from carelessness||        |        |      |
  +-----------------++--------+--------+------+
  |Bad              ||    205 |    116 |  321 |
  |construction,    ||        |        |      |
  |weak tubes,      ||        |        |      |
  |defective        ||        |        |      |
  |fittings or      ||        |        |      |
  |stays, or want   ||        |        |      |
  |of repair        ||        |        |      |