them all, was only about half way to wards its greateft elongation from the body of Saturn. How easily, with an inferior telescope, this might have been taken for one of the arms of Saturn, I leave thofe to guess who know what a degree of accuracy it must require to diftinguith objects that are fo minute, and at the fame time fo faint, on account of their nearness to the duk of the planet. Upon the whole, therefore, I cannot fay, that I had any one inftance that could induce me to believe the ring was not of an uniform thickness; that is, e qually thick at equal distances from the centre, and of an equal diameter throughout the whole of its conftruction. The idea of protuberant points upon the ring of Saturn, indeed, is of itfelf fufficient to render the opinion of their existence inadmiffible, when we confider the enormous fize fuch points ought to be of, for us to fee them at the distance we are from the planet. From thefe fuppofed luminous points I am, by imperceptible fteps, brought to the discovery of two fatellites of Saturn, which had efcaped unnoticed, on account of their little diftance from the planet, and faintnefs; which latter is partly to be afcribed to their fmalliefs, and partly to being fo near the light of the ring and difk of Saturn. Strong fufpicions of the existence of a fixth fatellite I have long entertained; and, if I had been more at leifure two years ago, when the difcovery of the two Georgian fatellites took me as it were off the fcent, I should certainly have been able to announce its exiftence as early as the 19th of Auguft 1787, when, at 22 h. 18′ 56′′, I faw, and marked it down as being probably, a fixth fatellite, which was then about 12 degrees paft its greatest preceding elongation. But, as I obferved before, not having time to give my thoughts to the fubject, I reserved a full investigation of the number of faellites, and the nature of the ring of U VOL. XII. N° 71. Saturn for a future opportunity. Be fides, not having any tanies of the fatellites, I could not confidently say, whether the fifth fatellite was not one of the five which I perceived in motion that night, though afterwards I found, that the real fifth had also been in view, and was marked down as a ftar, by the letter b, in a figure I'delineated of Saturn and its fatellites that evening. In the year 1788, very little could be done towards a difcovery, as my twenty-feet fpeculum was fo much tarnished by zenith fweeps, in which it had been more than ufually expofed to falling dues, that I could hardly fee the Georgian fatellites. In hopes of great fuccefs with my forty-feet fpeculum, I deferred the attack upon Saturn till that should be finished; and having taken an early opportunity of directing it to Saturn, the very firft moment I faw the planet, which was the 28th of last Auguft, I was prefented with a view of fix of its fatellites, in fuch a fituation, and fo bright, as rendered it impoffible to mistake them, or not to see them. The retrograde motion of Saturn amounted to nearly 4 minutes per day, which made it very eafy to ascertain whether the flars I took to be fatellites really were fo; and, in about two hours and an half, 1 had the pleafure of finding, that the planet had vifibly carried them all away from their places. I continued my obfervations conftantly, whenever the weather would permit; and the great light of the forty-feet fpeculum was now of fo much ufe, that I alfo, on the 17th of September, detected the feventh fatellite, when it was at its greatest preceding elongation. As foon as I had obfervations enough to make tables of the motion of these new fatellites, I calculated their place backwards, and foon found that many fufpicions of thefe fatellites, in the fhape of protuberant points on the arms, were confirmed, and ferved them more perfect. to correct the tables, fo as to render lefs be remembered, that a fatellite once discovered is much easier to be feen than it was before we were ace quainted with its place. By comparing together many obfervations of the fixth fatellite, I find, that it completes a fidereal revolution about Saturn in one day, 8 hours, 53′9′′. And if we fuppofe, with M. de la Lande, that the fourth is at the mean distance of 3' from the centre of Saturn, and performs one revolu. tion in 15 d. 22 h. 34′ 38′′, we find the distance of the fixth, by Kepler's Jaw, to be 35,058. Its light is confiderably ftrong, but not equal to that of the first fatellite; for, on the 20th of October, at 19 h. 59′ 46′′, the first, notwithstanding it was nearer the planet than the fixth, was ftill vifibly brighter than the latter. It would, however, be worth while to try whether a good achroma ic telescope, of a large aperture, might not poffibly fhew it at the time of its greatest diftance from the planet, and when no other fatellite is near; that is, provided it will fhew the other five fatellites with great eafe, as otherwife there will be no reafon to expect it fhould fhew the fixth In the period of this fatellite I have employed the obfervation of the 19th of August 1787, as, from other calculations, it seems the revolution is determined near enough to reach back fo far. The most diftant obfervations of the feventh fatellite, beng compared to gether, fhew, that it makes one fide. real revolution in 22 hours, 40 minutes, and 46 feconds: and, by the fame data which ferved to ascertain the dimenfion of the orbit of the fixth, we have the diftance of the feventh, from the center of Saturn, no