was very distinct by two dark spots. Nitsch took these spots for eyes, which they certainly are not, but the origin (if not the exit, against which, however, we have all analogy) of the ovaria, which are here destitute of the granular tissue. These spots sometimes assumed a spiral, sometimes a forked or double forked, form. With regard to the organs of motion, the mouth is large, more or less extensile, and surrounded with a notched or plain margin or ring; it is bell-shaped, and is attached to the body by a narrow pedicel. The tail has lateral indentations and longitudinal striæ or fibres towards its middle. I am inclined to suppose a union of transverse and longitudinal fibres, true muscular tissue, for I am convinced that these animals possess muscles quite analogous to those of the higher animals, and Ehrenberg has demonstrated it with regard to the hydatina. How the tail is fixed to the body, I have not been able precisely to determine, but it is probable that a sort of prolongation is fixed into a notch in the hinder part of the trunk.

These few observations on the infusoria cannot be compared with those of Nitsch, Baer, and Ehrenberg; they are only a fragment towards the completion of the history of this vast kingdom of nature. In conclusion, I must add a word to explain why I have taken no notice of two recent observers of infusoria-Bory St Vincent and Muncke; but so much do I value the labours of O. F. Muller and Schrank, that I owe it to truth to assert that I consider the communications of the former as quite lost for science. When Muncke says that he has devoted from three to four hours every day for three weeks to the observations which were laid before the meeting of Naturalists at Hamburg, we can only lament that a man of acknowledged reputation as a natural philosopher has spent so much time uselessly upon matters with which he was quite unacquainted. It is worse with Bory; for it is quite incomprehensible how any man can be considered as a naturalist of eminence, who displays in his writings an inaccuracy, superficiality, and ignorance, that must be frightful to every one who examines closely his works, and as revolting as the frivolity of which his Histoire Naturelle de l'Homme is so striking an example.

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An Exposition of some of the Laws and Phenomena of M10NETIC INDUCTION, with original Illustrative Experiments. By the Rev. WILLIAM SCORESBY, F. R.S. Lond. & Edin., Correspondent of the Institute of France, &c. &c. &c. Communicated by the Author.

THE

HE magnetic principle, like the electric, to which it is nearly allied, is not a mere attribute of a particular class of bodies, but a principle or influence pervading, doubtless, the whole of the terrestrial creation. For it is not likely that such an influence has been ordained by Infinite Wisdom for the comparatively minor purposes to which men are able to apply it; but rather that it is an essential constituent in the economy of the globe,—and not of this globe only, but of the entire system, perhaps, of created nature. Wherever the exploring traveller has urged his way, there its influences have been marked; wherever the adventurous mariner has traversed the ocean, there its agency has availed him; wherever the laborious miner has penetrated the earth, there its energy has been found undiminished; and wherever the daring aeronaut has ascended into the atmosphere, thence its power has been extended.

Though a universal agent, however, and a part of the constitution of our globe, it is chiefly in ferruginous bodies, and in bodies in a peculiar electric condition, where its phenomena become sensible, and its influences capable of being controlled. In ferruginous bodies, its strongest and most permanent energies are exhibited.

In iron, the magnetic principle has evidently permanent residence,-capable, indeed, of exerting external influences, but not capable of being abstracted or increased. Each portion and description of iron has its own constant and unalterable quantity, abiding apparently in its individual particles,—the two qualities possessing northern and southern polarity existing in every particle. The usual condition of the magnetisms is generally neutral, or nearly so, so that but a slight and imperfect energy is naturally evinced. Yet the latent energies, especially in the softest kinds of iron and steel, are readily developed by electric influence, or by the touch, or even the mere juxta-posiVOL. XIII. NO. XXVI.-OCTOBER 1832.

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tion of an active magnet. For though the natural quantity of inherent magnetism cannot be altered, yet an active and constant influence may be, and is, externally exerted, without in the smallest degree diminishing the original power of the actuating magnet. That this is the case, a well known fact sufficiently proves, that a magnet may elicit the magnetic condition in ten thousand bars of steel, and yet retain its original strength perfectly unimpaired. And that the magnetisms reside inalienably in their own particles of the metal, is equally evident from another familiar fact,—that if a bar in a magnetic condition be broken in the centre, so as to separate apparently the northern and southern polarities, each portion, instead of comprising one quality only, will be found to be a distinct magnet, exhibiting, like the original mass, the two different polarities at the extremities.

