Wednesday Evenings at 8 o'clock. MAY 3.—“ On Colonization; its Aspects and Results." By WILLIAM STONES, Esq. MAY 10.-"On the Art of Laying Submarine Cables from Ships." By Captain JASPER SELWYN, R.N. CANTOR LECTURES. The Third Course for the present Session, consisting of six Lectures, "On Some of the Most important Chemical Discoveries made within the last Two Years," by Dr. F. CRACE CALVERT, F.R.S., F.C.S. (Corresponding Member of the Royal Academy of Turin; of the Société Industrielle de Mulhouse; of the Société Imperiale de Pharmacie de Paris, &c.), is now being delivered on Tuesday evenings, at Eight o'clock, as follows: MAY 2ND.-LECTURE 4.-On the Discoveries in Physiological Chemistry. MAY 9TH.-LECTURE 5.-On the Discoveries in the Chemistry of Rocks and Minerals. MAY 16TH-Lecture 6.-On the Discoveries in the Chemistry of Metals and Alloys. These Lectures are free to Members (without ticket), and every Member has the privilege of admitting ONE Friend to each Lecture. Proceedings of the Society. TWENTIETH ORDINARY MEETING. Wednesday, April 26, 1865; Peter Graham, Esq., Member of Council, in the chair. The following candidates were proposed for election as members of the Society :Bean, Alfred W., Danson-park, Welling, Kent. S.E. Dent, William, 21, Newcastle-street, Strand, W.C. 402 Lighting ... 403 403 ... 404 ... 404 Gover, William, Italian-villa, Lee-road, Blackheath, S.E. Hancock, C., 6, St. Germain's-villas, Lewisham, S.E. The following candidates were balloted for and duly elected members of the Society :Brackenbury, Capt. C. B., R.A., 1 Adelaide-place, WoolBourne, Geo. S., R.N., Royal Hospital, Geenwich, S.E. wich-common, S.E. Brady, Francis, 12, Limes-villas, Lewisham, S.E. Dolan, H., Park-hill, Clapham, S. Eden, Frederick Morton, Capel-house, Kew, W. Folkard, Augustus, Haslam-house, Lewisham, S.E. Gowan, George D'Olier, Wood-lawn, Dulwich, S.E. Hawley, Henry J., 4, Foxgrove-road, Beckenham, S.E. Hoblyn, Thos. Hallam, Rickling, Bishop's Stortford. Martley, W., 15, Cedars-road, Clapham-common, S. Milnes, William S., 11, Devonshire-road, Greenwich, S.E. Painter, Henry, 1, Arlington-villas, Loughborough-park, Papengouth, Lieut. Oswald C., 12, Bloomsbury-sq., W.C. The Paper read was ON THE WEAR AND TEAR OF BY FREDERICK ARTHUR PAGET, ESQ., C.E. correct. There are two principal causes that tend to exert impulsive strains on the sides of a boiler:-1. The sudden checking of the current of steam on its way from the boiler to the cylinder; 2. quick firing, attended with too small a steam room; and both may sometimes be found to act in combination. To the first of these causes, the explosion, for instance, of one of the boilers of the Parana steamer, at Southampton, a few years ago, has been ascribed by the Government engineer surveyor;* to the second, the explosion of the copper boiler of the Comte d'Eu yacht, in France. According to Dr. Joule, the mere dead pressure of an elastic fluid is due to the impact of its innumerable atoms on the sides of the confining vessel. According to the published report of the engineer of the Manchester Boiler Assurance Company, forty-three explosions, attended with a loss of seventy-four lives, occurred in 1864 in this country. The engineer of the Mid-When the motion of a current of steam is suddenly land Boiler Assurance Company gives the number as forty-to the cylinder, its speed and weight cause a recoil on the checked, as by the valve in its passage from the boiler eight, causing the deaths of seventy-five and the injury sides of the boiler analogous to the effects of the, in this of 120 persons. These statistics are confessedly incomplete, being, from obvious causes, numerically under- case, almost inelastic current of water in the hydraulic stated. The Royal Commissioners on the metallic mines ram. This action is necessarily most felt with engines in report that, in the districts of Cornwall and Devon, boiler which the steam is let on suddenly, as in the Cornish and explosions are of very frequent occurrence; and, in these suddenly affording a wide outlet, and as suddenly closing. other single-acting engines, working with steam valves sparsely populated districts, they easily escape the public It produces such phenomena as the springing or breathing attention. Explosions, again, which only injure without of cylinder covers, and the sudden oscillations of gauges, killing outright, and therefore do not call for a coroner's noticed long ago by Mr. Josiah Parkes and others.