arranged that they each and all may be shut off and tested severally, in order to ascertain whether they are working alike, and in harmony with the mercurial gauge. I have a copy of the law with me, and I think no objection to it can be seriously made. It says: "The law that each boiler must have a separate gauge remains in force, but the choice of construction is free, and we recommend, on account of their practicability, and because they can easily be read off, the use of spring gauges, but we require them to be often compared with the controlling gauge." That is to say, suppose you have twelve boilers, every boiler would have one gauge; and in a corner of the house an open mercurial column is placed, and is connected with all these gauges. You have a tap to shut off each of them, but from time to time you have to test them, or perhaps the inspector comes round and sees whether they are all working alike. That is the present Prussian law.

On the Wear and Tear of Steam Boilers. By FREDERICK ARTHUR PAGET, Esq., C.E.

(Continued from page 20.)

From the Journal of the Society of Arts, No. 649.

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. The 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

VOL. L.-THIRD SERIES.-No: 2.-AUGUST, 1865.

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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 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 instrument for registering higher temperatures were in existence, that the powers of conduction are still less. Some of Mr. Péclet's experiments also seem to be vitiated by his disregard of Dr. Joule's discovery that water is heated by being mechanically stirred up. It is, however, certain that water can only moisten a metallic plate when at a lower temperature than 171° C. As soon as the water gets thus repelled, the heat radiated by the metal is reflected back from the surface of the liquid; the metal gets hotter and hotter, with a corresponding diminution of its conducting powers; its outside, exposed to the fire, would more or less oxidize, and with similar result; and a like effect is produced on the inside-on the roughened surface of which incrustation would rapidly adhere, forming a calcareous coating, conducting with about sixteen times less power than iron. All these tendencies are of a progressive character, leading to very high temperatures in the plate, even to a red-heat. This tends to explain how rivet-heads close to the fire are soon burnt away by the friction of the current of heated gases on the red-hot metal; how thick fire boxes are sooner burnt out than lighter ones, the process being often arrested at a certain thickness; how internal flues of thick plates so often give trouble; how externally fired boilers are most deteriorated at the corners from the junction of the three plates; and similar results well known to practical men. Another proof that thin plates conduct more heat than thick plates is afforded by some experiments lately made in Prussia, with two egg-end boilers, exactly similar in every respect, except that one was constructed of steel plate 4-inch thick, while the other was of wrought iron about 2-inch thick. The steam generating power of the steel boiler was to that of iron as 127.49 to 100§-a result which can only be accounted for by the thickness of the plates. Thick plates are also more liable to blisters, one of which would considerably diminish the conduction power of the spot where it happened to form.

*Traité de la Chaleur. Vol. II, page 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."

Traité de la Chaleur. Vol. I, page 391.

? Verhandlungen des Vereins zur Beförderung des Gewerbfleisses in Preussen, 1862, p. 140.

While it is certain that boiler plates can assume very high temperatures, even up to red-heat, authorities differ as to the diminution of ultimate strength which is caused by heat, while its effect on the elasticity of the plate has been scarcely attended to. The experiments on the ultimate tenacity of iron at high temperatures, conducted by Baudrimont,* Seguin, and the Franklin Institute, can scarcely be looked upon as of much value, for they were made on a very small scale, and with no regard to the temporary and permament elongations—or to the effect of heat on the elasticity and ductility.

Mr. Fairbairnt observed no effect on the strength of plate iron up to almost 400° F. At a "scarcely red" heat the breaking weight of plates was reduced to 16.978 tons from 21 tons at 60° F.; while at a "dull red" it was only 13.621 tons. MM. Tréméry and P. Saint Brice, aided by the celebrated Cagniard Latour, found that at nominally the same temperature (rouge sombre), a bar of iron was reduced in strength to one-sixth of its strength when cold. This is much greater diminution of strength than that found by Mr. Fairbairn. Apart from other causes this might easily be due to the fact that incandescent iron affords a different tinge during a dull day to what it does in a clear light. In fact, the great impediment to all these inves tigations is the want of a thermometer for high temperatures; but M. Tréméry's result is perhaps more conformable with daily experience. Mr. Fairbairn's data would show that the ultimate strength of wrought iron is reduced to about one-half; but M. Tréméry's result explains the generally instantaneous collapse of flues when red-hot, and which have been, of course, originally calculated to a factor of safety of six.

A most important question is the effect of temperatures, whether high or low, on the elasticity of the material-whether iron will take a permanent set with greater facility at a high temperature? These data are really more important than those on the ultimate strength, as they would show the influence of temperature on the elastic limit. Here again is a vacancy in existing knowledge, which can scarcely be said to be filled up by the few experiments of the late M. Wertheim on very small wires.§ He found, however, that the elasticity of small steel and iron wire "increases from 15° C. to 100°, but at 200° it is not merely less than at 100°, but sometimes even less than at the ordinary temperature."

There is, however, another important point with respect to wrought iron, which has scarcely received the attention it deserves. As would appear from a number of phenomena, there seems to be a sort of thermal elastic limit with iron. When heated, and when its consequent dilatation of volume does not exceed that which corresponds to (perhaps) boiling point, it returns to its original dimensions. Beyond a certain * "Annales de Chimie et de Physique," 3, s. 30, page 304, 1850.

On the Tensile Strength of Wrought Iron at Various Temperatures. Reports British Association, 1856, page 405.

