Lingula grits, followed by blue slaty rocks of the Bala or Caradoc series, capped by (a), which is a boss of porphyrite without visible quartz; the lines b and c denote respectively the planes of stratification and cleavage.

FIG. 2. Diagram showing the relations of the cleavage lines when cutting across a broad but thin quartz bed; the upper plane is an attempt to represent the distortion of ripple marks occurring on a plane of stratification affected subsequently by cleavage, which renders them jagged and irregular by the compression they have undergone. A fine example of this may be seen on the roadside between Pont Aber Glaslyn and the Croesor Valley, North Wales. FIG. 3. Shows the edge of a slate from Llanberris, North Wales, which, instead of being straight, has numerous bends, owing to the cleavage lines being deflected when passing through thin slate beds of unequal hardness.

FIG. 4. Ideal section of a slab of slate from the Penrhyn slate quarry at Bangor, containing a hard pebble of diabase inclosed in the cambrian slate: although interrupting the cleavage lines, its major axis lies parallel to them.

FIG. 5. Yellowish concretions in the purple cambrian slates of Llanberris, (a) in plan, (b) in section, showing how they lie flattened out in the line of cleavage and cut through by the cleavage planes. FIG. 6. A farthing in its normal condition (a); ditto, after passing it longitudinally between the rolls (b); and the same, transversely (c), showing how the image becomes distorted lengthwise or crosswise, according to the direction in which it is rolled. FIG. 7. Section of a mass made up of pipe-clay and smithy scales (a), and a section of the same edgeways, after being compressed to onehalf its volume (b) (Sorby).

FIG. 8. Microscopic section of ordinary slate rock, (a) uncleaved, (b) cleaved.

FIG. 9. Asaphus Homfrayi (Salter), a trilobite from the Tremadoc beds, (a) but slightly flattened out, (b) distorted by cleavage. FIG. 10. Sections of devonian encrinite limestone from Devonshire, (a) cleaved; (b) uncleaved (Sorby).

FIG. 11. Vertical section seen in the cliffs near Ilfracombe (Sorby), showing the crumpling up of hard arenaceous beds when they occur in well-cleaved slate.

FIG. 12. Figures (a, b, c) of normal Euomphalus pentangulatus, and the same distorted in two directions, from Little Island, co. Cork (Haughton).

FIG. 13. Diagram of a stone quarry, which shows the occurrence in one mass of rock of stratification (b b), cleavage (c c), and parallel joints (j) (Murchison).

FIG. 14. Lozenge or rhomb-shaped fragment of cambrian purple slate from Llanberris, North Wales, showing the cleavage planes (bc), bounded by two smooth joints (a b), and the colour bands or stratification (d d), all in a hand specimen of about 4′′ × 2′′ × 1′′ in dimension.

127

THE PLANET JUPITER, 1869-1870.

BY THE REV. T. W. WEBB, M.A., F.R.A.S.

T

HE singular appearances presented during the late season by the planet Jupiter have naturally received, as indeed they have deserved, much attention from astronomers; and in many instances no doubt from those possessed of optical appliances, as well as eyes, superior to my own. This consideration, however, has not induced me to withhold a series of observations which I undertook as far back as the middle of October; for every original contribution to our general store of astronomical knowledge may possess a certain, though it may be a very subordinate value. As regards astronomical phenomena in general, a comparison of descriptions and representations by various eyes and hands, and in the use of different telescopes, points very distinctly to the conclusion, that where so much of personal and instrumental equation exists, a collection of independent details from any fairly reliable quarters may not only be permitted but recommended: in such an accumulation, though error must necessarily be involved, there will be a continual increase of the elements of truth. The readers of the present paper will therefore be content to take my statements as they find them. I have attempted to describe simply what I have seen; not doubting that others may have seen more, and understood it better.

An important distinction has been repeatedly pointed out, between the group of interior and exterior planets, as referred to the wide interval now known to be occupied by a multitude of minuter bodies. Either group, as far as observation extends, or fair analogy will carry us, has a character peculiarly its own; the outer being distinguished from the inner by inferior comparative density, but superior magnitude, velocity of rotation, and attendance of satellites. These remarkable differences, though increasing the interest of such researches as may be permitted to us by the Great Ruler of the universe, add materially to their difficulty; and we find it impossible to carry on to remoter planets the analogies which have apparently served

us so efficiently in the case of our closer neighbour Mars. It is fortunate, therefore, for the purpose of our study, that Jupiter, the nearest at once and the largest of that external group, presents a disc so broad and so luminous as to invite examination even with telescopes of moderate size; and it may be hoped that a persevering scrutiny will produce some interesting result in the end.

μ

The instrument employed in the following observations is a Newtonian silver-on-glass reflector, with a speculum by With, of 91 inches aperture. It is capable of separating y2 Andromedæ and μ2 Böotis, and showing steadily the minute comes of Andromeda; and its definition of Jupiter with a power of about 212 was always as satisfactory as the state of the air would permit; the latter, I regret to add, was frequently such as to offer great and occasionally insuperable obstacles. It was very seldom that I attempted to employ a power of 450, but even when its definition was tolerable, the light was too much diminished to render it of service. I had no micrometer available for such minute details, but trusted to the estimate of a tolerably experienced eye, accustomed to drawing as well as to observation. The series was commenced on Oct. 15, 1869, and continued till March 11, 1870; during which interval the planet was examined, though sometimes to little purpose, on forty nights.

