A Table of the Eclipfes of Jupiter's firft Satellite, reduced to apparent Time, 1757.

Emerfions Emerfions Emerfions Immerfions

AUGUST

d. h. m. 1. d. h. m. f. d. h. m. f. d. h. m. f.

The Times of the Eclipfes in this Table are adapted to the Meridian of the Royal Obfervatory near London, and afford an excellent Method of difcovering the Longitude, or Difference of Meridians, between that and any other Place whatsoever, which I fhall illuftrate by an Example.

Suppofe on the 9th Day of October, the Time of the Emerfion of Jupiter's first Satellite, be obferved by a Telescope at 44 Min. 22 Sec. paft 10 at Night; I find by the Table, that the Time of this Emerfion will happen at the British Obfervatory, the fame Night, 48 Min. 42 Sec. paft 8: The Difference, of the Times is 1 Hour, 55 Min. 40 Sec. which converted into Deg. and Min. of the Equator, will make 28 Deg. 55 Min. the Longitude of the Place of Obfervation, to the Eaft of the British Obfervatory.

Euphrof. Well, I fuppofe by this Time you are tired of talking fo much about the Longitude, to one of our Sex, who are fo feldom employed in putting in Practice any of the great Discoveries of the Philofophers; but as we have naturally a Curiofity of Enquiry into every Thing that we hear of that is of a public or wonderful Nature, we are oftentimes folicitous to be fatisfied about Things that do not fo immediately concern us. Of this I fhall give you a farther Inftance by a Query or two concerning Saturn, his Moons, and Ring. I fee they move, after the fame Manner with thofe of Jupiter, about their Primary; but I obferve this Syftem of Moons has not the fame Pofition with refpect to the Plane of the Orrery as those of Jupiter have; but as the Time is now far advanced, I shall beg a more particular Account of what relates to Saturn and his Syftem the next Opportunity we have for Converse on thefe Subjects.

I

DIALOGUE VIII.

The Ufe of the Orrery continued.
Cleonicus.

Remember you took Notice, that Saturn's Moons were not alike pofited with thofe of Jupiter, in regard to the Ecliptic. And it is true, they are not; for those of Jupiter are parallel to that; but thofe of Saturn are inclined thereto in an Angle of about 31 Degrees; as is alfo the Plane of his Ring.

Euphrof. Then, I perceive, that the Shadow of Saturn's Moons, caft on the Pafteboard behind them, will not appear to move backward and forward in a right Line, like thofe of Jupiter.

Cleon. You rightly obferve, they cannot appear fo to move to an Eye placed upon the Earth; as you will eafily fee by the Experiment; for having placed the Pafteboard properly behind this Planet, and taken all the Candles but one out of the Room-lay your Hand on the Winch, and put them in Motion, and then you fee on the Pafteboard, that each refpective Satellite describes an Orbit of an oval Figure, and what the Geometers properly call an Ellipfis, one Part of which lies above, and the other below the Planet, and its Ring in the Center,

Euphrof. It greatly delights me to observe these curious Appearances; and now I fee, in Fact, how Things are in Nature performed, and brought about: I fee the Reafon why the Moons, while they defcribe the remote Part of their Orbit, appear direct in Motion, and retrograde, while they describe that Part next to us:-I fee likewise, that the Shadow of the Ring, in like Manner, is not circular, but elliptical, including the Body of Saturn, very much like what formerly appeared in the Heavens through the Telescope.

Cleon. It muft undoubtedly be pleasant to fee the wonderful Machinery of Nature thus reprefented in Epitome, and yet, at the fame Time, fo perfectly; but there is one Thing which you have not yet adverted to, and that is, that Saturn and his whole Syftem, and the Motion about the Sun, observe a Paralism of Pofition; or in other Words, the Planet, his Ring, and Satellites, always respect the fame Part of the Heavens in every Part of their Orbits; and this you will eafily perceive, if you attend to it but a very fhort Time; but as this is a curious Phænomenon, I fhall reprefent it to you in the Orrery, by Means of the Lamp in the Place of the Sun, and the Pafteboard, connected with the Stem of the Planet; fo as always to be behind it, by which Means the Shadow of the Ring will caft a Shadow upon the Pafteboard, and thereby the feveral Phafes of the Ring will plainly appear, as they are obferved through a Telescope in the Heavens during the feveral Parts of his long Period *.

Euphrof. This will be a delightful Spectacle, indeed; but, I fear, it will coft you a good deal of Time and Trouble; fince the Motion of this Planet is fo very flow, even in the Orrery itself.

