which he was thereby led. With reference to the form in which he enunciated the law regulating the action of a magnet on a uniaxal crystal-that the optic axis is attracted or repelled by the poles of the magnet-he disclaims any intention of assigning a physical cause to the phenomenon, or doing anything more than expressing the results of observation, which are as if such a force existed. In the case of crystals of a more complicated character, he was led, in the first instance, to assume the existence of two magnetic axes, possessing a similar character as to attraction and repulsion with the one axis of optically uniaxal crystals. But finding that the proposed law did not hold when the crystal was examined in all directions, and not solely along peculiar axes, he abandoned, nearly two years ago, a hypothesis respecting which serious doubts had arisen long before. For the hypothesis of one or two axes acted upon by the magnet, he substituted another similar hypothesis. In the case of uniaxal crystals he now conceived an ellipsoid of revolution, consisting of an amorphous paramagnetic or diamagnetic substance, and having within the crystal its principal axis coincident with the principal crystallographic axis. It is easy to verify that both crystal and ellipsoid, the poles of the magnet not being too near each other, will be directed between them in exactly the same way. In the generalization, an ellipsoid with three unequal axes, having a determinate direction in the crystal, must be substituted for the ellipsoid of revolution. In this hypothesis too, two "magnetic axes" are met with, that is, according to the new definition, directions which possess, in common with the single crystallographic axis of uniaxal crystals, the property that if the crystal be suspended so that either of these axes is vertical, and the body is at liberty to turn freely round it, no extraordinary magnetic action is exhibited, but the crystal behaves like an amorphous substance.

According to observation, a crystal under favourable circumstances is directed in the same way as the smallest of its fragments. Hence, according to the new hypothesis, each of its particles may be regarded as acted on like an amorphous ellipsoid. But such an amorphous molecular ellipsoid, when influenced by a magnetic pole at a finite distance, will be directed like an ellipsoid of finite dimensions under the influence of an infinitely distant pole. Here Poisson's theory presented itself for the verification of the hypothetical conclusions and their consequences, to which the author had been led by considerations of a different kind. This verification had the most complete success. But before proceeding to it, it was found necessary to confirm Poisson's theory itself (or rather the results following from it), with respect to an ellipsoid of finite dimensions influenced by an infinitely distant pole. By means of a beautiful theorem lately published by Professor Beer, by which the results relating to the influenced ellipsoid are simply and elegantly expressed by means of an auxiliary ellipsoid, the author was enabled to deduce immediately the analytical expressions. These were afterwards compared with experiment, by obser vations made on two carefully worked ellipsoids of soft iron, executed by M. Fessel of Cologne.

The results thus obtained from theory, and verified by experiment, with reference to an amorphous ellipsoid, were compared with the results obtained from the observation of crystals, and manifested a complete agreement. According to this theory, the magnetic induction within a crystal is, like the elasticity of the luminiferous ether, determined by means of an auxiliary ellipsoid. As there are three rectangular axes of optical elasticity, so there are three principal axes of magnetic induction, characterized by the property that if a crystal be suspended along any one of them, the two others set, one axially, and the other equatorially. As there are two optic axes, situated in the plane of the axes of greatest and least elasticity, so there are two magnetic axes, characterized by the property already mentioned.

Among crystals, the author selected for special examination red ferrocyanide of iron, sulphate of zinc, and formiate of copper. The first is paramagnetic, the second diamagnetic, and in both cases the principal axes of magnetic induction are determined by the planes of crystalline symmetry. The setting of elongated prisms, as well as of long cylinders and short cylinders or circular plates, cut in various selected directions from the crystals, is described in detail. The use of both cylinders and circular plates, cut with their axes in the same direction, obviated any objection which might be raised attributing the setting to the external form, since, so far as was due to mere form, a cylinder and a circular plate would set with their axes in rectangular directions.

Formiate of copper differs from the former crystals in having but one plane of crystalline symmetry, and accordingly in having but one principal axis of magnetic induction determined by the crystalline form. The existence of three principal magnetic axes, having the property already mentioned, was demonstrated experimentally, and the directions of those two which were not determined by the crystalline form, were ascertained by experiment. In this crystal the axes of greatest and least induction, and consequently the magnetic axes, lie in the plane of symmetry; and the existence of two magnetic axes was demonstrated, and their positions were determined.

In conclusion, the author gives a list of crystals, classified according to their paramagnetic or diamagnetic characters, and the order of magnitude of the magnetic inductions in the direction of their principal axes. He also remarks that some crystals, of which instances are given, though belonging according to their form to the biaxal class, have two of their principal magnetic inductions so nearly equal that they cannot be distinguished from magnetically uniaxal crystals; while others, though not belonging to the tesseral system, have all their principal inductions so nearly equal that they cannot be distinguished from amorphous substances.

LV. Intelligence and Miscellaneous Articles.

ON A NEW STEREOSCOPIC PHENOMENON. BY M. A. CIMA.

TAKE a drawing of a head seen in front, of 3 or 4 centimetres in

height; it may be lithographed, or engraved, or drawn with a pencil. I cut this drawing in the direction of the vertical axis of the nose, and arrange these two parts of the drawing in the same vertical plane, before the eyes, at a less distance than that of distinct vision. I remove or bring together the two parts of the drawing until the two images which result from their duplication, combine so as to form the entire face. The image of this face thus obtained presents the appearance of a solid object, or of a modelled figure in which the nose, the cheek-bones, the chin, and the eyebrows are detached, as in an object in relief. This sensation of relief increases in proportion to the length of time during which the two images are viewed; to obtain the maximum of effect, the two halffaces must be held at a suitable distance, which varies according to the observer. A similar, but much less perfect effect is obtained by looking at the drawing of the entire face, either with one or both eyes, at a distance much less than that of distinct vision.

