P L A
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P L A
accommodated to the various Forms and Profiles of the Mouldings; (See Moulding) as the Round Plane, the .'Mot, the OG, the Snipe's Bill, &c. which are all of fe- Vvral Sizes, from half an Inch to an Inch and half. See Moulding, Crc.
To life the Moulding Planes on foft Wood, as Deal, Pear-Tiee, &c. they let the Iron to an Angle of 45 with the Bafe or Sole of the Plane. On hard Wood, v. gr. Ebo- ny, Box, &c. they fet to an Angle of 80° ; fofrfetimes quite upright. To work on hard Wood, the Edge or Bafil is ground to an Angle of 18 or 20 Deg : on foft Wood, to an~Angle of about 12°. For the more acute the Bafil, the fmoother the Iron cuts •, but the more obtufe, the ftronger.
Fn« e, among Fowlers.— To plane, is to fly or hover as a Bird does, without moving its Wings. See Flying, Hawk and Hawking.
PLANET, P L a n E ta, in Afironomy, a Celeftial Body, revolving round the Sun as a Centre, and continually chang- ing its Pofition, with refpeft to the other Stars ; whence its Name mho/wan, Wanderer; in oppoBtion to a fix'd Star. See Star.
The Planets are ufually diftinguifhed into Primary and Secundary.
The Primary Planets, call'd alfo, fimply, Planets, by way of Eminence, ace thofe which move round the Sun as their proper Center. — Such are Saturn, Jupiter, Mars, the Earth, Venus, and Afercury.
Secundary Planets are Inch as move round fome Primary Planet, as their refpeetive Center, in the fame manner as the Planets do round* the Sun.— Such are the Moon moving round our Earth ; and thofe others moving round Saturn and Jupiter, properly call'd Satellites. See the Doctrine of Se- cundary Planets, under the Article Satellites.
The Planets, or Primary Planets, are in Number fix; which are again diftinguilhed into Superior and Inferior.
The Superior Pl'anets are thofe further off the Sun than our Earth is Such are Mars, Jupiter, and Saturn.
The Inferior Planets are thofe nearer the Sun than our
Earth, and fituate between the Earth and Sun Such
are Venus arid Mercury. See the Order, Pofition, &c. of the Planets reprefented to the Eye, Tab. Ajlronomy, Fig 44.
The Planets are reprefented by the fame Characters as the Chymifts ufe to reprefent their Metals by, on account of fome fuppofed Analogy between thofe celeftial and fubterra- neous Bodies. See Metal and Character.
Saturn is reprefented by the Character ^..< — This Planet, by reafon of its great Diftance appears to the Eye with a feeble Light.— It. performs its Revolution round the Sun in about 30 Years. See Saturn.
Jupiter, mark'd It, is a bright refulgent Star, finifhing its Courfe round the Sun in about 1 2 Years. See Jupiter.
Mars, characterise £ , is a ruddy fiery colour'd Planet, fi- nifhing its Courfe in about two Years. See Mars.
Venus, s, is the brighter! of all the Planets, conftantly attending the Sun, and never diftant from him above 47 Degrees.— It finifhes its Courfe in about leven Months. See Venus.
When it goes before the Sun, 'tis call'd Phofphorus, and Lucifer; and when it follows him Hefperus. See Phospho- rus, &c. ,
Mercury, V, a little bright Planet, the Sun's conftant Companion, from whofe fide it never departs above 28 , and by that means ufually hid in his Splendor. — It performs its Courfe in about three Months. See Mercury.
To which we now add, Tellus, the Earth, mark'd £ft or ?, performing its Courfe about the Sun, between Mars and Venus, in the Space of a Year. See Earth.
From thefe Definitions, a Perfon may eafily diftinguifh all the Planets — For if after Sun-fet he fee a Planet nearer the Eaft than the Weft, he may conclude 'tis neither Mer- cury nor Venus ; and may determine whether 'tis Saturn, Ju- piter, or Mars, by the Colour and Light : By which alfo he may diftinguifh between Mercury and Venus.
Nal ure s/flisPLANETS.
From the fevera! Phafes and Appearances of the Planets, they are found to be all perfectly like the Moon ; which, we have fiiewn to be perfectly like our Earth; whence it follows, that the Planets, too, are dark, opake, fpherical, C$*c. Bodies, like our Earth. See Moon.
This may be fbewn almoft a to Demonstration t Venus,
obferv'd with a Telefcope, is rarely found full, but with va- riable Phafes like thofe of the Moon; her illumined Part ftill turn'd towards the Sun, viz.. toward the Eaft when fhe fe the Morning-Star, and the Weft when the Evening-Star. — And the like Phafes are are obferv'd in Mercury and Mars.
2° Caffendus firft, and after him others, have obferved Mercury on the Face of the Sun, a-crofs which he appeared to pals like a black round Spot. See Transit.— Norrox in 1639, alfo obferv'd Venus in the Sim ; where Die made the fame Appearance.
3° De U Hire, in 1700, with a Telefcope of 16 Footj difcover'd Mountains in Venus, larger than thofe of the Moon, See Mountain.