more than 27'366 It is incomparably fmaller than the fixth; and, even in my forty-feet ref ctor, appears no big. ger than a very finali lucid point. I fee it, however, alfo very-wel' in the twenty-fect reflector; to which the exquifite figure of the fpeculum not a little contributes. It must neverthe The revolution of this fatellite is not nearly fo well afcertained as that of the former. The difficulty of having a number of observations is uncommonly great; for, on account of the fmalinefs of its orbit, the fatellite lies generally before and behind the planet and its ring, or at least fo near them, that, except in very fine weather, it cannot easily be feen well enough to take its place with accuracy. On the other hand, the greatest elongations allow fo much lautude for mi taking its true fituation, that it will require a confiderable time to divide the errors that muft arise from imperfect estimations. The orbits of these two fatellites, as appears from many oblervations of them, are exactly in the plane of the ring, or at least deviate fo little from it, that the difference cannot be perceived. It is true, there is a poshibi, lity that the line of their nodes may be in, or near, the prefent greatest elongation, in which cale the orbits may have fome fmail inclination; but as I have repeatedly feen them run along the very minute arms of the ring, even then the deviation cannot amount to more than perhaps one or two degrees; if, on the contrary, the nodes fhould be fituated near the conjunction, this quantity would be fa confiderable that it could not have escaped my obfervation. From the ring and fatellites of Sa+ turn we now turn our thoughts to the planet, its belts, and its figute. April 9, 1775, I observed a northern belt on Saturn, which was a little inclined to the line of the ring. May 1, 1776. There was another belt, inclined about 15 degrees to the fame line, but it was more to the fouth, and on the following fide came up to the place in which the ring croffes the body. July pretty bright white one. ed; with 200, 250, 300, 400, faint appearances of a fecond and of a third belt. Aug. 27. The belts less inclined. Sept. 2. A darkish belt, but very little inclined; and a fine white belt, close to the ring. 5. The belt a little inclined. 6. The belt not inclined. 8. The bright belt close to the ring, and two dark equatorial belts. It will not be neceffary to continue the account of thefe belts up to the prefent time; but I have conftantly obferved them, and found them generally in equatorial fituations, though now and then they were otherwise. We may draw two conclufions from what has been reported. The firft, Jan. 21, 1780. Two belts; the moft which relates to the changes in the north clouded. 22, Faint belts. May 17, A ftrong, equatorial June 20, 1780. 10 h. 10. The fame figure is on the difk, but feems to be more central than it was yefterday. -21, 9h. 25'. The fame two belts; a ftrong, dark spot, near the margia of the disk. 10 h. 1. The fpot not so remarkable as it was at 9 h. 25'. 26, Small twenty-feet telescope; an equatorial belt, and another lefs marked. 29. Two dark equatorial belts. April 19. Two belts. August 23. Two belts, a little declining from the equatorial pofition. 20. A broad belt much inclin U u 2 appearance of the belts, is, that Sa turn has probably a very confiderable atmosphere, in which thefe changes take place; juft as the alterations in the belts of Jupiter have been shewn, with great probability, to be in his atmosphere. This has alio been con firmed by other obfervations: thus, in occultations of Saturn's fatellites I have found them to hang to the difk a long while before they would vanish. And though we ought to make fome allowance for the ene croachment of light, whereby a fatel lite is feen to reach up to the difle fooner than it actually does, yet, without a confiderable refraction, ic could hardly be kept fo long in view after the apparent contact. The time of hanging upon the disk, in the feventh fatellite, has actually amounted 19 twenty minutes. Now, as its quick motion during that interval carries it through an arch of near fix degrees, we find, that this would denote a refrac tion of about two feconds, provided the encroaching of light had no fhare in the effect. By an obfervation of the fixth fatellite, the refraction of Saturn's atmosphere amounts to nearly the fame quantity; for this fatellite remained about 14 of 15 minutes longer longer in view than it should have done; and as it moves about 24 degrees in that time, and its orbit is larger than that of the feventh, the difference is inconfiderable. It is not my prefent intention to enter into a confideration of the amount of thefe refractions, otherwife we might perhaps find data enough to fubject them to fome calculation. But what has been faid will fuffice to fhew, that very probably Saturn has an atmosphere of a confiderable density. The next inference we may draw from the appearance of the belts on Saturn is, that this planet turns upon an axis which is perpendicular to the ring. The arrangement of the belts, during the courfe of fourteen years that I have obferved them, has always followed the direction of the ring, which is what I have called being equatorial. Thus, as the ring opened, the belts began to advance towards the fouth; and to fhew an incurvature answering to the projection of an equatorial line, or to a parallel of the fame. When the ring clofed up, they returned towards the north; and