‡ inquest, also happen without attracting much notice. The Some years ago, while standing on a boiler working a figures cited thus understate the destruction and injury to single-acting engine, and with a deficient amount of steamlife through boiler explosions, while only a guess can be hazarded as to the annual loss of property they cause. room, the writer noticed the boiler to slightly breathe Each explosion testifies to the probability that a number been observed by others with boilers the steam-room of with every pulsation of the engine. The same action has of boilers have been prevented from exploding by mere which is out of proportion to their heating surface. The chance, as also to much unchecked decay and deteriora-intensity of the instantaneous impulses thus generated tion, which might have been prevented by greater care would be, as Mr. Parkes observes, difficult to measure, but and more knowledge. Besides, apart from the disas- their repeated action must rapidly affect the boiler at its trous results of an explosion itself, the undue wear and mechanically weakest points. The more or less sudden tear of boilers means the suspension of the workshop or closing of a safety-valve while the steam is blowing off factory and the demurrage of the steam-vessel. With would evidently produce the same effect; and this view is respect to the causes of explosions themselves, are," to use the words of the late Mr. Robert Stephenson,† motive boilers-in which while at work there is no such strengthened by the fact that the great majority of loco"but few cases which do not exhibit undue weakness in sudden call on the reservoir of steam as in the Cornish some part of the boiler;" and the same opinion appears engine-explode while standing with steam up at the to be held by Professor Faraday.‡ The opinion that an stations. It is not denied that, in the case of a locomoexplosion is rather due to the weakness of the boiler than tive, the mere extra accumulation of steam from the to the strength of the steam may in fact be said to be safety-valves being screwed down above the working universal. There is, indeed, a very complex train of mechanical, chemical, and physico-chemical forces, lead- pressure will also come into play. But there can be little ing to the deterioration and consequent destruction of a and with more or less frequency, to an impulsive doubt that most boilers are subjected sooner or later, steam-boiler, and it is probable that no other metallic structure is subjected to such complicated conditions. would demand a factor of safety of six in the This being the case, this consideration alone The pressure of the steam and the heat of the fire produce designing of steam boilers. mechanical effects, while both the burning fuel and the the application of iron to railway structures, in their The Commissioners on water react chemically on the plates and in accordance third conclusion on a with their varying chemical properties. Each of these made their investigations the most valuable ever conagents play, so to speak, into the other's hands, furthering ducted on the strength of materials, say :-"That, as it and quickening the other's progress. It is difficult to has been shown, that to resist the effects of reiterated distinguish with strictness between the effects of each; flexure, iron should scarcely be allowed to suffer a deflecand it is mainly for the sake of convenient examination equal to one-third of its ultimate deflection, and tion that they can be classified into:-1. The effects of the pressure of the steam; 2. the mechanical effects of the heat; 3. the chemical effects of the fuel; 4. the chemical effects of the feed-water. "there load. mass of evidence which has since the deflection produced by a given load is increased by the effects of percussion, it is advisable that the greatest load in railway bridges, shall in no case exceed one-sixth of the weight which would break the beam when laid on at rest in the centre." Emerson showed, more than sixty years ago, that the stress tending to split in two an internally perfectly + Rudimentary Treatise on Marine Engines and Steam Vessels, etc. By Robert Murray, C.E., Engineer Surveyor to the Hon. Board of Trade, p. 74-78. + Instituto di Scienze. Milano, 1829. Transactions of the Institution of Civil Engineers. Vol. 3. § Reports of the Inspecting Officers of the Board of Trade, 1850-64. (The four locomotive boilers which burst last year all did so while standing. Neither the primary rupture leading to the explosions, nor the secondary rupture caused by the explosion, took place through the rivet holes.) Report of the Commissioners appointed to inquire into the Application of Iron to Railway Structures. xviii. cylindrical pipe, submitted to the pressure of a fluid from the interior, is as the diameter of the pipe and the fluid pressure. He also showed "that the stress arising from any pressure, upon