Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/456

 454 THE BUILDING NEWS. ‘May 31, 1879. eee TRON. Notwithstanding the great advance which has of late been made towards a more perfect acquaintance with the properties of irons of different classes, and notably by means of the numerous experiments made by Mr. Kirkaldy, and the stimulus which has been given to the manufacture of high-class irons by the rival contests between iron guns and iron shields, it must be admitted, even by those who have made it a subject of special study, that there is very much yet to be learnt about iron; whilst, if we except a small circle, whose special employment has caused them to follow with interest in the track of every experiment which could throw any light upon the nature and properties of the material with which they are chiefly called upon to deal, there is a general lack of knowledge about the whole subject, besides much misconception, gehich the clear proof of prac- tical experiment will alone be able to sweep away. As a building material, iron is day by day forcing its way everywhere, and many who not long ago would have set their faces against its use in structures aiming at a high class of art, no longer hesitate to call in its valuable assistance in order to solve con- structive problems which would be beyond the reach of wood, brick, or stone, at any rate within any reasonable limits of expenditure. Such being the case, it is essential that its properties should be tho- roughly understood by all those who are likely to make use of it for constructive purposes, and that they should not merely order a girder, for instance, to carry a given load, leaving the designing of it to the manufacturer or his agent, whose interest is torun up the weight, and hence the price, at the expense of quality and good workmanship. It may safely be said that there is no material more dangerous to trust to, without a full knowledge of its behaviour under different conditions, than iron; whilst there is none which varies so much in quality, or in the manufacture of which there is more knowledge, ex- perience, skill, and care required, or which admits of more deception being practised upon the unwary by unscrupulous and dishonest manufacturers. Now, in the author's opinion, beyond the difference between east and wrought iron, and the inferiority of the former when exposed to the effects of sudden shocks, there is very little accurate knowledge on the subject of the properties and powers of resistance of different classes of iron under varying conditions of stress. Their behaviour under different circumstances, such as tension, compression, shearing, bending, torsion, either suddenly or gradually applied, varies so widely, according to the description of iron under trial, that the strongest proof which could be adduced of the necessity fora far wider acquaintance with the subject is given by the ordinary formule in use for calculating the strength of iron girders, &e. Turn- ing to Hurst’s “ Architectural Surveyors’ Hand- book”’ as a likely source from which a formula might be taken for calculating the strength or the requisite dimensions of (say) a plate girder, we find the following formula given for ascertaining the central breaking-weight of a plate girder—viz., W = CAD L inches; D = depth of girder in inches, L = length of bearing in feet; and a constant C, in this case taken equal to 6, is made to include such a variable quantity as the breaking strength of wrought iron per square inch of section, without one word of ex- planation as to the quality of iron to which this constant is specially applicable. Nor is the corre- sponding formula in Molesworth’s “ Engineering Pocket Book” one whit better. If we analyse the above formula, we shall find the tensile strength of the iron to which the constant is applicable is not more than 18 tons per square inch; for L being in feet, and A and D in inches, ,»in which A = area of tension flange in 9! 12 L W SrGAnD 2x2 &.LW = CAD Wyle ORES 31, in which for C to = 6, the value laid down, the tensile strength must equal 18 tons. Such formule, to those who are unable to unravel them, and see how they are arrived at, are but a delusion and a snare; they leave out of consideration the varying resistances of different classes of iron, and encourage the false idea that the strength of wrought and cast iron, or steel, may be safely represented by constant quantities. In point of fact, we might ignore altogether the ultimate strength of a girder, since it is never, in practice, intended to be loaded so as to bring it anywhere near the breaking point; in addition to which the ultimate strength of iron is, by itself, no proof whatever of its suitability for the work it may be called upon to perform. What really would be quite sufficient for us to know is the degree of elasticity, combined with the elastic limit of the metal employed, the former being measured by the amount of temporary alteration of form the metal will allow of under a temporary stress, and the latter by marking the point beyond which, under uniform increments of stress, the visible work done, such as compression or tension, keeps sensibly increasing, instead of remaining uniform, and beyond which the alteration of form becomes permanent. When strained beyond this point, which is its elastic limit, the metal is permanently injured, and its power of resistance decreases in an accelerating ratio, until it finally gives way altogether. The testing of iron, however, for its elastic limit is a matter of great delicacy, more especially as it is necessary to observe its reduction of area or elongation at the moment it reaches its elastic limit, and for this reason the breaking strength of iron is adhered to in practice, as giving, when observed in combination with its alteration of form, all the information required, The degree of elasticity required in iron depends entirely upon the nature of the stress to which it is to be subjected, as, whether the load upon a girder is to be dead or live, and how the latter is to be applied—namely, gradually or suddenly. If, then, we take proper steps to ensure the use of an iron sufficiently elastic for our purpose, and limit the stress to be put upon any of its fibres, so as to keep it well within the elastic limits of the particular iron employed, we may be sure of steering clear of any possible failures, except such as might arise from faulty design or faulty workmanship. Keeping these points in view, formule should be used in which the maximum stress to be put upon the metal takes the place of constants. Every one would then know the amount of stress they were actually putting upon the iron, and would take care that a proper class of metal, capable of safely resist- ing that stress, was made use of. Instead, then, of the formula already condemned, we might safely use the following, viz. :— Wl Yr =fAD,oW = ebeD ’ ’ in which W = Joad at centre of girder in tons, 7 = span in inches, A = area of tension or lower flange, in inches; D = depth of girder in inches, f = limit- ing stress in tons per square inch, which, for rail- way