times Stationary, and fometimes Retrograde in its Course, as all the fuperior Planets moft remarkably do; which Appearances, their higher Situation, and flower Angular Motion, or longer Periods, muft neceffarily produce, without the leaft Alteration of their real progreffive Motion about the Sun all the while.

1000

Jupiter, the largest of all the Planets, is much higher in the Syftem, and has four Satellites or Moons revolving about it; and all by their common Center of Gravity defcribe a very great Ellipfis about the Sun. The Eccentricity of its Orbit is of its mean Distance from the Sun. It comes to its Conjunction, Quadratures, and Oppofition, as well as Mars; but at fo great a Diftance can never appear other than full, or nearly fo. It is in Diameter 81.000 Miles; in Surface it contains 20.000,000.000 of Square Miles, and in Solidity 280,000 coo, 000.000 Cubical Ones; and revolves about the Sun in Eleven Years and Ten Months, or 4332 Days and a Half, at the Middle Distance of 424,000.000 Miles. It revolves about its own Axis in 9 Hours, and 56 Minutes, which makes its Figure that of an Oblate Spheroid, having the Diameters of its Equator confiderably longer than its Axis. The Quantity of Light and Heat it receives from the Sun is but one Twenty-feventh Part fo great as ours on the Earth. Its Quantity of Matter is about 220 Times fo great as that of the Earth. Its Denfity is about one fifth Part of the Earth's, and fo the Weight of all Bodies on its Surface is about double to that with us. It is alfo Direct, Stationary and Retrograde as Mars, but not in fo great a Degree. It has Belts, like Clouds,

lying fomewhat regularly along the Equatorial Parts, but fubject to many Changes and Variations.

Saturn, the highest and most remote of all the known Planets, has five Satellites or Moons, and a vast but thin Ring encompaffing his Body, as an Horizon does a Globe; all which, or rather the common Center of their Gravity, defcribes an Ellipfis about the Sun. The Eccentricity of its Orbit is of its mean Distance from the Sun. It comes to its Conjunction, Quadratures and Oppofition, as well as the two former, but with no vifible Decrease of its Light at its Quadratures, which is scarce to be expected at fo great a Diftance. It revolves about the Sun in about 29 Years and a half, or about 10.760 Days. It is in Diameter 68.000 Miles; in Surface it contains 14.000,000.000 of Square Miles, and in Solidity 160,000 000, 000.000 of Cubical Ones, at the mean Distance of 777,000.000 Miles from the Sun. Its Quantity of Matter is about 94 Times as great as that of the Earth, tho' its Density be only between a fixth and a feventh Part fo great as that of the Earth's; and the Weight of Bodies on its Surface is to that on the Surface of the Earth as about five to four. It is not yet certainly known to revolve about its own Axis, tho' its Ring is faid to do fo. The Light and Heat communicated to it by the Sun are not quite the Ninetieth Part fo great, as thofe bestowd on the Earth. It is alfo in fome measure Direct, Stationary and Retrograde, as well as the two former Planets, tho' ftill in a lefs Degree,

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Of the Satellites, or fecondary Planets; the most eminent as to us is the Moon. It defcribes an Ellipfis about our Earth, (or rather both Earth and Moon defcribe their own fimilar Ellipfes about the common Center of their Gravity, as is the Cafe in all fuch Syftems) in a periodical Month of 27 Day s7 Hours 43 Minutes. The Mean Eccentricity of its Orbit is of its mean Distance from the Earth. It makes a Lunation or Synodical Month in 29 Days 12 Hours 44 Minutes. At a Mean it is diftant from the Earth about 240,000 Miles, tho' with confiderable Difference on Account of its great Eccentricity. The Moon's Diameter is 2175 Miles; in Surface it contains 14,000.000 of Square Miles; and in Solidity 5.000,000.000. of Cubical Ones. It has between the 39th and 40th part of the Quantity of Matter of the Earth; its Denfity is to that of the Earth as about 5 to 4, whilft the Weight of all Bodies is but about a third Part fo great on its Surface, as on that of the Earth. It revolves from Welt to Eaft upon its own Axis, exactly in a periodical Month, and thereby turns in general the fame Face towards the Earth continually; yet does the Inequality of its Motion, and the Obliquity of its Axis, occafion fome unequal Librations here alfo. It has very high Mountains, and very deep regular Valleys, and has lately had an Atmosphere difcover'd about it ; nor does it feem unlike the Earth as to Sea and Land. It has all variety of Phafes, according to its various Pofition with refpect to the Sun, or according as we on the Earth can fee the whole, the half, or only fome Part of its enlighten'd Hemisphere. Its own Day and Night