any part, to split it longitudinally, transversely, or in any direction, is equal to the pressure upon a plane drawn perpendicular to the line of direction." As in a boiler the thickness of the metal is small compared with the radius, the circumferential tension has been assumed to be uniformly distributed; and the strain per unit of length upon the transverse circular joint being only half that upon the longitudinal joints, the strength of the latter has been taken as the basis of the calculations for the tensile strength of the joints. But in taking the internal diameter of the boiler as the point of departure, the internal section has been assumed to be a correct circle, which would only be practically true in the case of a cylinder bored out in a lathe, and never in that of a boiler. Two of Emerson's corollaries from his first proposition have in fact been neglected. He shows that if one of the diameters be greater than another, there will then be a greater pressure in a direction at right angles to the larger diameter; the greatest pressure tending to drive out the narrower sides till a mathematically true circle is formed. The second is that, "if an elastic compressed fluid be enclosed in a vessel, flexible, and capable of being distended every way, it will form itself into a sphere."* A number of proofs can be adduced that both these influences are more or less at the bottom of the wear and tear caused by the direct action of the steam. From 1850 to 1864 forty locomotive explosions causing a loss of human life have occurred in the United Kingdom. The Board of Trade reports in the Bluebooks presented to Parliament, and more especially those by Captain Tyler, R.E., probably form the most valuable and connected series of records extant on boiler explosions. This is more especially the case with regard to wear and tear caused by the direct action of steam unmasked by the effects of the fire, as the barrel and outside fire-box of a locomotive cannot be said to be under the direct action of the heat. Perhaps the vibration of the boiler through the motion on the line may intensify this action, but it is clear that vibration cannot be a primary cause. The majority of the reports are illustrated by careful drawings. Eighteen of the forty boilers gave way at the firebox-eleven from the crown of the inside fire-box being blown down upon the tube plates; seven from the shells or sides giving way. Twenty burst at the barrel; and two explosions may be ascribed to miscellaneous causes, from an originally defective plate, and from running off the line. Leaving out all those which occurred at the firebox, as the majority of these might be ascribed to other influences than direct pressure, all the twenty explosions of the barrel could be traced either to internal furrows or to cracks, both running parallel with one of the longitudinal joints of one of the rings forming the barrel. All the joints which thus gave way were lap-joints; and the furrows or the cracks (and the former greatly preponderate in number) occur at the edge of the inside over-lap, and, therefore, just at the point where the diminution of diameter caused by the lap-joint would be most affected by the pressure of the steam. (See Fig. 1.) The plate at the channels shows distinct traces of lamination through the cross-bending, and it is probable that plate of a good material will gradually laminate, while *The action of a fluid pressing with equal forces in all directions can be evidently represented as to force and direction by innumerable radii of equal length led from a single point in all directions. Upon this principle may be explained the spherical shapes of soap bubbles, of the bulbs of thermometers (blown while the glass was in a plastic state), of the thin india rubber balls, used as playthings, and which are formed by forcing air into india rubber tubes closed at one end. Gas and air bubbles in water are necessarily flattened by the hydrostatic pressure. It is upon that principle that a gun of soft ductile iron often bulges out at the breech. FIG. 1. (Full size cross section of the furrowed longitudinal joint in the fire box ring of a boiler which exploded at Overton station, on the 30th May, 1864. It does not differ from other furrows.) inferior metal will crack through in much less time. Nor are these furrows found with only lap-joints. Butt-joints, with a strip inside the boiler, and thus destroying the equilibrium of internal pressure, have been found to be attended with similar furrows. Channels of exactly the same character have been observed in locomotive boilers with lap-joints, which have exploded in Germany.