Heat,' that if the condition of metal at a certain temperature depended exclusively on that temperature, no distribution or movement of heat could possibly give rise to a current of electricity in a circuit of one metal; nevertheless I find, as above stated, that in a circuit of one metal wire a current is maintained for five minutes at a time, gradually vanishing to nothing when the two ends of the homogeneous wire have been for some time in contact, but recommencing if one wire is cooled for a minute and then again applied to the hot one. One explanation of this might be that the condition of the wires does not solely depend on their temperature, but is influenced to a considerable extent by the time during which they have remained at that temperature. Nor is this a gratuitous assumption: Dr. Matthiessen has proved that wires of several metals do not attain a constant conducting power until they have been kept for some time at a constant temperature; he finds that the conducting power of bismuth increases, while that of tellurium decreases when kept for a time at 100°. Quite similarly, some metals may rise and some may fall in the thermo-electric scale after being heated for some time, a supposition which is necessary to account for the metallic contact currents by the theory I suggest. Another possible explanation of the metallic contact currents may be found in a partial hardening on the one side and annealing on the other, caused by the sudden contact of the hot and cold metal. If this be so, the current between annealed and unannealed wires of the same metal would correspond with the contact current between two homogeneous wires, in a way which it does not seem to do. I am, however, now engaged in investigating this subject, and hope before next year to be able to give facts which may decide whether either of these theories is tenable. There is great difficulty in forming any conclusion from experiments hitherto made, inasmuch as none of the observers, except Dr. Matthiessen, have used chemically pure metal, and it is found that the electrical properties of a metal are affected to an extraordinary degree by the presence of impurities in very small quantities. Explanation of the Table. The names of the metals of which the loops were made are entered at the side and top of the Table. The experiments made with each combination are entered in the subdivision at the intersection of the horizontal and vertical columns corresponding to the two metals. The metals named at the top formed the right-hand loop, those at the side the left-hand loop. The arrows show the direction of the current across the joint. The first entry in each subdivision shows the deflection observed when the right-hand metal was heated and the wires held loosely together. The second entry shows the deflection when the same metal was heated but the wires drawn tightly together. The third entry gives the maximum deflection, and the direction of the current, when the middle of the joint is gradually heated and the two wires held tightly together. The fourth entry (where given) shows the maximum deflection from a current in the opposite direction when greater heat was applied. The two last entries show the common well-known metallic thermo-electric effects. The first entry shows the new loose-contact effect. The second entry shows an uncertain combined effect of metallic and imperfect contact effects. An example will perhaps make this clearer. When copper and iron were 1862. N used and copper loop heated, a loose contact produced a current from copper to iron across the joint, giving a deflection of 100 divisions. A tight contact gave nothing decided. When the iron loop was heated (the copper cold) the loose contact produced a current from iron to copper across the joint, giving a deflection of 90 divisions. A tight contact in this case gave a weak current in the opposite direction. When the joint was heated in the middle, as the temperature gradually rose, a maximum deflection of 3 divisions was first reached, showing a current from copper to iron across the joint; and as the heat increased still further this current was reversed, and finally, at a white heat, gave a maximum deflection of 3 divisions with a current from iron to copper. On the Mechanical Properties of Iron Projectiles at High Velocities. By W. FAIRBAIRN, F.R.S. A VALUABLE series of experiments were made at Manchester upon portions of plates fired at by the Iron Plate Committee at Shoeburyness. These experiments comprised the determination of the resistance to punching, to a tensile strain, to impact, and to pressure. They show that the tenacity varied from 11 to 29 tons per square inch in the iron plates, and from 26 to 33 tons in the homogeneous iron plates. The average strength of the iron plates between 1 and 3 inches thick varied from 23 to 24 tons per square inch, and this, or about 21 tons, may probably be insisted upon as a measure of strength in future contracts for iron plates. The elongation of the plates under a tensile strain may be taken as a measure of the ductility of the material; it varied in the