LIV. On the Relation of Mechanical Strain of Iron to MagnetoElectric Induction. By G. GORE, Esq., F.R.S.*

OR making experiments on this subject I have constructed the following apparatus:-A A' is a strong base or plank of oak about 120 centims. long, 18 centims. wide, and 7·5 centims. thick (the thinner part of which, marked A, may with advantage be left of the same thickness as that of A'). B is an iron lever about 72 centims. long, and 1.5 centim. thick at its lower end, capable of moving in a vertical plane upon a strong horizontal rod or axis of steel C, about 2 centims. diameter, driven tightly into a hole in the wooden base; the lever has a piece of iron Ď (shown in section in fig. 2) welded upon its upper surD

face, with a hole 6 millims. in diameter drilled through it for the reception of one end of the iron wire. E is a slot cut in the wood for the reception of the lever. The wire to be strained cannot be shown in fig. 1, because it lies in a narrow groove FF, about 8 millims. wide and 3 centims. deep, cut in the base-board, the wire being at a distance of about 1.5 centim. below the surface of the board; its position is shown by the horizontal dotted line in fig. 2. G is a coil of insulated copper wire; it is about 18 centims. long and 43 centims. in diameter, with a hollow axis of glass tubing of about 8 millims. internal diameter; it contains about 3295 turns of wire 0.54 millim. thick (=No. 26 Birmingham brass wire-gauge), the two ends of the wire being attached to the binding-screws H, H. The coil lies tightly fixed by wedges of wood in a rectangular space in the wood, with its axis coincident with that of the iron wire. The iron wire is kept in the same horizontal plane by passing over two brass rollers (without axles) I, I, shown in section in fig. 2; these rollers are about 2.5 centims. long and 1.5 centim. in diameter, and lie upon the bottom of the hollow rectangular spaces in the wood shown at J, J. K, K are two moveable rectangular pieces of oak which slide vertically and tightly in narrow grooves formed

* Communicated by the Author.

in the sides of the rectangular space in the base-board; they have fixed to them two small vertical plates or armatures of soft iron, the lower edge of each of which has a small notch filed in it which fits and presses upon the iron wire; and when the apparatus is going to be used, a permanent bar-magnet L is placed with its poles resting upon the upper ends of these armatures. The iron wire employed is usually about 4 millims. in diameter, and the apparatus is sufficiently powerful to readily elongate iron wire of that size. M is a rectangular or T-piece of wrought iron of the form and dimensions indicated by the figures; it is moveable, and may be readily taken off the end of the base-board by removing the screws N (of which there is one on each side). The ends of the wires require to be held very securely during the strain; this is perfectly effected by having in D and in M two steel screws each 6 millims. diameter. The level of the top surface of the small brass rollers is slightly above that of the ends of the iron wire.

By means of this apparatus I have repeatedly verified the fact that a magnetized soft iron wire, during the act of being stretched (either with temporary or permanent elongation), increases in magnetism, and produces a current of electricity in the coil of wire G in a contrary direction to that of the hands of a watch, i. e. in

the direction of

when we are looking at its south pole, cor

responding to that of the north pole of the magnet above it. With a galvanometer of moderate sensitiveness, the amount of deflection of the needles was generally about 14 degrees on one side by a single stretch of the wire, or 17 degrees on each side by repeated synchronous stretches. No deflections were obtained by stretching thick wires of zinc or copper. The apparatus was placed in a direction of east and west during the experiments.

The results obtained with iron agree with the fact discovered by Mr. Joule, that straining an iron wire lowers its temperature, and also with the well-known fact that lowering the temperature of a piece of iron increases its magnetic capacity.

LV. On the Bearing of Recent Observations upon Solar Physics. By G. JOHNSTONE STONEY, M.A., F.R.S., &c.*

97.

THE

HE main difficulty in dealing with solar and stellar physics arises out of the scantiness and fragmentary character of our data. This makes the inquiry a very treacherous one-so much so, that some of the most eminent men have

* Communicated by the Author.

gone

astray in attempting it, and no one that has ventured upon the enterprise can deem himself secure. For, on the one hand, that which at any stage of information appears the most obvious account of a phenomenon is often not the true one; and the meagre knowledge we have of solar phenomena seldom suggests much beyond the obvious account of them; and, on the other hand, if we are led astray on any point, we have little to set us right, as we cannot select our test; and all we can do is to lay down correct principles of interpretation, to bring to bear from every quarter such shreds of knowledge or information as can be laid under contribution, and to review all, from time to time, in the most cautious and painstaking manner.

We may therefore well hail with satisfaction the additional news of the sun which the late eclipse has brought us. These fresh items of intelligence are chiefly two :-that the light of the corona, so far as it was examined (that is, in its upper regions), is much of it polarized in planes passing through the centre of the sun; and that the spectrum of the prominences which were examined contained bright lines. It is with these particulars that I wish at present to endeavour to deal.

