Euphrof. Well, I long to fee the Machine in Motion. But do you not place the Planet Mercury alfo in that Part of the Ecliptic, which it really poffeffes at

that Time?

Clean. Yes, or else we do Nothing: But in order to do that rightly, you will obferve, that in the Table of Heliocentric Motions (Page 86) the Planet's Place for the first Day was in 18° 40′ of, and on the 6th Day in 16° 51' of Leo SL; therefore in five Days the Planet moved 28° 11'; one fifth Part of which is 5° 40', the Motion in one Day; in two Days therefore it moved thro' 11 20, which fubducted from 16° 51′ leaves 5 31 of Leo St, for its Place on December the 4th, 1755, when it began to be direct in Motion; and of Course to that Part of the Ecliptic the Arm which carries Mercury must be fet, which you fee I have done.Next I pafs the Wire thro' the Body of Mercuryand then I place the Planet on its Stem-laftly, I fix a fmall, white, fhining Bead on the End of the Wire by the Stars.And thus the Planetarium is rectified for our Purpose. — You now obferve the Earth, Mercury and Venus all in Motion.

Euphrof. I do; the Planet Mercury goes the fame Way with the Earth, from Weft to Eaft-The white Bead alfo moves among the Stars the fame Way.

Cleon. Very well, Sifter; and you will fee the Planet, and its reprefentative Bead both keep moving the fame Way for a confiderable Time.You will obferve it thro' all the prefent Month December.

-Let us now

take the new Ephemeris for the Year 1756, and you will fee the Bead keep moving on the fame Way thro' all January. Now take the Book, and as there will be Time enough, let me fee you find the Day when the Planet begins to be Retrograde.

Euphref. I fhall fee that foon - let me look in Fe-· bruary- fee no Letter R there- Next, let me look at March-Oh, there I find it, against the 2d Day I see the Letter R of Retrogradation.-Hence I obferve the Planet is direct in Motion from December 4, 1755, to February 2, 1756.

Cleon. Admirably good, Sifter; and now attend to the Planet in the Machine.- -The Earth and the two VOL. I, Planets

H

Planets ftill keep moving forward; but Mercury's Reprefentative among the Stars begins to flacken its Motion-do you not observe a fenfible Difference between it and the Planet itself, my Euphrofyne?

Euphrof. Yes, indeed, I do; and it is very pretty and pleasant to behold it.— Let me look, what Day the Earth is upon-The 11th of February-very well; now let me fee the new Year's Ephemeris--How much do I find it there move from the 11th to the 12th? — about 2 Degrees, and that is not half so fast as its true Motion--it now moves flower, - now very flow indeed not one Degree in a Day, it is now the 23d of February; It scarcely now moves at allthere it is quite at Rest the Earth is now upon March the ift; Well, and in the Ephemeris, March the 2d fhews it begins to be retrograde by the Mark R. All this is wonderfully natural, inftructive, and entertaining, beyond any Thing of this Sort that Ĩ have feen before.

Cleon. It is not ftrange the Phænomenon this Way fhewn should please you so much; because it is so natural as to be the very Thing itfelf. You'll fee by the Ephemeris, that the Planet is really not Stationary at all, but only in Appearance; and that as foon as the Bead ceafes to move forwards, it begins to move backwards.

Euphrof. I do; for I obferve that from February 29 to March the 1ft, it moved two Minutes of a Degree forwards, but from the 1ft to the 2d of March, it moved 6 Minutes back again. Whence I learn that the Planets are (as you fay) flationary only in Appearance, and that arifing from the Motion being infenfibly flow a few Days only. -But behold, it is now going backwardsGood Heavens, how faft it moves! - 'twill not be long ere it gets quite back again. --How very fwift

the Motion, and fhort the Time of its RetrogradationWhy, before the Month is out, it ftops again—it moves no longer.- -The Earth is now at March the 25th The Letter D is there too, in the EphemerisHow exact is the Agreement between the Book, the Planetarium, and the System itself? The fame Things appear in all, and force themfelves upon the

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