thicker iron plates from 0.91 to 0.27 per unit of length, and averaged 0.27 inch in the homogeneous metal plates. The maximum observed was 0.35. The most important results in connexion with the question of the resistance are, however, those obtained by combining the tensile breaking weight with the ultimate elongation, as first indicated by Mr. Mallet in a paper read before the Institution of Civil Engineers. By finding in this manner the product of the tenacity and ductility, numbers are obtained which, though not identical with those expressing the resistance of the plates in the experiments with guns at Shoeburyness, are yet in close correspondence with them. The average value for Mr. Mallet's coefficient in the thicker iron plates was about 6500 lbs., and in the steel or homogeneous plates 8300 lbs. But the resistance of the iron plates increases with the thickness, whilst that of the homogeneous metal diminishes. The correspondence of these numbers is indicated in the Report addressed to the War Office and the Admiralty; but a more extended series of experiments are yet wanting to determine the true value of the coefficient as a guide to be insisted upon in the manufacture of iron plates. 9000 foot-pounds is the maximum for iron given by the results already obtained; but an extended series of experiments might develope new features of resistance and new improvements in the manufacture. The experiments on punching afford an explanation of the greatly increased perforating power of the flat-headed shot over that of the round-headed projectiles. They also lead to a formula for the ordinary cast-iron service shot, which appears to give with approximate accuracy the law of the resist ance of plates of different thicknesses to missiles of various weights and velocities. These investigations led to inquiries into the state of the manufacture of plates calculated to resist heavy and powerful projectiles directed against the sides of an iron-plated ship, and, moreover, to determine the exact thickness of plates that a vessel was able to carry. Again, they had reference to the quality of the plates and their powers of resistance to impact. There were three conditions necessary to be observed in the manufacture: 1st, that the material should be soft and ductile; 2nd, that it should be of great tenacity; and, lastly, that it should be fibrous and fough. All these conditions apply to the manufacture of plates, and they also apply, with equal force, to the projectiles in their resistance to pressure and impact. In the experiments at Shoeburyness, it was found that the ordinary castiron service shot were not adapted for penetration, as they invariably broke into fragments when discharged against a sufficiently thick armour-plate. In most cases when delivered at high velocities, they had the power of damaging and breaking the plates; but owing to their crystalline character and defective tenacity, a considerable portion of the power was expended in their own destruction. To some extent the same law was applicable to wrought-iron shot, as part of the force, from its greater ductility, was employed in distorting its form, and depriving it of its powers to penetrate the plate. Cast and wrought iron are therefore inferior as a material for projectiles intended to be employed against iron-plated ships and forts. With steel hardened at the end the case is widely different, as its tenacity is not only much greater than that of cast and wrought iron, but the process of hardening the head prevents compression and its breaking up by the blow when the whole of its force is delivered upon the plate. Steel, although much superior to cast or wrought iron in its power of resistance in the shape of shot, is, nevertheless, susceptible of distortion and compression, and in every instance when employed against powerful resisting targets the compression, and consequently the distortion, was distinctly visible. There is another consideration besides the material which enters largely into the question of the resisting powers of shot, and that is form. It will be recollected that, some years since, the late Professor Hodgkinson instituted a series of experiments to determine the strength of iron pillars, and the results obtained were in the following ratios: 1st. That pillars of about 20 to 30 diameters in length, with ith} 2nd. Pillars with one end rounded and one flat broke with lbs. 3000 2000 being in the ratio of 1, 2, 3. Now in order to ascertain the effects of form on cylindrical shot, a series of experiments were instituted to determine the force of impact and statical pressure produced upon shot of different shapes, and from these experiments the following results were obtained. The description of shot experimented upon was cast-iron of the cylindrical form, with flat and round ends; and it is interesting to observe that the results correspond with those where both ends are rounded and one end only rounded, as obtained by Mr. Hodgkinson on long columns; but in the short specimens with both ends rounded the results are widely different, as may be seen by the following Table. |