are

are each half a fynodical Month, or near 15 of our Days long.Its Orbit is inclin'd to that of the Ecliptick, at the leaft in an Angle of 5 Degrees; fo it but fometimes paffes juft between the Sun and Earth at the new Moon, and but fometimes falls into the Earth's Conical Shadow at the Full. Yet when it is in, or near the Nodes, or Interfection of thofe Plains,it cannot avoid thofe Accidents; from the former of which the Solar, and from the latter of which the Lunar Eclipfes are derived.

I

Jupiter's four Satellits, or fecondary Planets, are visible with an ordinary Telescope, and fometimes pafs like Spots on the Face of Fupiter, and fometimes enter into his Shadow; which to an Eye in Jupiter would cause Appearances juft like our Solar and Lunar Eclipfes. They revolve about him in Circles, or Ellipfes very little Eccentrical; the Innermoft at 130.000 Miles distance, in 1 Day 18 Hours and a half; the next at 364,cco Miles distance, in 3 Days 13 Hours and a quarter; the Third at 580,000 Miles distance in 7 Days 3 Hours; the Fourth at 1,000.0co Miles diftance, in 16 Days 16 Hours. The Cubes of their Diftances are alfo as the Squares of their periodical Times: and fo they are kept in their Curvilinear Orbits by their Gravity towards Jupiter's Center, in a duplicate reciprocal Proportion from it; as is the Cafe of all the Planets, both Primary and Secondary about the Sun.

Saturn has five Satellites or fecondary Planets. The Fourth in order from Saturn is the largest, and was difcovered by the celebrated Hugenius; the Third and Fifth are vifible in the next Degree; but the Knowledge of the two Innermoft are wholly owing C 3

το

to Caffini's extraordinary Glaffes and Diligence. They all revolve in Orbits almost Circular, and are all in or very near the Plain of his Ring, which is inclined in an Angle of 31 Degrees to that of the Ecliptick. The innermoft revolves about Saturn at 146,000 Miles distance, in 1 Day 21 Hours one third; the next at 187. 000 Miles, in 2 Days, 17 Hours, and two thirds the Third at 263,000 Miles, in 4 Days, 13 Hours, three Quarters; the Fourth or large one at 600, cco Miles in 15 Days 22 Hours two thirds; the laft at 1,800.000 Miles, in 79 Days 22 Hours. Nor is it improbable, that the large Interval between the Fourth and Fifth may have a Sixth, which is yet to us invifible, as Hugenius conjeEtures. As to that strange and unparallel'd Phanomenon of Saturn's Ring, which is commonly vifible through an ordinary Telescope ; its Thickness may well be 500, or perhaps 1000 Miles, tho' it be at that Distance almoft invifible; its Breadth is certainly about 21,000 Miles, and its diftance from the Body of Saturn on every Side as much. It caufes many different Appearances, not only to us on Earth, but much more to the Inhabitants of Saturn, if any fuch there are; all which Hugenius has defcrib'd in his Syftem of that Planet, and others from him.

As to the Syftem of the Comets, it appears now to be very confiderable, and indeed they are the moft numerous Bodies of the entire Solar Syftem. They appear both by their Bignefs and Motions to be a fort of Planets, revolving about the Sun in Ellipfes, fo very oblong, that their vifible Parts feem in a manner Parabolical; but have fuch vaft Atmospheres about them,

and