I think that the explanation of these phænomena must be founded upon the two following facts, which led me to make the observation which is the subject of this note. The eye sees of a gray colour a series of very small alternate black and white spaces, very close to each other, and seen at a distance less than that of distinct vision. In looking at a single face divided into small squares alternately black and white, at a distance less than that of distinct vision, the white squares appear larger, the lines of contact between the white and black squares become of a gray colour, which extends gradually, and the black squares appear larger; at the same time the white squares appear to be raised at the centre and the black squares to be hollowed. This double property of our eye, which is due to ocular irradiation, furnishes us with a sufficient explanation of this new stereoscopic appearance.-Comptes Rendus, October 26, 1857, p. 664.

NOTE ON M. RUHMKORFF'S INDUCTION COIL.

To the Editors of the Philosophical Magazine and Journal. GENTLEMEN, Boston, Mass., Oct. 20, 1857. I notice, in the September Number of your Magazine, my paper from Silliman's Journal, giving a description of my construction of the Ruhmkorff induction coil. I have since (in July last) constructed one with 60,000 feet of wire on the same principle, which gives sparks of 10 inches in the atmosphere with two cells, Bunsen's battery. This instrument is in the possession of Columbia College, New York.

Respectfully yours,

E. S. RITCHIE.

LONDON, EDINBURGH AND DUBLIN

PHILOSOPHICAL MAGAZINE

AND

JOURNAL OF SCIENCE.

SUPPLEMENT TO VOL. XIV. FOURTH SERIES.

NOT

LVI. Observations upon the Structure of Glaciers.
By JOHN BALL, M.R.I.A., F.L.S.*

WOT a few of those who have paid attention to the interesting questions connected with the movement and structure of glaciers, have long felt that, in spite of the efforts of many able men who had observed the phænomena on the spot, and of the progress made towards understanding their physical theory, for which we are chiefly indebted to Professor J. Forbes, much still remained both of fact and theory that called for further investigation on the part of men who should combine habits of careful observation with a mastery over the principles of physical science.

The paper read last January before the Royal Society, by Professors Tyndall and Huxley, followed by a lecture delivered at the Royal Institution by the first-named gentleman, has been welcomed by such persons as a material advance towards the completion of our knowledge on these subjects.

The theory of glacier motion may, indeed, be now considered complete. Professor Forbes had proved that glaciers in their gradual descent move after the manner of viscous fluids; Prof. Tyndall has now brought to light that property of ice by which rigid masses of glacier are enabled to conform themselves to the laws that regulate the motion of imperfect fluids.

The question remains, whether the able and ingenious authors of the paper read before the Royal Society have been equally successful in explaining the physical causes of the singular and complicated structure which careful examination detects in all known glaciers; and with a view to satisfy some doubts on this subject, I endeavoured, during a recent visit to the Swiss Alps, to observe with attention such peculiarities in the structure and arrangement of those glaciers which I was able to visit, as might help to test the sufficiency of the new theory.

* Communicated by the Author.

Phil. Mag. S. 4. No. 96. Suppl. Vol. 14.

21

Although circumstances did not permit me to undertake any continuous series of observations, accompanied by accurate measurements, it has seemed to me that some notes made at the time, and of the accuracy of which, as far as they go, I feel confident, may deserve the notice of those who are interested in the subject. If I have ventured to draw some conclusions from facts, or to point out what appear to be objections to the deductions of other more competent physical observers, I am well aware that it becomes me to do so with great hesitation. That hesitation is somewhat lessened by the expectation that the subject is about to receive further illustration from the continued labours of Professor Tyndall, and to no more able and candid judge could any facts or arguments seeming to tell against his own conclusions be proposed.

The chief, but not the only peculiarities of glacier structure that have attracted the attention of observers, are the following:

1. Stratification of the Névé.

2. Veined structure of the middle and lower region.

3. Dirt-bands of Professor Forbes.

4. Dirt-bands of M. Agassiz*.

5. Crevasses.

6. Capillary fissures in the compact ice.

7. Air-bubbles included in the ice.

These are all so familiar to those who have either observed for themselves, or who have studied with care the writings of Forbes, Agassiz, and other glacier authorities, that it would be superfluous to describe them, and I shall merely offer such remarks in regard to each of them as have some bearing on the theory of glacier structure.

I. The stratification of the névé, its arrangement in parallel layers of more and less crystalline snow, the diminished thickness of the layers in the lower part of exposed sections, accompanied by a consolidation of the substance of the névé into ice, more or less filled with air-bubbles, are facts familiar to those who have dwelt much in the upper regions of the ice-world, where in concave hollows, or still more often, on slightly convex plateaux, the vast accumulations are formed which give birth to the greater glaciers.

On one point, many of those who have described the formation of these beds seem to be certainly in error.

Each of the horizontal beds seen in an exposed vertical section of the névé is supposed to correspond with the whole annual fall of snow at the spot where the bed was formed. But this inference rests either on a great exaggeration of the effects of * Recently named Dirt-Streaks by Professor Huxley.