are now, while the ring and the Earth, is flattened at the Poles; and therefore ought to be fupë pofed to turn on its axis. July 22, 1776. I thought Saturn wa not exactly round. May 31, 1781. It appears as if the body of Saturn was at least as much flattened as that of Jupiter; but as the ring interferes, this may be better afcertained eight years hence. August 18, 1787. The body of Sa turn is of unequal diameters, the equatorial one being the longeft.. Sept. 14, 1789, 23 h. 36′ 32′′. Having referved the examination of the two diameters of Saturn to the prefent as the most favourable time, I measured them with my twenty-feet reflector, and a good parallel-wire micrometer. Equatorial diameter, ift mea- " 21,94 23,11 21,73 22,85 paffes over the center, exactly ranging Polar diameter, 1ft measure, 20,57 with the fhadow of it on the body; generally one on each fide, with a white belt clofe to it. When I fay, that the belts have always been equatorial, I pafs over trifling exceptions, which certainly were owing to local caufes. The ftep from equatorial belts to a rotation on an axis is fo eafy, and, in the cafe of Jupiter, fo well afcertain ed, that I fhall not hesitate to take the fame confequence for granted here. But, if there could remain a doubt, the obfervations of June 19, 20, and 21, 1780, where the fame fpot was feen in three different fituations, would remove it completely. There is another argument, of equal validity with the former, which now I fhall bring on. It is founded upon the following obfervations, and will thew that Saturn, like Jupiter, Mais, 3d 20,10 21,16 Mean 20,61 By this it appears that Saturn is confiderably flattened at the poles And as the greateft meafures wer taken in the line of the ring and of the belts, we are affured that the axis of the planet is perpendicular to the plane of the ring; and that the equatorial diameter is to the polar one nearly as 11 to 10. We may also infer the real diame ter of Saturn from thefe measures, which are perhaps more to be depend ed upon than any that have hitherto been given. But as in my journal I have measures that were repeatedly taken these ten years paft, not cas of of the diameter of Saturn, but of the ring, and its opening, whereby its inclination may be known; as well as of the diftance of the fourth, and fifth, and other fatellites, which will be of great ufe in afcertaining the quantity of matter contained in the planet, I referve a full investigation of thefo things for another dpportunity. Some Account of the Strata and Volcanic Appearances in the Weflern Ilands of Scotland. In a Letter from Abraham Milis, Efq; to John Lloyd, Efq; F. R. S*. THE HE volcanic country, which we explored together in the ifland of Mull, having hitherto remained undescribed by any traveller, I shall now beg leave to remind you of what we faw there, and in our Voyage to Staffa. Sailing from Freeport, in the island of 1lay, at ten o'clock at night, of Wednesday, July 2, 1788, we paffed Colonfay, without being able to diftinguish the substance of its shores; but entering the found of Iona, we faw that the rude coaft of Mull, and the lefs elevated fhore of Iona, was compofed of red granite. At the landing place in Iona is laminated hornftone; and a quarter of a mile north from the ruins of the Cathedral is a ven of coarfe red granite, two feet wide, ftanding nearly vertical, and ranging with the hornítone E. N. E. and W. S. W. on the furface are tumblers of red granite, and fome few of lava. About a mile N. W. from the Cathedral, and near the fhore, is a vein, two feet wide, containing feldfpath and white mica, ranging E. and W. between granite fides. Many of the rocks are tinged with iron, and there is fome bog iron ore in the moffes. In the S. W. part of the island, is a body of white marble, veined with pale green. At the Cove, where it is faid St Columb landed, the cliffs are of red granite, and the fhore is covered with great variety of pebbles of ferpentine, bafaltes, granite, quartz, and other fubftances. The N. W. part of the Island is very rocky, affording little pafture, except in fome low fpots, where the foil is fandy, and produces not only grafs, but likewife corn and potatoes. The whole extent of the island is three miles in length N. E. and S. W. and one mile in breadth; and it entirely confiits of alternate barren crags and little fertile vales. Having engaged a boat with four rowers, we went. from Icolmkill through the Bull Sound, which runs between Nun's Ifland and the island of Mull; on both fides the cliffs are of red granite, ragged and broken, without any regular beds or fiffures, and having no particular range or inclination. Hence we fteered for Ardlun Head, which forms the S. W point of Loch Leven. When we approached the Head, we ftopped the rowers, and fat fome time contemplating the wonderful arrange ment of the bafalt columns; and as we again rowed along fhore to the eastward, had a fine view of the various fituations into which the columns were thrown. The coaft being every where fteep, it was fome time before we could find a convenient place to land; but having at laft got on fhore, we walked to the extreme point or head: here, ftruck with the errors of our maps, which placed the islands in fight very differently from their true fituations, I took the following bear ings by the compafs. The N. E. point of Iona N.W. by W. The Dutchman's Cape N.N.W.. Cairnborough N. by W.- Staffa from N. to N. W. diftant, by eftimation rom the Same. |