These general principles being premised, we are prepared for the consideration of the particular objects of this Essay.

CHAP. I.-EXPOSITION OF SOME OF THE LAWS AND PHENOMENA OF MAGNETIC INDUCTION.

INDUCTION is that well known property of magnets, of producing in contiguous ferruginous substances the magnetic condition. It is not, however, strictly speaking, the communication of any thing previously foreign to the ferruginous bodies, as nothing (as we have shewn) is abstracted from the inducing magnet, and nothing in reality infused into the iron thus magnetized. Induced magnetism, therefore, may be defined,—the development of the latent magnetism in iron or steel, by the juxtaposition of any substance in a magnetic condition. For this property is elicited not only by actual magnets, but by all electro-magnetic or thermo-magnetic arrangements. And the inductive effect is produced upon all substances capable of a magnetic condition, according to their respective susceptibility. The degrees of capacity of different ferruginous bodies for magnetism by induction may be ranged, beginning with the least susceptible, in the following order; iron-ores, hard cast iron, hard cast steel, hard blistered steel, soft steel, common malleable iron, best Swedish iron,-the most susceptible of all being the softest and most ductile iron. Hence it is probable, that the measure of

the relative capabilities of iron for induced magnetism, may be employed as a satisfactory test of quality in its several kinds.

At small distances from a powerful magnet the inductive energy is very striking, and productive of a number of well known and interesting phenomena. Let a piece of soft iron, for instance, such as a common key, be placed near the extremity of a bar-magnet, either in continuation of the same line or inclined to each other at any angle, provided either of the extremities of the key and magnet are the parts nearest to each other, and the key so placed will instantly acquire such a magnetic condition as to be able to support another smaller key or other portion of iron at either of its extremities. And though the magnet be placed beneath the table, or under a slab of marble or any other solid substance, the inductive influence will be precisely the same, so as the distance and position of the masses are similar.

But these phenomena, which have usually been observed only in circumstances of juxtaposition, may be satisfactorily exhibited at considerable distances, as shewn by the following experiment.

A pair of very fine bar-magnets, 3 feet in length, 24 inches in breadth, and 4th of an inch in thickness, being placed over each other, a quarter of an inch asunder, and with similar poles adjacent, were employed, with a view to their inductive effects, at different distances, on the nearest end of a soft bar of square iron, 13 inches long, and 1 inch in thickness. In order that the inductive influence might be separated from any magnetism of position derived from the earth, the iron bar was placed in the east and west magnetic line, and in a horizontal position; and that the more delicate experiments might not be affected by the magnetism acquired under the more powerful influences, the most distant and weakest inductions were first tried, and each experiment verified by taking the mean power of each end of the bar alternately presented to the magnets. The iron bar thus situated (Plate II. Fig. 1.) sustained by the extremity nearest to the magnets (when all the points of contact were polished) the substances and weights, at the distances respectively annexed to each, as given in the first three columns of the following table:

But these results, if we consider the nature of the polarities induced in the bar and the suspended substance in their relation to each other, will be seen to be less than what might be obtained by simple induction at the respective distances. For, let the acting end of the magnets be the north pole, then the magnetism induced in the nearest end of the iron bar will be the contrary, or southern polarity; but the tendency of the magnets on the end of the nail, or other suspended substance, in contact with the bar, will be to produce also southern polarity, because of this end of the nail, under the circumstances, being nearer to the magnets than the lower end; consequently, the induced magnetisms of the parts of contact of the bar and nail, being, as far as derived from the magnet, of the same kind, must have a tendency to diminish the action of the bar on the nail.

In order to compensate this defect in the results, another series of experiments was made with the same apparatus, in which the iron bar, whilst kept in the same vertical plane, was raised on each trial so much above the level of the magnets, namely, one half the length of each suspended nail,—that the influence of the magnets upon the suspended body might be similar at both its extremities, and consequently neutral. Under this arrangement (Plate II. Fig. 2.) the inductive power, as had been anticipated, was much greater, so that the different. substances were now suspended according to the distances in the last column of the table.