* Similar furrows, again, have been noticed in marine boilers, and in old boilers generally, longitudinal farrows being of course about twice as dangerous as those appearing transversely. The smoke-box tube-plates of inside cylinder-locomotive engines have been found to be similarly influenced by the racking action of the engines, showing furrows around the cylinder flanges. A parallel case is often found in Lancashire with the endplates of double-flued Fairbairn boilers, which may have been too stiffly stayed to the barrel. Circular furrows, caused by the confined motion of the end-plates are sometimes found at the base of the angle iron rings jointing the internal flues to the end-plates. But furrowing seems with no kind of boiler to be more felt than with locomotive boilers. This is due to the higher pressure, to the thicker plates causing a coarser lap, and more especially to the fact that the unstayed barrel cannot be thoroughly examined without drawing the tubes, thereby enabling the furrow to enlarge itself unnoticed. The inside fibres of a plate bent up while cold are necessarily initially in a state of compression. When the pressure from the inside comes on, striving to form a perfect cylinder, the plate gets bent to and fro by its own elasticity on one side, and by the pressure on the other. If the iron be brittle, it may crack right through; if ductile, the outside fibres gradually lose their elasticity, and, necessarily aided by other causes, crack away. This action is progressive, and probably very rapid towards its later stages. Once a weak place formed itself it would have to do more and more of the work. Even when pulled by the direct tension of the testing machine, a lapjoint behaves in a somewhat similar way. For instance, a half-inch lap, solidly welded by Bertram's process, has only half the strength of the solid plate; while the inch lap-weld has actually two-thirds of the strength of the entire plate. through with a knife, or that with which a column under compression is broken by a blow from a hammer, or by the similar ease with which a tube under tension is split by a sharp blow; in fact, the operation of caulking a defective boiler under steam seems thus to often give it the finishing stroke which causes an explosion. The new boiler which burst from a defective plate at the Atlas Works, Manchester, in 1858, and that which burst through a crack at a longitudinal joint last January, at Peterborough, both gave way whilst being caulked. This again accounts for the fact that adjacent boilers sometimes explode one after the other, pointing, at the same time, to the danger into which a sound boiler may be thrown by an explosion. Upon the same principle it is probable that the modern guns, built up from strained rings, will Messieurs Jean Piedboeuf and Cie., of Aix-la-Chapelle, Düsseldorf, and Liege, who turn out annually upwards of one thousand steam boilers, use a lap-joint which proba-be easily put hors de combat by shot. The probability is bly gives slightly better results as to furrowing, while it s much easier to caulk, and must be therefore less inured by that process. (See Fig. 2.) FIG 2. (The edges of the plates are cut to an angle of 659 by means of inclined shears.) There is, however, another important appearance to be noted with respect to these furrows. An iron cylindrical vessel under internal pressure would of course rupture long before it could assume a spherical shape, from its ranges of elasticity and of ductility being so short. But it may be said to be undergoing three distinct stresses in as many directions. There is a stress acting on the ends, and tending to rupture the boiler in two halves in a direction parallel to the axis; there is the stress which is hoop tension in a true circle, but which acts with a cross-bend ing strain in an ordinary boiler; and there is the stress which tends to make it assume the shape of a barrel, or to bulge it out in the centre of its length. The precise action on a material of several strains like this is a portion of the strength of materials which is still completely unknown. Its probable effects might be illustrated by the ease with which a stretched india-rubber ring is cut "Recent Practice on the Locomotive Engine," p. 5. that a number of simultaneous strains in different directions diminish the elasticity of the material that would allow it to yield in any given direction. However this may be, it will be seen that it is only the pressure on the ends of the boiler acting parallel to the axis, and tending to tear the cylinder through transversely, which bears fairly on the rivetted joint, or rather on that metal between the rivets which is left after punching. Unless the cylinder be perfectly