Cleon. I fhall think no Time or Trouble too much, or ill-spent, to inform the Mind of my dear Euphrofyne. Befide that, we need only obferve the Phases of the Ring through one fourth Part of its Period; and feven Years, you know, will foon be over in the Orrery; but it will be neceffary, in the first Place, to bring this Planet to that Part of its Orbit where the Plane of its Ring is most

of

The Reader will here caft his Eye upon Plate XX. where thefe Phafes are delineated.

of all open to the View of the Spectator upon the Earth. This is about the 20th Degree of Sagittarius, which Place Saturn poffeffed at the latter End of the Year 1752, to which Place, you fee, I now bring the Planet; and the Shadow of the Ring on the Pafteboard is fuch, as shews near the whole Body of the Planet within it.

Euphrof. I fee it does, and fhould have been glad to have had an Opportunity, at that Time, of feeing fo fine an Appearance of the Ring through a Telescope, and the large Space between it and the Body of Saturn. I think it is confiderably different from what I obferved it fome Months ago.

Cleon. You will not wonder at that, when you confider, that a few Years make a great Alteration in the Appearance of the elliptic Figure of the Ring. For as the Planet moves on, at the Rate of about 12 Degrees in a Year, and the Ring always keeps parallel to itself, it must be in a Situation, at this Time, much more oblique to the View. Keep turning the Winch, and you will fee the Shadow of the Ring becomes more and more elliptical, 'till at length you obferve the Planet is in the 11th Degree of Aquarius, where it is at this very Time, as feen from the Earth.

Euphrof. Then here I will stop and view it; and, indeed, I fee it much the fame as I did through the Telefcope, and by this Means am fully convinced how just a Reprefentation this is of Nature. But will not the Pleafure of viewing this Ring decrease with its Dimensions?

Cleon. Perhaps not; fince Variety always is productive of Pleasure, in fome Degree. For now turn the Handle again, and you will obferve, that, when the Planet arrives at about 10 Degrees of Pifces, the Ring feems little more than as a Line that lies over the Planet, and at a fmall Distance on either Side; and thus it will appear in the Beginning of the Year 1759, foon after which, it entirely difappears; and in the Beginning of the Year 1760, it arrives to the Place of the Nodes of the Ring, which is in about 20 Degrees of Pifces, then the Plane, paffing through the Eye of the Spectator, projects the Shadow of the Ring into a ftraight Line, juft over the Middle of the Disk of the Planet; and if you live to view the Planet at that Time, through a Telcfcope, you will VOL. I.

P

fee

fee nothing at all of its Ring. From this Situation keep him moving on, and it will be nearly fix Months in the Orrery before the Ring will appear any otherwise than a right Line. But you will obferve, that foon after he enters the Sign of Aries, the Shadow of the Ring will again appear to be curved, which will be in the Year 1761, when the other, or fouthern Side of the Ring will be enlightened by the Sun. Thus every fucceffive Year 'the Phafes of the Ring will return again 'till seven Years are compleated, when the Ring will be again most open to the Telescopic View of the Spectator, which will be in the latter End of the Year 1767, where we may now leave that gloomy Planet; for we can fee no more Variety in the remaining half of its Orbit than you have observed

in this.

Euphrof. I would not give you any unnecessary Trouble, to infift longer on this Phænomenon, as it must neceffarily be a Repetition of the fame Thing, which I can as eafily conceive without feeing it, as with. I fhall be extremely defirous often to view this Planet with a Telescope; efpecially, as the Time is now approaching, when the greatest Variety of these Phafes will entertain the Eye. But methinks it will be a long Time before I fhall have the Pleasure of viewing the Ring in its moft ample Extent.

Cleon. Why that is the moft curious of all the Phafes of Saturn's Ring, and it may poffibly happen, that you may fee a fixed Star in the Space between the Planet and its Ring; for this we are affured of by Mr. Whiston, that Dr. Clarke's Father once faw a Star in that Situation +.

Euphrof. There would be no Reafon for me to expect fuch a Sight, as, I prefume, it must be a very rare Appearance, if only one Man has been obferved to see it; but, pray, who was the first Discoverer of this wonderful Phænomenon of Saturn's Ring, and how many Years is it fince ?

Cleon. We owe this happy Discovery to the Inventor of the Telescope itself, with which it was feen, the famous Gallileo, an Italian Mathematician and Philofopher, who first of all viewed it, and publifhed it to the World in the Year 1610, in an ænigmatic Latin Sen

+ Whifton's Memoirs of Dr. Clarke's Life.

tence,