quality, as everything depends on the skill of the operator in closing the furnace at the precise moment of time, before the mass is deprived of its carbon. This precaution is necessary in order to retain the exact quantity of carbon in the puddled bulb, so as to produce by combination the requisite quality of steel. It will be observed that in the Bessemer process this uncertainty does not exist, as the whole of the carbon is volatilized or burnt out in the first instance; and by pouring into the vessel a certain quantity of crude metal containing carbon, any percentage of that element may be obtained in combination with the iron, possessing qualities best adapted to the varied forms of construction to which it may be applied. Thus the Bessemer process is not only more perfect in itself, but admits of a greater degree of certainty in the results than could possibly be attained by the mere employment of the eyes and hands of the most experienced puddler. Thus it appears that the Bessemer process enables us to manufacture steel with any given proportion of carbon, or other eligible element, and thus to describe the compound metal in terms of its chemical constituents.

Important changes have been made since Mr. Bessemer first announced his new principle of conversion, and the results obtained from various quarters bid fair to establish a new epoch in metallurgic manipulation, by the production of a material of much greater general value than that which was produced by the old process, and in most cases of double the strength of iron.

These improvements are not exclusively confined to the Bessemer process, for a great variety of processes are now in operation producing the same results, and hence we have now in the market homogeneous and every other description of iron, inclusive of steel, of such density, ductility, &c., as to meet all the requirements of the varied forms of construction.

The chemical properties of these different kinds of steel have been satisfactorily established; but we have no reliable knowledge of the mechanical properties of the different descriptions of homogeneous iron and steel that are now being produced. To supply this desideratum, I have endeavoured, by a series of elaborate experiments, to determine the comparative values of the different kinds of steel, as regards their powers of resistance to transverse, tensile, and compressive strain.

These experiments have been instituted not only for those engaged in the constructive arts, but also to enable the engineer to make selections of the material as will best suit his purpose in any work proposed. In order to arrive at correct results I have applied to the first houses for the specimens experimented upon, and judging from the results of these experiments, I venture to hope that new and important data have been obtained, which may safely

be relied upon in the selection of the material for the different forms of construction.

For several years past, attempts have been made to substitute steel for iron, on account of its superior tenacity in the construction of ships, boilers, bridges, &c.; and there can be no doubt as to the desirability of employing a material of the same weight and of double the strength, provided it can at all times be relied upon. Some difficulties, however, exist, and until they are removed it would not be safe to make the transfer from iron to steel. These difficulties may be summed up in a few words, viz. the want of uniformity in the manufacture, in cases of rolled plates and other articles, which require perfect resemblance in character, and the uncertainty which pervades its production. Time and a close observation of facts in connection with the different processes will, however, surmount these difficulties, and will enable the manufacturer to produce steel in all its varieties with the same certainty as he formerly attained in the manufacture of iron.

In the selection of the different specimens of steel, I have endeavoured to obtain such information about the ores, fuel, and process of manufacture as the parties supplying the specimens were disposed to furnish. To a series of questions, answers were, in most cases, cheerfully given, the particulars of which will be found in the experimental Tables, published in the Transactions of the British Association for 1867.

I have intimated that the specimens have been submitted to transverse, tensile, and compressive strain, and the summaries of results will indicate the uses to which the different specimens may be applied. Table I. gives for each specimen the modulus of elasticity, and the modulus of resistance to impact, together with the deflection for unity of pressure; from these experimental data the engineer and architect may select the steel possessing the actual quality required for any particular structure. This will be found especially requisite in the construction of boilers, ships, bridges, and other structures subjected to severe strains, where safety, strength, and economy should be kept in view.

In the case of transverse strain some difficulties presented themselves in the course of the experiments, arising from the ductile nature of some part of the material, and from its tendency to bend or deflect to a considerable depth without fracture.

