If our attention is directed to the bridge class of structures, we perceive that, previous to the introduction of railways, the more general form was the simple arch, which, whether constructed of cast or wrought iron, there is left indisputable proof that the arch, as a principle, cannot be excelled, either for stability or durability, and that, simply because there are in it fewer elements of self-destruction than in any other form or principle which has been applied for the like purpose. Hence it is not too much to presume, that just in proportion as we leave the principle of the arch, and approach that form which necessity, in many cases, has rendered imperative, we introduce elements which, if not carefully watched, must sooner or later prove fatal to the stability of the mass.

To determine what these elements of self-destruction are, and to what extent they are in operation, has lately occupied the highest mathematical and practical talent of this kingdom; and they have recorded as the result of their experiments and investigation, that to resist the effects of reiterated flexure, iron should scarcely be allowed to suffer a deflexion equal to onethird of its ultimate deflexion, for should the deflexion reach one-half of its ultimate deflexion, fracture will sooner or later take place. It is, therefore, reasonable to conclude that the greater the amount of rigidity it is practicable to introduce into structures of this nature, the fewer will be the self-destroying elements in operation, and, consequently, the greater their durability.

These deductions will receive very considerable support from the history of the various descriptions of shafting employed in the different manufactories of this country; previous to the introduction of the slide lathe, the shafting employed in the spinning manufactories was a constant source of vexation and expense; the want of that perfect parallelism which is now obtained exposed the shaft to vibration or bending at every revolution; the consequence was constant fractures. The same results have been observed and recorded in reference to the shafting in use in the iron works of this district; and if we pass to the main shafts of water wheels, or the intermediate shafts of marine engines, we see that what at one period of their history was considered good in principle, (viz.—parallel shafts,). have had to give place to others, more generally of increased diameter at the centre, or if parallel greatly in excess of former practice. Perhaps no better case could be selected than that of the intermediate shaft of a marine engine to illustrate the subject now before the institution, for there might be traced an almost perfect agreement in all the forces which act upon that shaft, and on the axles of either engines or carriages on a railway.

The author of this paper being a manufacturer of railway axles, has had his attention drawn to the subject of the form of axles for some considerable time; and from his knowledge of the properties of iron, and his observations of the fractures of shafts and axles, has concluded that various forms of shafts and axles possess elements of self-destruction-that the fractures which take place are generally confined to given parts, and that those parts where fracture takes place exhibit errors of mechanical construction, or errors of mechanical arrangement, when in motion.

A very extensive course of experiments has been gone through by the author, approximating as closely as possible to the forces on axles when

in use; and these have satisfied his mind, that just in proportion as there are departures from certain fixed principles of construction in either shafts or axles, in the same proportion will be their liability to fracture.

Before passing to an examination of the experiments, it may assist to a more correct elucidation of the subject if the railway axle is viewed as having certain relations to a girder in principle. Girders generally have their two ends resting on two points of support, and the load is either located at fixed distances from the props, or dispersed over the whole surface; just so with the axle; it has its points of support and its loaded parts, but it is not clearly evident which are the loaded parts and which the props. It has been stated that the wheels may be considered the props, and the journals the loaded parts; but it is thought that with equal propriety the journals may be considered the props, and the wheels the loaded parts: if this latter opinion is at all admissible, we then have the load brought much nearer the centre of the axle than in the case where the journals are considered the loaded parts; and, besides, it brings more immediately before us the influence which the inclined bearing surface of the wheels will necessarily have in increasing the power of any lateral or vertical blow, which the axle will receive through the wheels. It is found that the inclined surface of the wheel tire ranges from 1 in 12 to 1 in 20, and, as a matter of course, the direct tendency of the wheels under a load is to descend that incline, so that every vertical blow which the wheels may receive is compounded of two forces, viz.:-the one to crush the wheels in the direction of their vertical plane, and the other to move the lower parts of the wheels together; it will be seen that these two forces have a direct tendency to bend the axle somewhere between the wheels; should that yielding or bending extend no farther than one half the elastic limit, if long continued, fracture will ultimately take place; but should the elastic limit be exceeded, the axle takes a permanent bend, the wheels are then diverted from their vertical plane, and, as a matter of course, leave the rails. To demonstrate this is the object of the first experiment. An axle reduced in the middle to 13 inch dia. was placed upon two props 4 feet 9 inches apart, and loaded in the middle, the utmost of its deflexion without a permanent set was 232 inches, the load carried 7 tons. An axle reduced to 4 inches in the middle, and then placed upon the props 4 feet 9 inches apart, its utmost deflexion without a permanent set was 281 inches, the load carried 9 tons. Another axle, but parallel, 4 inches diameter, was placed upon the props 4 feet 9 inches apart, its utmost deflexion, without a permanent set was 343 inches, the load carried 14 tons. Hence, by reducing an axle of 4,5 ins. diameter in the middle to 3 inches, its limit of elasticity is reduced from 343 inches to -232 inches, and the load, to produce that elasticity, from 14 to 7 tons. Fig. 1 shows the position of the wheels to the rails when the bending of the axle has exceeded its elastic limit.

