11 I N 



Saturn goes from G to A, his ring turns more and more 

 edgewife to us, and therefore it Teems to grow narrower 

 and narrower ; and at A it difappears as before. Hence, 

 while Saturn goes from A to E, the fun (Junes on the 

 upper fide of his ring, and the under fide is dark ; and 

 wh.lft he goes from E to A, the fun mines on the under 

 fide of his ring, and the upper fide .s dark. The ring dif- 

 appears twice in every annual revolution of Saturn, «w. 

 when he is in 19th degree of Pifces and of Virgo, and 

 when Saturn is in the middle between thefe points, or in the 

 inth degree either of Gemini or of Sagittarius, his ring ap- 

 pears moft open to us ; and then its longei .diameter is to 

 its fhorteft, as 9 to 4. Fergufon's Aftr. feft. 204. 



Dr. Herfchel (Phil. Tranf. for 1790, vol. lxxx. pt. I.) 

 obferves, that the black difc, or belt, upon the ring of 

 Saturn, which we have already mentioned, is not in the 

 middle of its breadth ; and that the ring is not fubdivided 

 by many fuch lines, as fome aftronomers have reprefented ; 

 but there is one tingle, dark, confiderably broad line, belt, 

 or zone, which he has conftantly found on the north fide 

 of the ring. (See PtoXIX. AJlronomy, fig. I J. ) 



« Since the year i 7 74> * the P^" 1 " me '. aS Dr \ H «T 

 chelfays (Phil. Tranf. for 1792, vol. lxxxn. pt 1.) 1 

 can find only four obfervations where any other black 

 divifion of the ring is mentioned, than the one which I have 

 conftantly obferved : thefe were all in June, 1 780. As this 

 dark belt is fubject to no change, it is probably owing, as 

 the dodtor remarks, to fome permanent conllruftion oi the 

 furface of the ring. This belt cannot be the fhadow of a 

 chain of mountains, fince it is vifible all round on the ring ; 

 for at the ends of the anfx there could be no (hade vifible 

 on account of the diredion of the fun's illumination, which 

 would be in the line- of the chain ; and the fame argument 

 will hold again* any caverns or concavities. It is, more- 

 over fully evident, that this dark zone is contained between 

 two 'concentric circles, as all the phenomena aniwer to the 

 projection of fuch a zone. Thus mfig.i6, we may fee 

 that the zone is continued all round the ring, with a gra- 

 dual decreafe of breadth towards the middle, anfwermg to 

 the appearance of a narrow circular plane projefted into 

 an ellipfis. The matter of the ring is undoubtedly no left 

 foUd than the planet itfelf; and it is obferved to call a 

 gon e ftiadow upon the planet. The light of the ring ; is 



R I N 



difference of the telefcopes, and of the ftate of the atmo- 

 fphere. The difappearance of the ring fecms to occur only 

 with the telefcopes in common ufe among aftronomers ; for 

 Dr. Herfchel, with his large telefcopes, has been able to 

 fee it in every lituation. lie thinks the edge of the ring 

 is not flat, but fpherical, or fpheridical. He obferves, that 

 the ring was feen in his telefcope, when we were turned 

 towards the unenlightened fide ; fo that he either faw the 

 light reflected from the edge, or elfe the rciltdtion of the 

 lio-ht of Saturn upon the dark fide of t'ue ring, as we fome- 

 times fee the dark part of the moon. He cannot, however, 

 fay which of the two might be the cafe ; efpecially as there 

 are very ftrong reafons for thinking, that the edge of the 

 rinc is of fuch a nature as not to reflect much light. M. de la 

 Lande thinks, that the ring is juft vifible, with the belt tele- 

 fcopes in common ufe, when the fun is elevated 3' above its 

 plane, or three days before its plane panes through the 

 fun ; and when the earth is elevated 2' 20" above the plane, 

 or one day from the earth's paffing it. In the Phil. Tranf. 