But this is always the case with tough bars, whether of iron or steel, and hence the necessity of fixing upon some unit of measure of the deflections, in order to compare the flexibility of the bars with one another, and, from the mean value of this unit of deflection, to obtain a mean value of the modulus of elasticity (E) for the different bars. This unit or measure of flexibility given in the Table, is the mean value of all the deflections corresponding to unity of

pressure and section. In order to determine the resistance of the bars to a force analagous to that of impact, the work in deflecting each bar up to its limit of elasticity has been calculated. These results differ considerably from each other, showing the different degrees of hardness, ductility, &c., of the material of which the bars are composed. The transverse strength of the different bars up to their limit of elasticity is shown by the amount of the modulus of strength or the unit of strength calculated for each bar.

Table II., on Tensile Strain, gives the breaking-strain of each bar per square inch of section, and the corresponding elongation of the bar per unit of length, together with the ultimate resistance of each bar to a force analogous to that of impact.

Table III., on Compression, gives the force per square inch of section requisite to crush short columns of the different specimens, with the corresponding compression of the column per unit of length, together with the work expended in producing this compression.

It will be observed from the following Tables that the results of the experiments show that the deflections produced by a transverse strain are in proportion to the pressures within the limits of elasticity.

În Table I., as in the other two on tension and compression, the value of the work done on each specimen has been determined, and the results recorded in the last column indicate the comparative strength of each particular bar; and the mean value of the deflections corresponding to unity of pressure and section will be found in column 3. These may be taken as the measure of flexibility, elasticity, and ductility of the different bars, and the uses to which the material may be applied.

The mean value of E, the modulus of elasticity taken for thirty of the best specimens, is about 31,000,000, which exceeds that of wrought iron by more than the thirtieth part. Steel having a much greater flexibility than malleable iron, accounts for the approximation of their respective values in D. This arises from the fact that the bars of the greatest flexibility-other things being the samehave the least value for the modulus of elasticity.

On tensile strain the mean result derived from thirty of the best specimens is 47-7 tons, or nearly 48 tons per square inch; and in this, as in the previous Table, the measure of ductility and strength is given in the last column, which indicates the utility of the material and the purposes for which it may be selected.

Comparing the best quality of steel with the best wrought iron at 24 tons as the breaking-weight per square inch, we find that we have a material of double the strength with the same weight, or what is the same thing, of only half the weight with the same strength, or as 47.7 to 24. In the art of construction these

COMPARISON of STEEL MANUFACTURED after the BESSEMER PROCESS, with that MANUFACTURED by other PROCESSES.

TABLE I-TRANSVERSE STRAIN on inch-square bars, and 4 ft. 6 in. between the supports.

Messrs. Naylor, Vickers, Melted in 0013007 30,278,000 65.049 6.548

& Co.

..the crucible

f

TABLE II. TENSILE STRAIN on bars inch diameter. Elongations taken on 8 in. length.

COMPARISON of STEEL- continued.

TABLE III.-COMPRESSIVE STRAIN on specimens in. diameter and 1 in. in length.

are considerations of great importance; and in every case where steel can be depended upon, it is entitled, on the score of economy and lightness, to the judgment and practical knowledge of the architect and engineer.

In Table III., on compression, each of the specimens were reduced, when cut from the bars previously experimented upon, to small columns of inch diameter and 1 inch in height. They were each loaded with weights equal to 100 tons per square inch, without undergoing any sensible appearance of fracture. On consulting the Table it will be found that with the above weight of 100 tons per square inch they were compressed, on the average, to two-thirds their original length; and from these facts we were enabled to find the value of u, recorded in the last column, as the value of work done by the load which produced the change of form in each of the specimens submitted to pressure. This, it will be observed, was the true test of the powers of resistance of the respective specimens to a compressive strain, and the conditions under which materials of similar properties may be safely applied in constructive art.

On comparing the mean tensile resistance to rupture at 47-7 tons per square inch, it will be seen that the resistance to com