ON THE ACTION OF LIGHTNING CONDUCTORS. BY CHAS. WALKER, ESQ. [Abstract of a paper read before the London Electrical Society, July 19th, 1842.] The object of an extensive series of experiments related in this communication is to illustrate the following opinions: 1st. That the discharge of a Leyden jar does not resemble a flash of lightning; and, therefore, that Leyden jars should not be employed in investigating the action of lightning rods. 2nd. That the discharge from a prime conductor does, in all essential points, resemble a flash of lightning; and is therefore admissible in such experi ments. 3rd. That a wire, on which sparks are thrown from a prime conductor, represents a lightning-rod. 4th. That sparks will pass off from such a wire, and, therefore, from a lightning-rod, to vicinal conducting bodies; and, 5th.-That these sparks may be prevented by connecting the vicinal bodies with the rod itself. The experiments were made by aid of the magnificent machine of the Polytechnic Institution, and are supported by a course of reasoning based upon the dissimilarity between the Leyden light and a lightning flash, and the resemblance between the latter and that from the prime conductor. It was shown that the length of the spark from the conductor is due to the action of one force in one direction; and that the shortness of the Leyden discharges is due to two forces, acting in opposite directions. The importance of excluding the Leyden experiments from the field of inquiry is very evident from the fact that on them hinges a large portion of the conflict of opinions. Using the new prime conductor and throwing sparks from it upon wires subJected to every variety of arrangement "lateral sparks" were always obtained, unless vicinal bodies were in close metallic connexion with the wire conveying the charge. This fact was illustrated by a most important modification of the wellknown experiment with the metallic discs ; and if this modification, which consisted in erecting an elevator rod beneath the great globe, forming the prime conductor, be legitimate, the danger of lateral flashes is evident. SUPPLEMENT TO PRACTICAL GLEANINGS, FROM THE TRANSACTIONS OF THE MANCHESTER MEETING OF THE BRITISH ASSOCIATION. LOCOMOTIVE AXLES. One Mr. Nasmyth brought forward (27th June) several specimens to illustrate some remarks which he intended for further illustration of his previous observations on this subject. (See Mech. Mag. No. 987, p. 34.) From late accidents, arising from breaking axles, the public were alive to the subject, and it was desirable that the question should be examined. In locomotive engines the axle was the chief point of danger; and it was therefore important, both as a scientific and practical question, to determine the nature and habitude of iron when placed under the circumstances of a locomotive axle. Experiment was the only way to discover this, and he would have wished to place iron under exactly similar circumstances; but the short time intervening since the previous meeting, had rendered it impossible to do so. opinion then was that the alternate strains in opposite directions, which the axles were exposed to, rendered the iron brittle, from the sliding of the particles over each other. To illustrate this, Mr. Nasmyth took a piece of iron wire and bent it back and forward, it broke in six bends. He had suggested annealing as a remedy for this defect in proof whereof, he took a piece of annealed wire, which bore eighteen bends, showing an improvement of three to one in favour of annealing. He should therefore advise railway companies to include in their specification, that axles should be annealed; he did not like the custom of oppressing engineers with useless minutiæ in specifications, but this was so useful and so cheap, that he considered it ought to be insisted on. To exhibit on a larger scale the effect produced on iron in our workshops, he showed a specimen of iron as it came from the merchant: being nicked with a chisel, it broke in four blows with a sledge, at the temperature of 60°, with a crystalline fracture; by raising the temperature 40° higher, it bore twenty blows, and broke with the fibrous or ligneous fracture; so that the quality of iron was not the only circumstance to be considered as influencing the fracture. I noticed also, said Mr. Nasmyth, at the last meeting, the injurious effect of cold swaging, as causing a change in the nature and fracture of the iron; and here let us take the practical workshop view of the case, and not run after the ignis fatuus of electricity or galvanism, but consider the practical effects. Swaging was necessary in many cases; for instance, when an axle had collars welded on, these could not be finished with the hammer, and certain tools called swages were used, from the action of which great condensation of the iron took place, and a beautiful polish was given to the surface, with what injurious effect he would show by the next specimen, which had been heated red hot, and then swaged till cold; it broke at one blow without nicking, and the fracture was very close and beautiful, like steel. This showed the fallacy of considering close fine grain a good test of excellence in wrought iron; but moderate swaging was often necessary, and not injurious, unless where an over regard to finish carried it to excess. To prove that annealing restored the toughness and fibrous texture, a portion of the last bar was heated, and cold-swaged till cold as before, then heated dull red, and left to cool gradually; it bore 105 blows without breaking, and at last was rather torn asunder than broken, as was shown by the specimen; this proved that the fibrous structure was restored by annealing, and he