ing dark spaces are exhibited, but the darkening is not bounded by a defined outline.* A similar darkening of the outer bright ring, near the extremities of the major axis of the great division, also appears in several pictures of the planet taken at this time. These dusky regions have been termed the shadows on the ring,' a term not very well chosen, as will presently appear. Figs. 2 and 3, Plate I., exhibit the general appearance of these dark spaces:† their nature is discussed in Chapter V.

In the years 1855, 1856, and 1857, Messrs. Bond and Dawes were occasionally able to trace dusky, ash-coloured, and mottled stripes, concentric with the outlines of the rings. These were not always visible, however; they reappeared along different circles on the rings' surface; and, in fact, were as variable and mysterious as the dark traces of division.

A singular discovery was made by Mr. Wray during the disappearance of the ring in the winter months of 1861-62. Observing Saturn on December 17th, 1861 (when the dark side of the ring was turned towards the earth), with an achromatic of only seven inches' clear aperture (with which he expected to be able to detect no trace whatever of the ring), Mr. Wray was surprised to find the illuminated edge of the ring distinctly visible, not only where it crossed the dark shade on the body, but also extending on each side of the planet's margin.' Continuing his observations, he was led * The same phenomenon is indicated in Plate I. of the first edition (1833) of Sir John Herschel's 'Outlines of Astronomy.'

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+ Mitchel, with a less powerful instrument than Bond used, saw these spaces less satisfactorily. He writes, 'I have sometimes been confident that the breadth of the dusky ring at the extremities of its longer axis was much greater than that which would be due to an elliptical figure concentric with the bright rings.'

It seems clear, however, that what Mr. Wray has described as 'the edge of the ring crossing the dark shade on the body,' was a strip of the planet's surface. For, at the time of this observation, the earth was on the northern or dark side of the ring, and therefore, in an inverting telescope, the bright edge of the ring must have been the upper boundary of the dark surface; and the sun being on the southern side of the ring, the shadow of the ring was north of the ring, or, in an inverting telescope, below the ring. Both the dark stripes being below the bright edge of the ring, of course this edge could not be seen between them. But the sun being much more elevated above the plane of the ring on one side of it than the earth on the other, it is clear that a strip of the planet's surface must have been visible between the two dark stripes, and it is this strip (I imagine) which Mr. Wray mistook for the rim of the ring. Mr. Wray's sketches do not, it is true, accord very well with this explanation; but it is possible that, in drawing these figures (which he expressly describes

F

(Dec. 23rd) to suspect that the edge of the ring was thicker and somewhat nebulous about the region on either side, where it joins the planet's limb.' Finally, on Dec. 26th, he completed the discovery of certainly the most singular phenomenon detected in the appearance of the rings, since the discovery of the dark ring. The atmosphere being fine, and the image of Saturn exquisitely steady and well defined,' he observed a prolongation of very faint light stretched on either side from the dark shade on the ball, overlapping the fine line of light formed by the edge of the ring, to the extent of about one-third of its length, and so as to give the impression that it was the dusky ring, very much thicker than the bright rings, and seen edgewise, projected on the sky.' He saw this faint overlapping light on four other occasions, in January, 1862. M. Otto Struve, using the magnificent refractor of Pulkowa, rediscovered these singular appendages when the plane of the ring was passing through the sun, in May, 1862. M. Struve observed that 5 hours before the computed time of this passage (which took place on the 18th of May, at 8h. 30m. A.M.), one ansa of the ring was plainly visible; and on the 19th of May he was able to trace the luminous appendages along the ansæ. He had seen them less distinctly on May 15th, when they appeared only on the southern (or, in an inverting telescope, the upper) side of the ansæ. He describes them as resembling' clouds of a less intense light lying on the ansæ.' On May 19th, they appeared to him to differ much in colour from the ordinary colour of the ring;' to be, not yellow, but more of a livid colour, brown, and blue.' Later, he saw them still more favourably; they appeared unequal in length along the two ansæ; extending on one side to a distance equal to about two-fifths of the planet's diameter, on the other half as far again; the breadth ‘of these appendages increased in the neighbourhood of the planet, giving them the form of sharp wedges.'

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as rough), he was more careful in giving the details of the appearances which the sketches were meant to illustrate, than the outlines of the thin lines of light visible across the disc, and on either side of it.

In Chapter IV. the reader will find a complete account of the disappearances and reappearances of the rings in the years 1861-1862.

* The observations of Wray and Struve are recorded in the 'Reports of the Astronomical Society,' for January, 1863, from which the above extracts are taken.

ances.