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The object of the second experiment was to ascertain what influence the reduction of an axle in the middle would have on its strength to resist sudden impact, compared to an unreduced one; this axle was made as represented by fig. 2, which shows the end A parallel to the centre 4 inches diameter, and the end B is drawn down from the back of the wheel towards the centre, where it is 4 inches diameter. The end A was then

subjected to impact-the relative position of prop and ram was the back of the wheel and the neck of the journal, this end received 46 blows of the ram, and bent to an angle of 18°. The end B was then subjected to impact the prop and ram in the same relative position, when it bent back to an angle of 22° with only 16 blows of the ram, (as shown by the dotted lines in fig. 2.) The object of the third experiment was to ascertain what Fig. 1.

influence a shoulder behind the wheel would have on the strength of the axle at that part, compared to one without a shoulder. Fig. 3 and 4 were one axle cut in two, the end E was turned from the neck of the journal, leaving a shoulder th inch deep as a stop to the wheel; the end F was turned from the neck of the journal to the same diameter, but no shoulder

left. The end E was subjected to hydraulic pressure, the load being in a direct line with the shoulder, when it broke in two with a load of 60 tons. The end F was subjected in the same way to hydraulic pressure, when it bent into the form shown by the dotted lines, with 84 tons. The object of the fourth experiment was to ascertain what influence the position of the wheel, in relation to the neck of the journal, would have on the strength of the journal under impact. Fig. 5 was a piece of an axle with Fig. 5.

H

a journal taken down at each end; the end G was keyed into a cast iron frame, the face of the frame in a line with the neck of the journal, the journal was then subjected to the impact of a ram falling 10 feet, when it broke off at the 7th blow. The end H was keyed into the cast iron frame in the same way, but with the neck of the journal projecting 14 inches from the face of the frame, the journal was then subjected to the impact of the same ram falling 10 feet, when it broke at the 24th blow.

From these experiments, and from the acknowledged deteriorating influence of vibration or bending on iron, especially when continued any great length of time, it is the author's opinion that neither shafts nor railway axles ought to be reduced in the middle, but rather, if there is to be a departure from the parallel form, they should be made thickest in the middle, and thus effectually prevent any vibration or bending whatever; for it is the introduction of this principle into almost every description of beam and girder, also into the connecting rods of every description of steam engine, and into a large quantity of the shafting now in use, that has rendered the whole of these articles so superior in point of durability to what they were when other principles of form were in use.

Mr. Thorneycroft gave a further illustration of the paper by reference to several specimens of axles which were exhibited to the meeting. Having obtained an axle which had a shoulder at both ends, he turned the shoulder off one end but left it on the other, and he found that in the instance where the shoulder was turned off, it required a pressure of 120 tons to break it, and 11⁄2 inch deflexion; while the other end, where the shoulder was not turned off, broke with a pressure of 105 tons and of an inch deflexion.-Proc. Lond. Inst. Mech. Eng.

Magnetic and Diamagnetic Condition of Gases.*

The Bakerian Lecture was delivered yesterday (Nov. 28, 1850) by Prof. Faraday to a crowded audience. At this late period of the month we can only glance at the highly interesting investigations laid before the Royal Society, reserving a fuller notice for our next number.

• From the London, Edinburgh, and Dublin Philosophical Magazine, December, 1850.

One of the conclusions arrived at by the author was, that the motions of magnetic and diamagnetic bodies in each other do not appear to resemble those of attraction or repulsion of the ordinary kind, but to be of a differential action, dependent perhaps upon the manner in which the lines of magnetic force were affected in passing from one to the other during their course from pole to pole, the differential action being in ordinary cases between the body experimented with and the medium surrounding it and the poles. A method of showing this action with the gases is described, in which delicate soap-bubbles are made to contain a given gas, and then, when held in the magnetic field, approach, or are driven further off, according as they contain gases, magnetic or diamagnetic, in relation to air. Oxygen passes inwards or tends towards the magnetic axis, confirming the results formerly described by the author.

Perceiving that if two like bubbles were set on opposite sides of a magnetic core or keeper cut into the shape of an hour-glass, they would compensate each other, both for their own diamagnetic matter and for the air which they would displace; and that only the contents of the bulbs would be virtually in a differential relation to each other, the author passed from bubbles of soapy water to others of glass; and then constructed a differential torsion balance, to which these could be attached, of the following nature:-A horizontal lever was suspended by cocoon silk, and at right angles to the end of one arm was attached a horizontal cross-bar, on which, at about 14 inches apart, and equidistant from the horizontal lever, were suspended the glass bubbles; and then the whole being adjusted so that one bubble should be on one side of the iron core and the other on the other side, any difference in their tendency to set inwards or outwards from the axial line causes them to take up their places of rest at different distances from the magnetic axis; and the power necessary to bring them to an equidistant position becomes a measure of their relative magnetic or diamagnetic force.

In the first place different gases were tried against each other, and when oxygen was one of them it went inwards, driving every other outwards. The other gases, when compared together, gave nearly equal results, and require a more delicate and finished balance to measure and determine the amount of their respective forces.

The author now conceived that he had attained to the long-sought power of examining gaseous bodies in relation to the effects of heat and the effects of expansion separately; and proceeded to an investigation of the latter point. For this purpose he prepared glass bubbles containing a full atmosphere, or half an atmosphere, or any other proportion of a given gas; having thus the power of diluting it without the addition of any other body. The effect was most striking. When nitrogen and oxygen bubbles were put into the balance, each at one atmosphere, the oxygen drove the nitrogen out powerfully. When the oxygen bubble was replaced by other bubbles containing oxygen, the tendency inwards of the oxygen was less powerful; and when what may be called an oxygen vacuum (being a bulb filled with oxygen, exhausted, and then hermetically sealed) was put up, it simply balanced the nitrogen bubble. Oxygen at half an atmosphere was less magnetic than that at one atmosphere, but more magnetic than other oxygen at one-third of an atmosVOL. XXI.-THIRD SERIES.-No. 1.-JANUARY, 1851.

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