In my Letter to Mr. Herschel, published in the Transactions, I communicated the results of some experiments which I had made, with the view of determining according to what law of distance the magnetism developed in copper and other metals during rotation, varied. I was aware, that all I could offer at that time must be considered as distant approximations towards useful results; but as I had witnessed several of the very interesting experiments in which Mr. Herschel and Mr. Babbage were engaged, I was desirous that they should be in possession of whatever results I had obtained. In these experiments, I had made use of a thick copper plate revolving under a magnetized needle, and of magnets revolving under a copper disc; so that, at different distances, some of the forces which were brought into action during rotation, had very different angles of inclination to the plane of the needle, in the one case, and to the plane of the disc in the other; and in consequence of this, the results were by no means uniform. In order to remedy this, I proposed, instead of a disc, to make use of a copper ring, as, in this case, the poles of the magnet revolving vertically under it, no lateral forces would be called into action; and I expected that the law, according to which the force acting during rotation varied at different distances, would be determined with great precision. In this expectation I was not disappointed; and I now propose laying before the Society an account of these experiments, and of the results which I obtained from them. In making some preliminary trials with thin flat rings, I found the effects produced so very much less than with a disc of the same weight, that, previously to pursuing the enquiry which I had at first in view, I was induced to make some experiments, in order to ascertain the effects that would be produced by a simple solution of continuity, in a circular disc, by concentric sections, and likewise by successively removing concentric portions. These experiments clearly show, that the intensity of the magnetism developed during rotation, is not alone materially affected by a separation across, what may be termed, the path of the induced pole, as has been found to be the case by Mr. Babbage and Mr. Herschel; but that a separation, concentric with that path, by which the pole is undisturbed in its progress, is equally efficacious in diminishing the intensity of that pole; and that in the magnetism of the whole, when all the parts are continuous, there is in all cases a great accumulation of intensity above the sum of the intensities of the separate parts. This is so important a feature in the phenomena depending upon rotation, that these experiments may not be considered uninteresting: I shall therefore give an account of them previously to entering upon the principal object of this communication.