therefore thought it should be insisted on in specifications. The effect of heating to welding-heat was very injurious, unless the iron was subsequently hammered to close the texture; a piece of the same iron heated to welding, and left to cool, broke without nicking, in one blow, showing very large crystals, especially in the centre. The effect of nicking was also very singular. strength of iron was generally stated to be equal to its sectional area; but a nick not removing of the area took away of the strength. Mr. Nasmyth broke a piece of nicked, or rather scratched wire, to illustrate this point. These, and similar things, did not prove that science and practice were at issue; but, as Halley reached the great accuracy of his prediction of the return of his comet by taking into account the disturbing forces of Jupiter and Saturn, and the other planets amongst which the body had to pass, so scientific men should seek in the workshops correctional formulæ, by learning there the practical occurrences which would clucidate their theories, and he hoped that these specimens might be of some use. The Prof. Willis was aware that many subjects of a purely physical nature could only be explained by practical research; and one great advantage of the British Association was, that it brought scientific and practical men together for this purpose.-Mr. Fairbairn was of opinion, that the two chief causes of breaking axles seemed to be bending and percussion, changing the fibrous into the crystalline structure; this last was the effect of cold swaging, and he hoped that his friend Mr. Hodgkinson would undertake a series of experiments on this very interesting subject. By nicking a bar the extended fibres were cut, which supported more of the weight than the compressed.—Mr. Worthington thought the additional friction in steps, given by annealing, would counterbalance the advantage gained in strength, as casehardening (the very opposite operation) was used to diminish friction, by giving a glassy hardness to the surface, the annealed axles would be laid aside after a few trips, from the friction: he would wish, as a security for life, that the springs should be made as long as possible, to diminish the effect of concussion.-A Member showed specimens of pins which had broken in machinery. They appeared very crystalline in fracture; the bar from which they had been made was fibrous and tough showed also specimens of tender axles broken on the Sheffield and Rotherham railway. Tender axles most frequently broke from the action of the brakes on the wheels; crystals larger in the centre of the axles than at the ends.-Mr. Mallet was quite at issue with the French committee on the very uncomfortable Report which they had made so authoritatively; he believed that the alternate strains, as long as they were within the limit of elasticity, did not injure the texture of the iron. Wire might be bent backward and forward to ipfinity, if we kept within this limit. The effect of nicking depended on a change of crystalline structure; that the effect of the nick in determining fracture, was according to the sharpness of the chisel, and the direction; a nick sloping, according to the natural direction of the fibre, was not so efficacious; a molecular change was effected by this cutting across the fibres: we, in fact, established a plane of cleavage in the iron; this took place in glass, when scratched with a diamond, although glass, from passing through the intermediate viscous state, did not crystallize so definitely as iron, which crystallizes per saltum. Iron, polished and placed in such a situation as just not to corrode, if scratched, immediately began to corrode; and iodide of mercury presented a curious example of entire disintegration from a slight scratch. Crystallization takes place in the direction of motion; in rolled iron the motion was in the direction of the length of the bar or plate, and percussion, in a direction perpendicular to that, had the effect of breaking up these laminæ or fibres of crystals, into their original molecular arrangement; and this effect was proportionate to the temperature caused, and extent of motion imparted. But he believed, that to effect this molecular alteration required more violence than was to be expected in any ordinary railway travelling, or, indeed, any circumstance of machinery in perpetual work. The chief danger was to be feared where any cutting perpendicular to the direction of the fibre took place, as, for instance, shafts, with square collars, would break at these collars, while a little rounding out preserved them. That rotation of iron induced magnetism, he was aware, but he did not believe that either rotation or vibration would affect iron which was sound when first applied. If this theory were correct, the engineer should discard wrought iron entirely; no engine was safe, no suspension bridge should be trusted.-Sir J. Robison considered that injuries did arise from vibration and alternate bending; he instanced tongues of musical instruments, and the effects of bending pure tin, which crackled and broke when very slightly bent in opposite directions. Mr. Mallet believed those tongues to be alloyed, and he found that alloys altered their crystalline nature from mere lying by, as tough brass became brittle, &c., which did not happen in simple metals.-Mr. Nasmyth showed that the effect of hammering bars was actually to make them hollow; every stroke had a tendency to make the bar an ellipse, and the intersection of all their axes was apt to be a hole, from the sliding of the laminæ over each other. THE HOT AND COLD BLASTS. Mr. Fairbairn read his Report On Experiments on the Transverse Strength of hot and cold blast Iron.'