correct inside, the circumferential strain resolves itself into cross-bending, shifting the dangerous strain from the iron left after punching to the metal at the over-lap. With respect to the stress tending to bulge the cylinder in the centre, it is clear that if we suppose a strip cut out from the entire length of the boiler, each portion of the length of this strip could be regarded as a beam under an uniformly distributed load. As, however, Iwith the lap joint, there is a double thickness of metal transversely, that joint is the strongest and stiffest portion to resist the stresses tending to bulge out the cylinder in the middle, and also to tear it into two halves. This affords some justification for the belief of old boiler-makers, before rivetted joints were tried under a direct tensional load, that the joints are the strongest parts of the boiler. And, indeed, this is what we find in practice. The thinnest portion of the longitudinal furrows is generally exactly in the middle of the plate, and this is caused by the longitudinal stress, which is acting at right angles to the transverse cross-bending stress. A strip cut from joint to joint is, in one respect, in the condition of a beam supported at both ends, uniformly loaded throughout its length, and, according to known principles, therefore giving way in the middle. (See Fig. 3.) The middle ring of the boiler which burst on the Metropolitan Railway last year, and the fragments of which were examined by the writer, also first given way at a furrow. Captain Tyler reports that at from 16 to 19 inches from the transverse joint, or just about the middle of the plate, there was "very little metal left holding," while it gradually got to its original thickness of, as the groove receded from the centre of the plate and towards the transverse joints at each side. It is impossible to deny the existence of an infinite number of stresses acting on the sides of a vessel undergoing fluid pressure, producing what, for want of a better term, might be called a "bulging strain," Instances of this action may be noticed in the sketch of the leaden pipes given by Mr. Fairbairn,* which were bulged out in the middle by internal pressure, as also in the fire-box sidest influenced by the same means, and in the centre of the surface. Unaccountably enough, the effect of such a strain on the ultimate resistance, and, above all, the elasticity, of materials, has been entirely neglected by investigators, and there are no published data on the resisted by a double thickness of plate at the joints, so matter. The effect of the internal pressure is evidently FIG. 3. (From Captain Tyler's report, dated 30th June, 1864, on the boiler explosion at the Overton station of the London and North-Western Railway The plate torn off is shaded, the course of fracture on the other side of the boiler is dotted, while the furrow is shown by the thick horizontal line.) that the load on the middle of a single ring may be considered as determining the weakest part of the boiler. One of the rings of the Great Northern boiler which exploded on the Metropolitan Railway last May had a length transversely of about (say) 36 inches from lap to lap, with an inside diameter of 45 inches. If we now suppose a strip one inch broad cut from the 36 inch long plate, parallel to the longitudinal axis of the boiler, this strip is, supposing there be a pressure of 100 lbs. to the square inch, uniformly loaded with 3,600 lbs.-equal to a transverse load of 1,800 lbs. at the centre. Supposing the plate to form a true circle, a hoop one inch wide of the plate would be subjected, circumferentially, to a tensile load of 6,000 lbs. per square inch, while (leaving out the diminution of area at the ends through the flue tubes) each portion of the circle, about 1 inch broad and inch thick, is subjected to a load of about 1,125 lbs. acting parallel to the axis of the boiler. To construct a general rule or formula that would take into account the distorting effects of the lap or of the welt of batt-joints would be impracticable; but it is clear that the usual mode of calculating the strength of a cylindrical boiler from the tensile strength of joints tested by weights, or hydraulic pressure, directly applied, is far from being correct. It is only tolerably correct with scarf welded joints, or with butt joints with outside welts. Even here, the hoop tension of the true cylinder is resolved into a cross bending strain, if the cylinder does not not form a correct circle internally. The usual formula would be practically correct, if the boiler were prevented from altering its shape during the impulses sometimes given by the stearn, and the quieter