a smaller magnet, 4 inches instead of 12 inches diameter, which will get through the business nine times as speedily. I will first of all throw upon the screen a diagram (Fig. 13) which shows the method of the experiment. Here is the source of electricity, marked "dynamo"; here are resistances for regulating the current, marked "rheostat "; and here is a liquid reverser intended to reverse the direction of the current by continuous steps. It consists of two copper plates, to which the current is taken, and of two moving plates revolving between these plates, which plates are connected to the electromagnet. As the moving plates revolve, the current will diminish from a maximum in one direction to nothing, and will then increase to a maximum in the opposite direction. The electromagnet is marked " magnet," the secondary coils of which are not indicated in the diagram. Only one coil in the centre of the magnet is connected. We will now turn the reverser faster. You see, the variations of inductions diminish greatly-indeed, the induction at the centre of the core is but little affected by the changing current in the magnetising coils (change leads). We might have tried three coils instead of one, but the experiment is a little confusing. If we had done so, we should have seen that the diminution of disturbance with increase of speed was less with No. 2 coil than with No. 1 coil, and that it had disappeared entirely with No. 3 coil, and we should further see that the current in No. 2 coil lay behind No. 3, and in No. 1 behind No. 2. I can show you the contrast more effectively with the Westinghouse transformer with a divided core. This is now connected to the lowest galvanometer. We turn slowly: notice the deflection. We turn faster: you see the deflection is increased instead of diminished, as it was with the central coil of the solid electromagnet. Experiments such as these are at once applicable to transformer cores and the cores of dynamo machines. They show that in practice manufacturers have divided the iron about enough, and not too much. la bi FIG. 14.-MAGNETIC CURVE-TRACER CURVES FOR SOFT IRON BARS. (a) Cycle performed slowly. (c) Period of cycle 0·43 second. I have here on the table an instrument designed by Prof. Ewing for the purpose of describing the curves which express the relation between induction and magnetising force in iron. It served in Prof. Ewing's hands to illustrate the point which we have been discussing. I will throw upon the screen curves taken from Prof. Ewing's paper read before the Royal Society. The curves have been taken from solid samples of iron-I mean iron which is continuous, and not divided for the purpose of annulling the current in it. In the first curve (a, Fig. 14) we have the result in which the cycle has been passed through very slowly, and is the true curve of magnetisation. We have b, where the cycle has passed through in three seconds-and you will observe that the amplitude of the 552 On the Effects of Electric Currents in Iron, &c. [April 26, induction has diminished, and we have, lastly, the curve c, where the period of the cycle is four-tenths of a second, and you will observe that it has much more diminished. This is owing to the fact that changes of induction in the centre of the iron hardly exist, and that therefore the total effect is materially diminished. The diagram (Fig. 15) is for steel. We have a the natural curve of induction; b the case when the period of the cycle is three seconds; and c when the period is six-tenths of a second. Curves such as these, of course, very readily give you the average effects upon the whole mass of the iron. The curves which I have shown you give the particular effects of different parts of the mass. a FIG. 15.-MAGNETIC CURVE-TRACER CURVES OF STEEL. (a) Cycle performed slowly. In conclusion, let us indulge in a little wild speculation, not because it is probable that it is in any sense true, but because it is interesting. Suppose a magnet were made exactly like the one on which we experimented, but of the size of the earth, and that some mighty electrician generated such a current in its copper coils as would give a magnetising force of 2.5, and then reversed it, it would take some thousands of millions of years before the rate of disturbance at the centre attained its maximum value. The speculation I suggest is this: is it not conceivable that the magnetism of the earth may be due to currents in its material sustained by its changing induction but slowly dying away? [J. H.] ANNUAL MEETING, Wednesday, May 1, 1895. SIR JAMES CRICHTON-BROWNE, M.D. LL.D. F.R.S. Treasurer and The Annual Report of the Committee of Visitors for the year Sixty-two new Members were elected in 1894. Sixty-three Lectures and Nineteen Evening Discourses were delivered in 1894. The Books and Pamphlets presented in 1894 amounted to about Thanks were voted to the President, Treasurer, and the Honorary The following Gentlemen were unanimously elected as Officers PRESIDENT-The Duke of Northumberland, K.G. D.C.L. LL.D. MANAGERS. Sir Frederick Abel, Bart, K.C.B. D.C.L. LL.D. Captain W. de W. Abney, C.B. D.C.L. F.R.S. Sir Benjamin Baker, K.C.M.G. LL.D. F.R.S. Edward Frankland, Esq. D.C.L. LL D. F.R.S. The Right Hon. The Marquis of Salisbury, K.G. 3asil Woodd Smith, Esq. F.R.A.S. F.S.A. VISITORS. John Wolfe Barry, Esq. C.B. M. Inst. C.E. Carl Haag, Esq. R.W.S. Victor Horsley, Esq. M.B. F.R.S. F.R.C.S. Sir Joseph Lister, Bart. M.D. D.C.L. LL.D. Lachlan Mackintosh Rate, Esq. M.A. Henry Virtue Tebbs, Esq. Silvanus P. Thompson, Esq. D.Sc. F.R.S. John Westlake, Esq. Q.C. LL.D. His Honour Judge Frederick Meadows White, Sir William H. White, K.C.B. LL.D. F.R.S. WEEKLY EVENING MEETING, Friday, May 3, 1895. SIR JAMES CRICHTON-BROWNE, M.D. LL.D. F.R.S. Treasurer VETERINARY-Captain F. SMITH, F.R.C.V.S. F.I.C. The Structure and Function of the Horse's Foot. THE structure and function of the horse's foot is a subject which is not only of theoretical interest but of supreme practical importance. When I tell you that half the unsoundness and at least half the lameness amongst horses in this kingdom are due to trouble either in or near the foot, you will, I am sure, agree with me that it is impossible to over-estimate the importance of the subject which I have to bring before you this evening. The reason why the foot should be such a frequent seat of unsoundness is not difficult to understand, when we remember the unnatural conditions under which horses both in town and country have to work, and, further, the risk they incur from shoeing. Shoeing is a necessary evil, but the harm resulting from the application of a shoe to the foot is not in itself great; it is the abuse of shoeing which constitutes the danger. The serious and senseless mutilations which are practised on the foot rob shoeing of much of its value, and constitute it a standing reproach to our civilisation. I regret that the time at my disposal will not admit of the question of shoeing being touched on; but during this discourse, as opportunity occurs, I will allude to some of the great evils which are practised in this indispensable art, evils which I may at once say might in a few months be swept away throughout the length and breadth of the land, if the horse-owning community possessed even an elementary knowledge of the manner in which the horse's foot is built up, and the use of its various parts. It is probable that the majority of laymen regard the foot as a solid block of horn placed at the end of the limb, and on which the horse stands. I shall hope to show you that the foot is a highly specialised structure, endowed with tissues possessing acute sensation, mechanisms by which concussion is warded off, a blood supply unequalled in any other part of the body, the whole being enclosed within a covering of horn known as the hoof. The foot is therefore divided into two parts, a core consisting of bones, blood-vessels, tendons and other tissues, which in shape resembles a miniature hoof, and enveloping this a covering of horn |