bridges exposed to the sudden shocks of alive load is fixed by the Board of Trade at 5 tons for tension, and 4 tons compression. The load and span being known, the requisite dimensions of the girder may thus be readily found; or the girder being given, the load it can carry, subject to the particular limits of stress which may be laid down, can be ascertained. In this case, the web of the plate girder, being thin, is not taken into account, but merely calculated to take the shearing stress ; but in rolled iron girders, and other forms in which, from the amount of metal in the web, it would not do to neglect it, the bending moment due to the external forces must be equated with the moment of resistance of the beam, arrived at by finding the moment of inertia of the section of greatest stress, a process which is a little more troublesome. The girder having been designed, the remaining points to be attended to are to ensure sound workmanship, and an iron capable, both as regards strength and elasti- city, of taking with safety the stresses to be put upon it. (To be Continued.) —_~>___——_ ON INDIAN ART. HURSDAY, the 23rd inst., Mr. William Tayler (British Bengal Civil Service, and late Com- missioner of Patna) exhibited, at the meeting of the Society for the Encouragement of the Fine Arts, a large and excellent collection of water - colour sketches, executed by himself during his long resi- dence in India, Mr. Eastwick, M.P., who presided, called upon Mr. W. Tayler to explain the many interesting sketches exhibited. The Leerurer then proceeded to dwell, from a general point of view, on the want of artistic power and conception in India. He attributed this to the two principal religious creeds of the Indians—the Mahometan and Brahmaniec. As to the Mahometans, they could have no art, as they looked upon the pictorial representation of anything living as asin. The second precept of the Deca- logue was observed by them with aven greater severity than by the Jews. Now such a precept was suflicient to obstruct pictorial art. The lec- turer wished to be understood that he did not touch architecture, nor mere ornamentation, as in these two branches the Indians produced perfect marvels. What he wished to call attention to was the absence of a correct appreciation of forms and perspective, of light and shade, in the educated classes of India. As to the other religious sects, especially the Brahmanie, the cause was less intelligible. The Indian mind early occupied itself with trying to represent the different forces in nature in visible forms, like the Greeks. Whilst the Greeks had assumed’the human body in its abstract beauty as the best representative of some phenomenon in nature, attributing it to some special divine individuality, the Indian delighted in divinities with four or twelve arms, three or four heads, or the representation of the beginning of all wisdom in an elephant’s head with a large trunk, as in Ganesé. This tendency to see in the most sacred forms grotesque monsters re- acted on pictorial art, and altogether obstructed the faculty of dealing with that wondrous nature, fall of glow and beauty. The lecturer then entered on a graphic description of the flora and fauna of India. He described the sacred female fig-tree (Ficus indica) and its male companion (the Ficus religiosa), exhibit- ing sketches of one of these trees stretching over many acres of ground, and having embraced more than thirty temples with its mysterious branches. What was with us a small shrub was in India a high tree, as the rhododendron or cotton-tree, the leaves of which disappeared, as if ashamed of their ugliness in comparison with the glorious beauty of the flowers. Theinnumerable species of horses having been referred to, the lecturer touched upon some incidents of every-day life: two instances of which formed most interesting topics—the snake-charmers, and the practice of mesmerism in India. The lecturer had witnessed, and sketched with the utmost fidelity, both phenomena. The cobra was in reality alive to the charm of music—a monotonous, plaintive kind of music—and the people, male and female, were subject to the influences of mesmeric action, Several in- stances were mentioned, of which the lecturer, in his official capacity, had been a witness. The influence of animal magnetism on the Indians was so powerful that he had seen the most frightful operations per- formed on patients whilst they were in mesmeric sleep without their ever having evinced the slightest feeling. The lecturer, in conclusion, regretted that want of time did not allow him to dwell more on details, but hoped that his few remarks would encourage his hearers to make themselves more acquainted with India. The CrarkMAN then invited discussion, and called on Dr. Zerffi to favour the meeting with some ob- servations. Dr. Zerrrt expressed his extreme gratifica- tion at the lecture he had had the pleasure of listen- ing to. Having made India for years a special study, he could endorse the many suggestions thrown out by the lecturer. Art and religion were in- divisible motors in the progress of humanity. Art was, in fact, the body; religion, the spirit. But art, as well as religion, had its first origin in nature. The lecturer had dwelt on the monstrous repre- sentations of the forces in nature as conceived by the Indian sculptors, and compared them with the symmetrical bodies of Greek art. We might find the solution of this problem in the very flora which the lecturer had exhibited. The marvellous and in- comprehensible twistings and windings of the created object impressed the Indian mind with a confused idea of the Creator himself. Jt was, however, not always so. The conceptions in the ancient Vedas were grand, sublime, and simple. The divinity was one, incomprehensible, unpronounceable, invisible = its forces the phenomena of nature. Only in later times, when an organised priesthood, no more satisfied with the simplicity of the Vedantie con- ceptions, began to turn every attribute of nature into a separate personal divinity, we saw the rise of that gorgeous Pantheon with all those grotesque and mon- strous gods and goddesses, through which the in- comprehensible was to be expressed in visible forms by means of mystic symbolism, and which tendeney had killed art. The Mahometans, who had erred in the very opposite direction, might teach us how narrow our own escape had been through modern science from the two evils. Had the ¢lerical spirit of the middle ages succeeded in forcing upon us those big-headed, thin-necked, spider-armed forms of divinities and saints with which our religious Pantheon had been peopled, or had we not broken in modern times the gloomy spirit of Mahometanism in the garb of our Puritanical fanatics, we should have killed art too. (Cheers.) We should either have remained stationary with our old forms, because they were sacred, or should have continued in square white-washed rooms to listen to a droning mono- tonous expounder of the doctrines of love and kind- ness, and have altogether neglected those higher faculties of the intellect, and that sense of beauty, with