grows smaller and feebler till the discharge is complete. When both sides of the charged jar are insulated, and each furnished with a pointed wire, the star will be seen on the negative point during the whole time that the brush appears on the positive point. These are beautiful theoretical facts, though by no means adapted for the lec

ture room.

When the discharge of a jar is made in the usual way through a metallic circuit, the velocity of the fluid is too great to allow of measurement: or rather, perhaps, there are circumstances in the way which frustrate all attempts to ascertain its velocity. When, for instance, a discharge is made on one end of a long metallic circuit, the entering fluid disturbs all that belonging to the conducting wire, a portion of which leaves the wire at one end of the circuit at the time the new fluid from the jar enters the other end; and this fact is, perhaps, the most formidable barrier against obtaining satisfactory experimental results. During lightning, however, the electric fluid may be traced by the eye as it traverses the air through long striking distances, and obviously occupies an appreciable period of time.

I have a beautiful experiment to bring forward, which has been often used as an illustration of the great velocity of the electric fluid whilst traversing metallic conductors, but for reasons already explained it is of no value in that capacity.

I have about fifty yards of iron wire chain suspended round the room by silken cords, and through this chain I discharge the battery of jars from a high intensity. The chain, you will have observed, was splendidly illuminated throughout by brushes of scintillating fire, which sprang simultaneously from every link in the circuit. The results of experiments of this kind are interesting, if it were on no other account than by their showing that a quantity of the electric fluid is thrown into the air from every sharp point in the circuit. The scintillating of the iron, however, adds much to the brilliancy of the display.

MEETING OF THE BRITISH ASSOCIATION.

(Selections continued from page 223).

Mr. NASMYTH brought forward several specimens to illustrate the remarks which he intended to make in further illustration of his remarks on Friday. From late accidents, arrising 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 cir

cumstances; but the short time intervening since Friday had rendered it impossible to do so. One opinion on Friday 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 and bent it 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 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 sixty degrees, with a crystalline fracture; by raising the temperature forty degrees 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, on Friday, 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 coldswaged 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. The 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 elucidate their theories, and he hoped that these specimens might be of some use.

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 case hardening (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 shewed specimens of pins which had been broken in machinery. They appeared very crystalline in fracture; the bar from which they had been made was fibrous and tough: he showed also specimens of tender axles broken on the Sheffield and Rotherham railway. Tender axles are 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 infinity, 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 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, 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, 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 engineer 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 lamina over each other.

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, three 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 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 iron-masters 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 one-sixth 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.

Mr. Hodgkinson then explained his apparatus for trying the strength of materials. He 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