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.

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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.

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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.

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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 Fig. 2, Fig. 3, Fig. 4, and Fig. 5, No. 35, 1870; No. 50, 1866; No. 46, 1869; and No. 25, 1870.

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.

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 Fig. 6 and Fig. 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 Fig. 8, Fig. 9, and Fig. 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 Fig. 11 and Fig. 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.

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.

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 Fig. 13 and Fig. 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.

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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.

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 Fig. 15, Fig. 16, and Fig. 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.

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.

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 Fig. 19 and Fig. 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.

Those shown in Fig. 21 and Fig. 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.

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.

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.

The same remark applies to some of the portable or agricultural boilers which have exploded, such as those shown in Fig. 30 and Fig. 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.

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 Fig. 15, Fig. 16, and Fig. 17, see page 70 and 71, No. 10, 1869, and Fig. 36.

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.

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 Fig. 44 and Fig. 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 Fig. 46 and Fig. 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 Fig. 49 and Fig. 50, so as to give the means of examining the plates near each seam by simply removing loose bricks.

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.

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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 Fig. 16, No. 35, 1868, No. 32, 1870, pages 70 and 74.

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.

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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.

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.

TABLE II.

TABLE III.

TABLE IV.

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