-The bars, as described in the former Reports, were supported by standards, 4 feet 6 inches apart, and were loaded with different weights; they were occasionally carefully examined, and showed a very slight progressive deflection. He had no doubt that they would ultimately break, but the progress was very slow. He read a table showing the weights laid on, and the deflections of each bar. Mr. Hartopp said, that Mr. Fairbairn's former experiments on hot and cold blast iron, had created a false impression with regard to the strength of hot blast iron. Mr. Fairbairn had found very little difference between the hot and cold blast; but his experiments, made with great accuracy, and in which the weights were laid on with great care, were of little practical advantage, as these were not the circumstances under which iron was tested in practice: there percussion, violent and sudden impact, should be expected, and here lay the great deficiency of hot blast iron. Even in Mr. Fairbairn's experiments, Oldberry, No. 2, cold blast, bore twice the percussion of Oldberry hot blast; and Milton hot blast was only half the strength of Elsicar cold blast, made of the same ore and smelted with the same coal. Experiments had been made in Yorkshire with great care; the results being, Low Moor cold blast bar iron, 3 inches diameter, broke with 6 blows, ditto Scrap, 3 blows, ditto hot blast, 1 blow; again, Low Moor cold blast 18 blows, Bierly, ditto, 18, hot blast of as good materials, 3 blows; again, Elsicar cold blast 21 blows, Milton hot blast 1 blow; therefore, in iron for axles this difference of at least th of the strength was very important. As to scrap iron it bore too high a character. Scrap, made on the old plan, was all charcoal iron, but the modern scrap iron was very inferior, being 32s. 6d. per ton cheaper, so that ironmasters put off as much of this cheap material as possible. Hot blast iron was rejected now for water pipes, &c. and even for cannon balls; and, in fine, he had been told by very eminent marine engine makers, that where any percussion took place, hot blast cast iron was only half the strength, and wrought iron only 4th the strength of cold blast.—Mr. Fairbairn explained, that he had found great difficulty in obtaining specimens from the different iron-masters, who would of course send, when possible, the best specimens, but every care had been taken to insure accuracy in the experiments.-Mr. Hodgkinson said, that the average strength of hot blast had been weaker than the cold, but the inferiority was chiefly in the softer irons; as the hardness increased the two kinds approached to equality, and in the hardest irons the hot blast was the best. He thought his experiments, made without any interest on either side, and with the greatest care, were more to be depended on than experiments made by those who had an interest in the result.Prof. Vignoles explained, that the question of hot and cold blast had nothing to say to the late contract for cannon balls. STRENGTH OF MATERIALS. He Mr. Hodgkinson explained his apparatus for trying the strength of materials. brought his apparatus forward, as he had made many experiments; and he was desirous to render them as trustworthy as possible, by convincing the members that every care had been taken to insure accuracy. Other experiments had been rendered unworthy of reliance, from injudicious methods of affixing the testing apparatus-as those of Rennie and Capt. Brown on iron; Girard's experiments, &c. In crushing specimens, it was necessary that both ends should be well bedded, and the pressure transmitted through the axis. To this, other experimenters had not always attended, and by using the pressure of bores directly on the substance to be crushed, they introduced the different errors arising from the pressure being oblique, transmitted through the side, or being exerted on mere points, instead of equably exerting its force over the entire top surface: to obviate these objections, he had devised apparatus by which all these errors were avoided. Mr. Hodgkinson explained the crushing apparatus by drawings, &c. In experiments on tearing asunder, he had also taken great care, by means of apparatus, which he exhibited and explained, that the strain should be through the axis, and otherwise free from causes of error. Mr. Hodgkinson explained his experiments on torsion, and illustrated his observations throughout by many models and specimens of the substances on which the experiments had been made. Professor Moseley asked whether, in the experiments on beams, care had been taken to obviate the effects of the friction of the beams on the supports, as this would affect the direction of pressure, altering it from vertical to inclined, and the neutral line only passed through the centre of gravity of the beam when the pressure was vertical: also if care had been taken in laying on the weights, as a weight suddenly laid on produced mathematically twice the effect in deflection. Theoretically, the weight should be increased by small additions, even as grains of sand.