Annales des Mines, 2 serie, Vol. III, page 513,

2 Comptes Rendus, xix, 231.

temperature it does not contract again to its pristine volume, but takes a permanent dilatation in consequence, apparently, of its elastic limits. having been exceeded. A number of observers* have determined the fact with cast iron, and though wrought iron has not been expressly investigated in this direction, there is no doubt that it exhibits a similar behaviour. Thus, a number of years ago,t an Austrian engineer named C. Kohn, remarked that a boiler about 12 metres long and 1.57 in diameter, with a thickness of plate of 0.011, permanently expanded, at a temperature corresponding to a steam pressure, of 5 atmospheres, (153° C.) by 0-07193, and did not, when cold, return to its original dimensions. The same thing has been noticed, by means of very accurate measurements, with other boilers. A number of experiments by Lt. Col. H. Clerk, of Woolwich, on wrought iron cylinders and plates, bear distinct evidence to a dilatation of volume in wrought iron, when repeatedly heated and suddenly cooled. In experiment 7, for instance," two flat pieces of wrought iron, each 12 inches long, 6 inches deep, and -inch thick, were heated and cooled 20 times, one being immersed to half, and the other to two-thirds, its depth in water. That immersed one-half contracted or became indented on the ends fully 3 inch; the other had similar indentations, but only to one-half the amount. They both turned up into the form of an arc," the convex side of which appeared in the portion heated and cooled. Unfortunately, the specific gravities of the different portions were not tried by Colonel Clerk. A succession of trials of the kind produced cracks in the metal, thus explaining how boiler plates are cracked by imperfect circulation and by cold feed-water let in near the fire; and, the thicker the plate, the more permanent dilatation of volume and consequent danger. Mr. Kirkaldy found that "iron highly heated and suddenly cooled in water, is hardened," being injured, in fact, if not afterwards hammered or rolled. This permanent dilatation of volume must be necessarily accompanied with a diminution of specific gravity, thus affording another close analogy, betweeen straining iron by loads in excess of the mechanical elastic limits, and straining by heat. Lajerhelm§ found, long ago, that the specific gravity of iron is diminished by strains in excess of the limit of elasticity, and this result has been completely confirmed by Mr. Kirkaldy's numerous experiments. The smith calls iron "burnt" which has been rendered brittle in working through the often repeated applications of heat, or through too high a temperature. Iron rendered brittle by strains in excess of the limit of elasticity has been long popularly termed "crystallized." Both these states are accompanied with a dilatation of volume and attendant hardness and brittleness, and both seem to be referable to very similar causes. In fact, a very general belief exists that very ductile good iron, used in the form of a steam boiler, soon gets brittle. There are some applications of metal to a steam boiler peculiarly liable to * Percy's Metallurgy, vol. ii, page 872. †Technologiste. 1850-51, page 102.

Proceedings of the Royal Society, March 5, 1863. ? Poggendorf's Annalen, 2, s., vol. ii., page 488.

be strained beyond the limits of elasticity; by mechanical force, by the mechanical force of expansion and contraction, and by dilatation of volume through heat-all three acting simultaneously. Such is the case with fire-box stay-bolts. Accordingly, they are found to get very brittle when of wrought iron-which is a much less ductile metal than copper. Mr. Z. Colburn states that he has "frequently found these stays (where made of wrought iron) to be as brittle, after a few years use, as coarse cast iron." He has "broken them off from the sides of old fire-boxes, sometimes with a blow no harder than would be required to break a peach-stone.'

The Chemical Effects of the Incandescent Fuel.

Whatever physical changes may be induced in iron by the long continuance of a high temperature which is not succeeded by the application of the impact of the hammer or the pressure of the rolls, it is certain that long-continued red-heat leads to the loss of its metallic consistency. Its surface gets converted to a greater or less depth into forge scales, which, according to Berthier, consist of a crystallized compound of peroxide and protoxide of iron. The mechanical action of the gases-and especially of the free oxygen contained in every flame-forced at a high velocity by the draft past the more or less® heated plates, would also aid these chemical combinations-upon the same principle as iron filings, thrown through a gas flame, burn in the air; and upon the same mechanical principle as the incandescent lime is worn away by the flame of the oxyhydrogen blow-pipe. These actions would take place with any fuel, even with pure charcoal. But when mineral fuel, which mostly contains more or less iron pyrites, is used, there is much more danger to the plates, especially over the fire, in getting red-hot, as the flames would then hold sulphurous acid, and often volatilised sulphur. A familiar illustration of an action of this kind is afforded by the fact that a piece of red-hot iron plate can be easily bored through by means of a stick of sulphur the combination forming sulphide of iron. Dr. Schafhaeutl, of Munich, has given great attention to the changes in plates subjected to the action of fire; twenty-five years ago he read a paper before the Institution of Civil Engineers, and more recently he has published an essay, both on this subject, in a Munich periodical. He has brought forward a number of facts, founded on chemical analyses of plates of exploded boilers, showing the danger, due to chemical action alone, when the plates of a boiler become red-hot. He notices that the iron of the inside of the plates, in getting red-hot decomposes the water, and combines with the oxygen thus freed. It also loses some of its carbon. The outside combines with the free oxygen and with any sulphurous acid in the flame. He states that iron made with pit coal is much more affected than charcoal made iron; becoming laminated at the original joints in the pile out of which the plate has been rolled. It is possible that * Steam Boiler Explosions, 1860, p. 32.

+ Transactions of the Institution of Civil Engineers. Vol. III, 1840, p. 435. Bairisches Kunst und Gewerbeblatt. June, 1863.