98. The protuberance upon which all the observers*, except Captain Haig, appear to have bent their chief attention was an unusually brilliant and lofty one on the eastern limb of the sun, nearly opposite to a solar spott. It emitted five bright lines, and apparently four fainter ones not seen by most of the observers. Major Tennant satisfied himself that the hydrogen line C was one of the bright lines, the sodium line D another, and the magnesium line b a third‡. He had not time to measure more; but his estimate of the positions of the two other lines which he saw suggests their being the two other principal hydrogen lines, viz. F and the hydrogen line close to G. Lieutenant Herschel's measures identify one line with D, and place another near F§. Mr. Lockyer, examining another prominence since the eclipse, has satisfied himself that he saw as bright lines C, F, and a line near D. M. Rayet, who records nine lines, supposed the brightest lines to be B, D, E, F, and G, “judging

* There is some uncertainty whether this was the prominence examined by Major Tennant, since he speaks of the prominence he examined as being on the preceding, i. e. the west, side of the sun. But his description of its aspect does not seem to accord with the appearance of any other than the great flame seen on the eastern limb, and he refers to its being conspicuous immediately before the sun reappeared as evidence of its great height, which also seems to imply that it was on the eastern side.

† Proceedings of the Royal Society, vol. xvii. p. 79.

Monthly Notices of the Astronomical Society, vol. xxviii. p. 245.

See The Engineer' of November 6, 1868, p. 346.

|| Proceedings of the Royal Society, vol. xvii. p.

91.

from their arrangement in the field of view, the intervals between them, their colour, and general aspect "*. To bring this into harmony with the preceding determinations in which measures were made, we must suppose that M. Rayet's B and E were in reality C and b. It is to be observed that the intervals between B and C, and between E and b, are the two smallest of the intervals between lines lettered by Fraunhofer, and therefore that these rays are the most liable to be confounded in such eye-estimates as M. Rayet made. The details of M. Janssen's observations have not reached Europe; but he describes hydrogen as detected in the principal protuberance; and therefore his report also inclines in favour of C. On the whole, then, we may with most safety conclude that the lines seen were probably C, D, b, F, the hydrogen line near G, and four other very faint lines. Those which seem to be identified are the principal lines of hydrogen, sodium, and magnesium-the three gases in the solar atmosphere which, as I have elsewhere shown†, project far beyond all other known constituents.

99. The most probable account to be given of this great prominence would appear to be that which I ventured to advance in a paper which was presented a year and a half ago to the Royal Society (see Proceedings of the Royal Society, vol. xvii.). [For convenience of reference the paragraphs of this paper are numbered in succession after those of the paper in the Proceedings of the Royal Society.] There is a low-lying shell of attenuated clouds outside the photosphere, at a distance from it of somewhat more than one semidiameter of the earth. Now, in the memoir already referred to, I have shown (§ 68) that if a disturbance in the lower parts of the outer atmosphere heave a part of this shell of cloud above its natural level into a cooler region, it will, from its continuing to be exposed to the unmitigated glare of the photosphere, import an enormous amount of heat into its new neighbourhood, which will have the effect of so heating the air through which the mist is dispersed, that the entire mass of cloudy particles and interposed gases will commence a violent ascent, and thus both produce a cyclone over the photosphere which may result in a spot, and give rise above to such great columnar flames as the one which has been found to give out these bright lines. When writing my paper I did not overlook the circumstance that the intermingled gases would emit bright lines; but I was under the impression that all such lines would be extinguished by the absorption of the surrounding colder parts of the atmosphere through which they should make their way

*Comptes Rendus, October 12, 1868, p. 758.

† See memoir on the "Physical Constitution of the Sun and Stars," passim, Proceedings of the Royal Society, vol. xvii.

Phil. Mag. S. 4. Vol. 36. No. 245. Dec. 1868.

2 G

before we could be spectators of them. It now appears that some, at least, of them escape-or, to speak more accurately, the marginal parts of some of them; for it will probably be found, if they can be submitted to examination with a sufficiently fine slit, that there is a central stripe of each blotted out by absorption, and appearing as a narrow dark line in the middle of each bright line. We must also bear in mind that there may be other lines emitted so narrow that they cannot cross the surrounding atmosphere.

100. Which lines then reached the earth? Plainly those which, as they were emitted by the incandescent gas, were broader than the corresponding dark lines of the solar spectrum, or at least than that part of the width of the latter which is due to the absorption of those strata of the sun's atmosphere through which we have to look in viewing the prominence. And as even lines of the same gas differ from one another in this tendency to spread out when heated, we see at once how one hydrogen line, C, could seem to Mr. Lockyer to extend further downwards in the solar atmosphere than another, F*. The lower part of F was no doubt effaced by the greater absorption of a hotter part of the atmosphere telling with more effect upon F than upon C, We must remember, too, that the ascending column may carry with it gases from lower regions of the solar atmosphere to heights not usually reached by them. Any rays which they would then emit while in the gaseous condition would be unobstructed in passing out through the surrounding atmosphere; and such may be some of the faint lines seen by M. Rayet, However, such constituents would probably, for the most part, soon cease to be gaseous, and augment the cloud by being chilled into a mist, or by hanging on the limits between their precipi, tated and gaseous conditions in the way explained in § 71. In any of these ways their spectral lines may be either enfeebled or suppressed.

101. Solar clouds may be of various kinds (see §§ 69 and 71). But the light from a sufficiently attenuated mist of particles giving a continuous spectrum would have the property of being so weakened in the spectroscope by being spread over the whole length of the spectrum, that it might be easily overlooked in such hurried observations as were made last August. It appears probable, therefore, that the protuberance which was most

* Comptes Rendus, October 26, 1868, p. 838.

Thus if a salt of sodium be burned in a candle-flame, though the sodium adds but little to the aggregate light of the flame, yet in a spectroscope with a sufficiently narrow slit the flame will give a background of almost insensible faintness upon which the two sodium lines are brilliantly rojected.