-Mr. Hodgkinson said he had taken all precautions with regard to the weights; they were added by small portions, and with great care; the beams rested on tolerably smooth cast-iron, on which he believed the friction would be of little importance. BROCKEDON'S CAOUTCHOUC STOPPERS FOR BOTTLES. Mr. Brockedon exhibited specimens of his patent India-rubber stoppers for bottles, explaining the late improvements in the construction of the cores on which the Indiarubber is spread. The present cores, he said, were made of cotton twisted into strands, &c., by means of a machine which he explained by a diagram. The cylindrical rope now consisted of several strands of tightly twisted cotton, lapped with flax thread, and laid together longitudinally, loose fine cotton rovings being placed between them; the entire was then lapped in a cylindrical form with flax thread, attaining by this method the advantages of perfect roundness and firmness; they also gave sufficient hold to the corkscrew, and bore the heating process well. These stoppers would slide on glass when wet, but not when dry, (although there was no cohesion in this latter state,) so that the bottler, by slightly wetting these stoppers with the liquor which he was bottling, could easily insert them; and when this slight film of moisture was dried up, the stopper required considerable force to withdraw it. VOL. XXXVII. THE RIVAL RAILWAY SYSTEMS RAILS Professor Vignoles read a paper "On the best Form of Rails, and the Upper Works of Railways generally."-He wished to compare the two chief systems of laying down rails, with chairs and without, and to do so he referred to two diagrams :-No. 1, exhibiting the heavy rail and heavy chair used on the South-Eastern Railway, the weight of rail being 80 lb. per yard, and the chair 20 lb. : the rail was fastened in the chair, not with iron, but with a longitudinal plug or key of wood; this mode of laying rails was found to answer very well.-No. 2 was invented by Mr. Evans; it was rolled with a slot or groove running along its under side; this slot, after coming from the first rollers, was rendered dovetailed by compressing the bottom edges of the rail towards each other, thereby narrowing the slot at the bottom. These rails required no chairs, having a continuous bearing on longitudinal wooden sleepers, being fastened down by bolts, with dovetailed heads slid into the groove, and which, passing through holes in the timbers, were secured with a nut and washer at the under side. He had suggested this improvement, as they had been previously fastened with a cotter. By this method, all the difficulties attendant on fastening down the chairs were removed. The chairs had been fastened with bolts and screws, but he had found that on the slightest loosening the bolt-heads flew off, from the continual percussions, and the screws very soon allowed vertical play, from the yielding of the fibre of the wood. By Evans's rail, we secured the rail without the intervention of the fibre of the wood. One inconvenience attending it was, the trouble of scraping away the earth to tighten the nuts when necessary; but this might be partially remedied by placing the bolts as often as possible in the transverse gutters for draining the road, by which at least one-half the bolts might be easily got at; and the difficulty of tightening the remaining bolts would be lessened, if, as he recommended, the timbers were left uncovered. He preferred thus giving a free circulation of air, and disliked burying the sleepers in ballast. The weight of Evans's rail was only 45lb. per yard, although quite strong enough, while the other was 100lb. The bevel in No. 2 rail might be given in the wood-bearing; in No. 1 it was caused by the casting of the chair; this latter rail, from having its top and bottom sides alike, had this advantage, that when it began to wear it might be turned round, or even turned upside down, which was a very great advantage. He had for many years advocated wooden sleepers versus stone, from his ex I perience on the Dublin and Kingstown Railway, where he found that the granite sleepers, the more massive they were the more injurious to the rails and carriages. These had all been taken up, and wooden sleepers laid down, and the saving in expense of repairs would in a few years reimburse the outlay. The railway costs now only 507. per mile per annum for repairs, notwithstanding the great traffic over it. The rails were the old 421b. rails, and, nevertheless, were still used, in consequence of the advantage gained by the wooden sleepers. He recommended keeping Evans's rail to the gauge by light iron rods passed through holes in the rails, and secured by nuts: he thought these transverse ties should never be used as supports. Mr. Bucke remarked, that the rails on the North Union had already been so worn as to require turning. The section of Evans's rails was bad, as, from the squareness of the top, the wheel would not bear on the centre of the rail. He had used these rails a little himself, and had no objection to them for light work. He had remedied the form of rail on the Manchester and Birmingham Railway, so as to give the wheels a bearing on the centre of the rail. He conceived a great disadvantage in the longitudinal continuous bearing was, that the foundation was so near the surface; the stability of the foundation was as the square of the depth, and therefore the necessarily narrow foundation of the continuous bearing was rendered still weaker by its proximity to the surface, and the plan of laying down a railway "like a ladder on the ground," had failed where it had been tried. -Prof. Vignoles considered that the failure to which Mr. Bucke alluded, was from making the transverse ties too strong.-From the Athenæum. THE CASE IN LIFE ASSURANCE- -(P. 95). Solution Sir, I send you a solution of Iver M'Iver's question on Life Annuities. First let us suppose the rate of interest to be 3 per cent. The number living at the age of 23 by the Northampton Tables is 4910, and at 43, 3404: 4910: 3404 100 : 69-33; that is, 69:33 will be the probable number alive at the end of 20 years, and 69:33 × 100 = £6933 will be the probable sum to be divided among the survivors at the end of 20 years. The next thing to be done is to compute the value of an annuity of £1 on the life of a person aged 23 for 10 years. |