4 Caffmi obferv'd two Spots in Venus ; four in Mars likewife obferv'd by Campani ; and feveral, at feveral times' in Jupiter : and from his Obl'ervations of thefe Spots found that they had a Rotation round their Axes ; and even deter- mine the Velocity of that Rotation, or the Period wherein it was effected, v.g. That of Jupiter, 9 Hours 56'. That of Mars 24 Hours 40'. And that of Venui, 24 Hours. See Spot — And fince the Sun, Moon, Jupiter, Mars, Venus, and the Earth, are found to revolve on their Axes, i. e. to have a diurnal Rotation ; no doubt Mercury and Saturn have the fame; tho' the great Nearnefs of the former to the Sun, and the great Diftance of the latter, prevent any Spots from being obferv'd on them, whence that Rotation might be demonftrated.
5° In Jupiter are obferv'd two Swaths, or Belts, brighter than the reft of his Disk, and moveable ; fometimes found in one part fometimes in another, fometimes broader fome- times narrower. See Belt.
6° In 1609 were firft obferv'd three little Stars, or Moons, moving about Jupiter by Sim. Marius ; and in 1610 the (lime were obferv'd by GalliUo : Thefe are now frequently obfer- ved todifappear in a clear Sky, when Jupiter happens to be
diametrically interpofed between them and the Sun
Whence it appears they are void of Light, at fuch Time when the Sun's Rays, intercepted by Jupiter, cannot be propagated to them in right Lines ; and hence alfo, that, like the Moon they are opake Bodies, illumined by the Sun ; and hence, again, fince Jupiter does not illumine his Satellites when found behind him, he himfelf, in that Part turn'd from the Sun, may be argu'd to be void of Light.
7 When Jupiter's Moons are diametrically interpofed be- tween Jupiter and the Sun, there is feen a round Spot on Jupiter's Disk, which is fometimes larger than the Satellite itfelf— Whence it appears, that the Satellites are opake Bodies, illumined by the Sun, that they projeft a Shadow upon the Sun, and that the round Spots feen in Jupiter are the Shadows of the Satellites. Whence, alfo, the Interfe- ction of the Shadow being found to be a Circle, the Shadow muft be conical; and therefore the Figure of the Satellites, at leaft as to Senfe, is fpherical.
8° The Earth being between Jupiter and the Sun ; if, at the lime time, any of the Satellites happen to be between Jupiter and the Sun, it is loft in Jupiter's Light; tho' fome- times appearing like a black Spot This Phenomenon has
been frequently obferved by Caffmi and Maraldi, who have likewife noted very confiderable Alterations in the apparent Magnitudes of the Satellites; for which no Reafon could be given from the Diitance of Jupiter, the Sun, or the Earth: e.gr. That the fourth, which is ufiully feen the fmalleft, is fometimes the largeft; and the third, which is ufually the
largeft, fometimes the fmalleft Hence, as the Satellites are
illumined by the Sun, even then wiien immerg'd in Jupi- ter's Light, yet do appear obfcuie, there muft be fome Alte- rations in their Atmofpheres, to prevent the Sun's Rays be- ing equally reflected from every Part of their Surface ; which muft likewife be theCaufe why their Shadow is fometimes larger than themfelves.
Now, to fum up the Evidence,— 1° Since in Venus, Mer- cury, and Mars, only that Part of the Disk illumined by the Sun, is found to fbine; and, again, Venus and Mercury, when between the Earth and the Sun, appear like dark Spots or Maculx, on the Sun's Disk ; 'tis evident, that Mars, Ju- piter, and Mercury, are opake Bodies, illumined with the bor- row'd Light of the Sun. And the fame appears of Jupiter, from its being void of Light in that Part to which the Sha- dow of the Satellites reaches, as well as in that Part turn'd from the Sun ; and that his Satellites are opake, and re- flect the Sun's Light, is abundantly fbewn. Wherefore fince Saturn, with his Ring and Satellites, do only yield a faint Light, fainter confiderably than that of the fix'd Stars; tho' thefe be vaftly more remote; and than that of the reft of the Planets : 'tis paft doubt, he, too, with his Attendants, are opake Bodies.
2° Again, fince the Sun's Light is not tranfmitted thro* Mercury and Venus, when placed againfl him ; 'tis plain they are denfe opake Bodies ; which is likewife evident of Jupiter, from his hiding the Satellites in his Shadow ; and, therefore, by Analogy, the fame may be concluded of Sa- turn.
3 From the variable Spots in Venus, Mars, and Jupiter, 'tis evident thofe Planets have a changeable Atmofphere; which changeable Atmofphere may, by a like Argument, be inferr'd of the Satellites of Jupiter; and therefore, by fimi- litude the fame may be concluded of the other Planets.
4 In like manner, from the Mountains obferved in Ve- nus; the fame may be fuppofed in the other Planets.
5 Since then Saturn, Jupiter, both their Satellites, Mars, Venus, and Mercury, are opake Bodies, Ihining with the Sun's borrow'd Light, are furniih'd with Mountains, and
encompafs'd