Mr. Carpenter, using the Greenwich equatorial, observed Saturn on the same days as M. Otto Struve, without detecting these appearIt may therefore be concluded that they are so little conspicuous, that a slight difference in atmospheric conditions affects their visibility. The earth was on the southern side of the rings throughout the observations of M. Struve, and raised more than five times as high above the plane of the rings as it had been during the observations of Mr. Wray; it was also nearer to the rings. On the other hand, the sun, which throughout the observations of M. Struve was nearly in the plane of the rings, was raised from 1° 50′ to 2° 16′ above that plane during Mr. Wray's observations. The discrepancies between the two accounts are not greater than we might expect from this difference in the circumstances under which Saturn was observed. The probable nature of these appendages will be considered in Chapter V.

On March 26th, 1863, Mr. Carpenter made an observation of some interest. When Saturn was passing across the field of view of the transit-circle of the Greenwich Observatory, it appeared to him 'that the dark space between the ring and the ball was much contracted.' Upon looking at Saturn with the equatorial, he found that this arose from a great increase in the brightness of the dusky ring, which appeared nearly as bright as the illuminated ring, and might easily have been mistaken for a part of it."* At the time of this observation, however, the earth was raised about 4° 23', the sun less raised above the plane of the ring; thus it is clear from the laws of refraction and reflection of light at the surfaces of transparent media, that the dark ring, whether we consider it to be a semi-transparent solid or fluid, would not, at the time of this observation, differ greatly in brightness from the outer rings. The results of Chapter V., however, will be found to afford a more probable explanation, both of the nature of this ring, and of the cause of its brightness when viewed at small angles.

Lastly, two observations by Mr. Dawes, rather singular than particularly valuable, are recorded in the February number of the • Astronomical Reports' of the year 1863. He observed the transit of the shadow of Titan (the largest and brightest satellite) across * Reports of the Astronomical Society,' April, 1863.

the disc of Saturn; and 'an eclipse of Titan itself in Saturn's shadow: the former a rare, the latter, in Mr. Dawes' opinion, an unique phenomenon.

Among telescopic discoveries may be classed the measures that have been taken of the planet's polar and equatorial diameters, of the diameters of the various rings, and of the orbits of the eight satellites. The absolute measures, and even the proportions between the several dimensions obtained by different observers, vary considerably. The following are the dimensions of the rings as given respectively by Hind* and Struve :—

Struve's measures were probably taken at a later period than those adopted by Hind (? Bessel's).

The most remarkable feature, at first sight, in the comparison of the two tables, is the excess of every measure but one in Struve's table over the corresponding measure in Hind's. This excess will not appear so remarkable, however, when it is considered that, at the immense distance to which Saturn is removed from us, a space of about 4,240 miles corresponds to an angle of one second of arc. † The single measure of Hind's table which exceeds the corresponding measure of Struve's, marks a more significant discrepancy. Thus the breadth of the system, which is given by Hind as only 27,500 miles, Struve estimates at 29,539 miles; yet the space between the planet and the inner edge of the inner bright ring is given as 19,250 miles by Hind, or 160 miles greater than

* Superintendent of the 'Nautical Almanac.'

That is, about 7th part of the angle subtended by the moon's apparent diameter (mean). Even in the most powerful telescopes an arc of 1" of a great circle of the celestial sphere appears a very small space; so that a double star whose components are 4th or 3rd of a second apart, severely tests their defining powers.

the corresponding measure of Struve. In other words, while Hind gives the ratio between the breadth of the system of rings and the breadth of the space between the rings and planets as 10: 7 exactly, Struve determines the same ratio as somewhat less than 10: 6. Such a discrepancy is not likely to be accidental; and it becomes still more significant when we compare the ratios just given with the corresponding ratios obtained by Huygens in the 17th, and by Herschel near the end of the 18th century. As already mentioned, these were respectively 10:10 and 10:8. Thus it would appear that from the first discovery of the rings to the present time, the ratio of the breadth of the rings to the space between the rings and planet has been continually increasing. As it does not appear that any perceptible change has taken place in the exterior diameter of the outer ring, it would follow that the rings have been continually spreading inwards. The absolute increase in the breadth of the rings, and the probable cause of this singular phenomenon, will be discussed in Chapter V.

At the time of the first discovery of the dark ring, its breadth was variously estimated-the lowest estimate being 6,000 miles, the highest not exceeding 8,000 miles; later, this ring appears to have grown broader, and the latest estimates of its breadth vary from 8,000 to 10,000 miles.

The compression of Saturn's globe has not been satisfactorily determined. It is usually given as th; that is, the polar diameter is considered to be less than an equatorial diameter by about 1th of such diameter. Herschel, on the other hand, considered the compression less than th; Hind gives it as about th; and in the 6 Nautical Almanac' it is assumed to be the same as the compression of Jupiter's globe, or about th. It is obvious that if the compression of Saturn's globe were accurately known, as well as the proportions of his equatorial diameter to the internal and external diameters of his rings, then the exact appearance of the system at any moment could be readily determined. We should only have to calculate (from the known orbits of Saturn and the earth about the sun) the elevations of the sun and the earth above the plane of the ring: these being known, the figure of the rings, the position of their shadow on the planet, and the position