 for 1790, Dr. Herfchel ventured to fuggeft a fufpicion that 

 the .ring was divided ; this conjecture was ftrengthened by 

 fubfequent obfervations, after he had an opportunity of 

 feeing both fides of the ring. His reafons are thefe ; 

 lit, the black divifion upon the northern fide of the ring, is 

 in the fame place, of the fame breadth, and at the fame 

 diftance from the outer edge, that it always appeared upon 

 the northern fide ; 2dly, with his feven-feet reflector and an 

 excellent fpeculum, he faw the divifion in the ring, and the 

 open fpace between the ring and the body, equally dark, 

 and of the fame colour with the heavens about the planet ; 

 idly, the black divifion is equally broad on each of the rings. 

 From thefe obfervations, Dr. Herfchel thinks himfelf au- 

 thorifed to fay, that Saturn has two concentric rings, 

 fitnated in one plane, which is probably not much inclined 

 to the equator of the planet. The dimensions of the ring 

 are in the following proportions, as nearly as they could 



be afcertained. 



Parts. 

 5900 

 7510 

 7740 

 8300 

 805 

 280 



"5 



Paris," 

 but Dr. 



Infide diameter of the fmaller ring 

 Outfide diameter - 

 Infide diameter of the large-it ring 

 Outfide diameter - 



Breadth of the inner ring - - - - 

 Breadth of the outer ring 

 alfo generally brighter than that of the t Janet tor the ^^ ^ 



ring appears efficiently bn,twhen^te^o P ^ ;;; ^ ^ ^ P^ ^ (< ^ fc ^ 



SS^S^JS^S, Dr. Herfchel fuppofes that the ring may .lave many div.t.ons ; 



cftablifhes the fad of the extreme thinnefs of the ring. He 



f -ther obferves, that there may be a refraft.on through a 



very rare atmoiphcre on the two planes of the ring, by 



which the fatelhtc may be elevated and deprefled fo as to 



become vifible on both fides of the nng even though he 



rin° mould be equal in thicknefs to the diameter of the 



fmSleft fatellite, which may amount to 1000 miles _ 1 



a feries of obfervations upon luminous point! ; ot this 



(Phil. Tranf. rol. lxxx. pt. 2.) £ h , aS d,fcovcrcd that 



it has a rotation about its axis, in io n 32 15 .4. 



The ring is invilible when its plane paffes through 

 fun, or the earth, or between them: xn the firft cafe, the 

 fun mines only upon its edge, which is too thin to n 

 fufficknt light to render it- vifible; in the fecond cafe, th 

 edeeS ,pofed to us, it is not vifible, for the 



S„,. Ltl rl cafe, the dark fide of the ring , 

 pofed to us, and therefore the edge being the only lull 

 Lt which is towards the 1 arth, it isinvifible, for the 1 m 

 reafon as before. Obfervers have differed ten or twel 

 days as to the time of its becoming invifibfe, owing to the 



UIIJUUIO LIKIL L1IV. 1 itig »iinjr »■*«.". ...w..y — , — -- 



Herfchel remarks, that no obfervations will juftify this iup- 

 pofition. 



From the mean of a great many meafures of the diameter 

 of the larger ring. Dr. Herfchel makes it 46".677, at the 

 mean diftance of Saturn. Hi the du- 



el' the earth :: 25.8914 : I. From the above pro- 

 here! , the 1 ter of the ring mutt be up- 



, of 204,80-3 miles ; and the diftance of the two rings 

 28.59 miles. 



The ring being a circle, appears elliptical from its oblique 



I Saturn ia 90 



.odes of the ring, upon th b oi Saturn; or 



1 Saturn's longitude IB about 2 s 17°, and 8* 1 7°. In 



.:, the lefler axis i« very m uJ to fa II 



the greati r, when the obfervations are reduced to the fun ; 

 confequ :ntl) the plane ot the ring makes an angle of about 



; , orl of S .'urn. 



he ring I I y Dr. Herfchel, Phil. 



Tranf. vol. xcvi. pt. 2. p. 467.) is to the fpace bet. 

 t h e r j :y of Saturn as about J to 4. 



Kepler, 