In order to render description more ready and intelligible, I have thought it allowable to assign, for my own purpose, and for the present season, names to the different features of the planet. I have called the brighter stripes, zones, the darker, belts. The central portion, which has recently exhibited such singular details, I have termed the equatorial zone; its two dark borders, the north and south torrid belts; the two bright regions on either side of these, the north and south temperate zones, the latter being subdivided by a dusky stripe, which may be called the south subtorrid belt; beyond these two zones respectively lie the north and south temperate belts; and on the further side of these, the two polar regions. The analogy of which I have availed myself makes of course no pretension to accuracy; but it will answer for present identification, if not for future recognition. A reference to the figures on the next page will exhibit these features to the eye.

We shall now attempt a slight description of these regions in order, beginning from the centre of the disc.

The Equatorial Zone, with the two Torrid Belts that enclose it. These boundaries were always continuous, uniform, and of a grey tint, which probably under more favourable circumstances might have been distinguished as "deepened yellow or yellow-grey, or faint chocolate;" such, at least, is the note

of their hue on occasion of the fine display on Nov. 16; and I thought them purplish on the following night. For variations in depth of shade, or breadth, no standard of comparison existed except the north temperate belt, which certainly wore a more uniform aspect; assuming the absence of change here, it was frequently perceptible in the torrid belts. For instance, on Dec. 11, I have noted that they were both much less conspicuous than formerly, and only about half the breadth and darkness of the north temperate belt; but in the next observation, two nights afterwards, the contrast was not so striking: subsequently, that belt is frequently recorded as stronger and darker than either of them, though without so striking a difference except on Jan. 5 and Jan. 12, when the recurrence of that proportion is expressly mentioned. These changes of aspect were at times very rapid; for instance, on Dec. 25, at 8h. G.M.T. the north temperate and the two torrid belts were for once equal, but at 9h. 45m. the latter had recovered its accustomed deeper tone, In such apparent variations it is not always easy

to distinguish between the optical effect due to rotation and the results of actual local disturbance; in many cases both may have been combined: a remark which may be extended to every portion of the visible hemisphere. On Feb. 25, for the first time, these two belts were noticed to be relatively unequal, the south being considerably darker than the north at 6h. 55m. and 7h. 45m., but at 8h. 15m. the difference had become but slight; and on the following evening at 7h. 20m. both were equally feeble. March 5 and March 11 the south was again rather the darker. The only estimate I find of their apparent dimensions was on Feb. 12, when I thought that the two together amounted to more than but less than of the whole central region.

The intervening space, which we regard as the Equatorial Zone, has been the scene of the phenomena which have attracted such general attention. In the first place its colour has been so marked that it would force itself upon the notice of even a careless observer. I saw it Feb. 5, with as low a power as 110.

There has been some difference in the estimate of its tone, which might indeed be expected, since eyes are known to differ widely in this respect-more, in fact, than instruments: for the same blue 66 which are rays outstanding" or external to the image, in achromatics, are, partially at least, transmitted instead of being reflected by the silver film; so that the result in either case will be similar-the predominance of the complementary orange in the focal image. But whatever might be due to personal or instrumental equation, the contrast of the central band with the rest of the disc was most obvious. To my eye the colour appeared to be a ruddy or brownish-yellow. I had an impression towards the end of the year that it was becoming less decided; but this idea was subsequently shaken, and the suspected differences may have been due to the difficulty of such comparative estimates, or perhaps to actual inequality of tint in different parts of the zone.

Across this coloured background, a number of grey shadings were projected from the inner edge of the south torrid belt, usually at right angles to it, though sometimes a little deflected eastward, extending about half-way across the yellow zone. These, in a bad state of the air, could not be made out, or only appeared as confused and shapeless markings; under better definition it could be perceived that their further extremities were bent right and left so as to form a succession of loops or festoons, whose undulating edge usually ranged along the equator. From a rough graphical estimate there may have been sixteen or eighteen of these loops around the girdle of the planet, and they appeared to exist on every side of the globe, with a surprising general similarity of character, though not exclusive of local differences as to distinctness and symmetry of form. One would frequently surpass in breadth all the others in sight, and at times they were darker or better marked on one side of the disc than on the other. Dec. 25, they were shorter and narrower than before, but the usual type soon returned. In one very remarkable view (Nov. 16), six of them assumed the aspect of a bridge; the piers being very clearly made out in the centre, though more difficult towards either limb; and as they were always broader at their origin in the south belt, the resemblance to elliptical arches was very striking, notwithstanding some want of precise correspondence. A faint idea of this appearance may be obtained from fig. 1. On this occasion, though I have noted that there was "a great profusion of minute detail," which I had unfortunately not sufficient time to analyse or sketch, the festoon form was only partially and slightly indicated. But on the following night, Nov. 17, another side of the globe, about 150° removed from that previously visible, exhibited, though feebly through a sheet of streaky haze, a most