A cyclopaedia of physical sciences : comprising acoustics, astronomy, dynamics, electricity, heat, hydrodynamics, magnetism, philosophy of mathematics, meteorology, optics, pneumatics, statics &c. &c / by J.P. Nichol with the assistance of eminent scientific men.

  • Nichol, J. P. (John Pringle), 1804-1859.
Date:
1860
Catalogue details

Licence: Public Domain Mark

Credit: A cyclopaedia of physical sciences : comprising acoustics, astronomy, dynamics, electricity, heat, hydrodynamics, magnetism, philosophy of mathematics, meteorology, optics, pneumatics, statics &c. &c / by J.P. Nichol with the assistance of eminent scientific men. Source: Wellcome Collection.

    796/952 (page 762)
    It will aid the rearler as be follows our discus- sion of detiiils.' (1.) General Description of the Rings.—Soon after liis discovery of tlie satellites of Jupiter, Galileo became aware that sometbing peculiar attached to the planet Saturn : but, as his tele- scope bad not power to discern the actual pheno- menon, be merely placed on record the facts which puzzled him. The ring being of course best seen at the sides of the planet, presented the ajjpearance of two anscB or handles attached to the central orb; nor was the reality ascertained until tlie time of Huyghens, who gave publicity to his discQvery in the following sentence (the letters of which, after the fashion of the time, he transposed), Annulo cingilur iemd, piano, misquam cohmrente, ad edipticam inclinato.' Not long after the discovery by Huyghens, the chief di\dsion of the ring was noticed by Mr. William Ball in England; and it was speedily placed beyond doubt by Cassini, that Saturn has two concentric rings of different breadths, sepa- rated all round by an interval easily distinguish- able. Tliese are the rings we shall refer to, as A and B; the latter the one nearer to Saturn. Sub- sequently, from Sir William Herschel's time downwards, statements have been made of visible divisions in the midst of these two rings. One in the middle of A, as represented in our plate, appears pretty well established ; but vari- ous others have been reported by Captain Kater, the Astronomers of the Collegia Romano, Encke, and several more. Tliese divisions, however, have not continued visible, and the solution ap- jiarently accepted was, that they do not exist •, but that through defects of his telescope or the condition of the atmosphere, the Observer report- ing them was deceived. This solution, it need scarcely be remarked, was far from satisfac- tory. Besides the apparent di^^sions or black lines, broad mottled and dark stripes, especially within the ring B, had been noticed, by Mr. Dawes among others.—But while the question as to the signilicauce of these various appearances still remained an unsettled one. Astronomers Avere startled by tlie discovery of a third ring c, with- in the ring b, of a slaty colour and therefore comparatively dark, but nevertheless sending out so much light that the marvel was it had not been discovered before. This third ring was first detected by Mr. Bond at Cambridge Massachussets, on November 15, 1850; and again, without any knowledge of what Mr. Bonu bad noticed, bv Mr. Dawes, on November 29 ; and a few davs later by M. Lassell. Ihe third ring c is represented in our engraving. It, too, ap- pears to present puzzling phenomena akin to those noticed above. Various accounts have been given of its breadth, which seems augment- ing. Mr. Dawes feels persuaded that he has seen at least one division in it; and at times it has looked as if it were separated from ring n. This ring is manifestly uo new or very recent fornia- SAT tion. Appearance.'! were seen by Galle at Berlin, —that can alone be explained by its existence— in the year 1838 : and in some of Sir William Herschel's drawings, a dark belt is found across Saturn, attributable to no other cause. Yet had it been as fully developed as now, why did so many admirable observers miss it ? It can now be seen in clear nights through a good achro- matic of four inches diameter; and we know that telescopes very greatly more powerful, had often been intently directed towards Saturn. What can be made of this accumulation of per- plexing circumstances ? Is there an actual in- staUlity in this singular system?—AVe shall attempt the solution in the remainder of this article; but, in the meantime, let us realize the dimensions of the formation, concerning which we are speculating. The best average measures of the rings, &c., are probably the folio whig :— Exterior diameter of a, Breadth of do., Interval between a and d, . Exterior diameter of b, Breadth of do.. Breadth of c, . . a^ont Distance of inner edjre of c I , ^ and tlie limb of Sntiil-n, f Breadth of entire System of Rings, Miles. 176,418 10,.57o 1,791 151,690 17,175 6,350 7,4G0 35,889 It is difficult to ascertain the exact thickness of this system. A bright line so thin, is scarcely measurable; and besides, there is no likelihood that this thickness is measurable. Bessel of Konigsberg determined the mass of the system of rings by their effect on the apsides of the satellite Titan, to be the -^th. part of Saturn's mass. Combining this determination with the best measures we have, which are probably those of Mr. Bond of Cambridge, it may be concluded that the thickness is somewhere about 100 miles,— that is, about the three thousandth part of the breadth. Conceive a cake one inch in thickness, and two hundred and fifty feet in breadth, its length being indefinite;—that is a similitude of the'mechanical condition of this Rmg-appendage of Saturn! (2.) The Slability of ihe peri-Sattirnian Rings. The character of an arrangement, apparently so unlike any other within our system, being realized, the question immediately arises as to the mode or the combination of forces, by wliicU it is secured in its position, and protected from the influence of disturbing causes. The theorems of Celestial THechanics, show distinctly enough how a planet or satellite suitably placed, must revolve for ever around its central Orb; but we cannot infer that the same stability may be extended, by the same agencies or any combination of thein to systems of Rings.—The question now pro- posed is divisible into two parts, the discussion of which leads directly to very miexpected con- clusions. 1. The subiect was, earliest of all, taken up bv Laplace. That gi-eat philosopher looked at it under two of its phases. The one case ue G2