buckling action caused by the alternate increase and fall of the pressure. In fact a boiler, like a girder, does not merely demand a high ultimate strength, but also a stiffness which is the protection against alternative strains-against buckling or collapse. Disregarding the effects of the thickness of the material, a perfect cylinder should theoretically afford the same ultimate resistance, whether exposed to external or internal pressure. Its resistance to collapse should indeed be greater, as most materials give more resistance to compression than to tension. This is not the case, as the distortion of form progressively weakens an internal flue, by increasing the load on its surface, while the contrary is rather the case with the boiler exposed to internal tension. Before Mr. Fairbairn showed the inherent weakness of flue tubes, their frequent explosions through collapse were ascribed to spheroidal ebullition and other similar causes. They are now, according to the engineer of the Manchester Boiler Association, stronger than the shells, by means of the T-iron and angleiron bands now generally used, and also by the excellent seams introduced by Mr. Adamson so long ago as 1852.* While T-iron and other bands could be used for the barrels of boilers not exposed to the fire (as is recommended in Francet and by the Board of Trade Inspector of Railways), Adamson's seams reversed would probably form excellent transverse joints for a shell fired from the outside, and, with a boiler like this, thin and narrow plates could be used, affording a stronger and tighter lap-joint. With a construction of this kind little or no deflection or bulging could occur, and the sectional area of the plate and the rings would really give the strength of the boiler. 2. THE MECHANICAL EFFECTS OF THE HEAT. While a maximum of stiffness to the mechanical action of the pressure is required in a steam boiler, a maximum of flexibility to the irresistible mechanical force of heat is of no less importance. For instance, a great advantage of some of the forms of strengthening rings for internal flues is that they allow the use of thinner plates; together forming a structure of great flexibility to complicated thermal influences. The longitudinal expansion of inside flues like this is taken up by a slight spring or swagging at each joint, and the end plates of the shell are not unduly strained by the combined efforts of the internal pressure and the expansion due to heat. This is one way in which defective circulation, or a sudden current of cold air or of water, can act on the structure, by unequally straining the plates; and, although it seems probable that the effects said to have been thus produced, are, to some extent, due to other causes, they point to the importance of keeping the temperature of the plates as low as possible. One protection against effects of this kind is the gradual diffusion of heat, produced by its conduction to and from the different plates. It is a general belief with engineers that a pressure of steam strains a boiler more than cold hydraulic pressure! but it is unsettled as to what amount and in what exact way. The basis of an examination of the kind would have to be sought in an exact determination of the temperature of a plate which is transmitting the heat to the water, and this has not yet been determined with any accuracy. fact is, as is remarked by M. Péclet, who has given great attention to these questions, the different phenomena involved are extremely complicated. It is clear that the plates must always be at a higher temperature than the water, as it is by the difference of temperature of the two surfaces of the plate that it is traversed by the heat. He supposes that, though the flow of heat through the plate is inversely as its thickness (while it is directly as the surface and as the difference of temperature between the outside and inside faces), yet the flow of heat would be the same through a thicker plate, from the greater difference of temperature between the two surfaces. He does not seem, however, to be aware of the important law demonstrated by Mr. J. D. Forbes, that the conducting power of, for instance, wrought iron, rapidly diminishes at the higher temperatures. At 200° C. it has little more than one-half the conducting power it has at 0°. At yet higher temperatures it might probably be proved, if an applicable instru p. Specification No. 14.259. The † Bulletin de la Société Industrielle de Mulhouse, 1861, 532. Traité de la Chaleur. Vol. 2, p. 393. Royal Society of Edinburgh, 28th April, 1862. "Experimental Inquiry into the Laws of the Conduction of Heat in Bars, and into the Conducting Power of Wrought Iron." |