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Beginning of January to the 3d of May, and after that, the Emertions out of Yupiter's Shadow, to December, for the several Days, Hours, Minutes, and Seconds, when they happen, as you here observe in their respective Columns.

Euphros. I see them all ; and take Notice, that at the End of the Table, he subjoins an Example of its Use, which is the very Thing I want to see explained.

Cleon. That you shall instantly; as thus ; fuppose you was in some particular Place on the oth Day of next October, and there, with a good reflecting Telescope, you observed the firft Satellite of Jupiter emerging out of the Shadow at 44 Minutes and 22 Seconds paft 10 o Clock at Night, by a good Watch, that shews Seconds; then you would take the Ephemeris, to see what Time the same Phænomenon happened at the Royal Observatory at Greenwich, (to which the Numbers in the Table are adapted ;) and you will find against Otober the oth, that it was 48 Minutes and 42 Seconds after 8 at Night. Now the Difference between the Times of Obfervation at Greenwich, and the Place you are fuppofed to be in, is 1 Hour, 55 Minutes, and 46 Seconds, which, converted into Degrees and Minutes, in the Equator, will make 28 Degrees, and 5 Minutes, allowing for every Minute of Time 15 Minutes of a Degree, as before mentioned.

Euphrof. So that these 28 Degrees and 5 Minutes are what you call the Difference of Longitude of those two Places; but Mr. White says, this is the Longitude of the Place of Observation to the East of the British Observatory: But why does he say to the East, Cleonicus? this I do not clearly understand.

Cleon. This you will easily apprehend, my Euphrofyne, if you consider, that the Díurnal Motion of the Earth is Eastward, and therefore, at whatever Moment of Time the Eclipse is seen at a Place 15 Degrees Eastward, it must necessarily be one Hour later than the same appears at the Observatory at Greenwich; and if the place were Westward 15 Degrees, then it would happen i Hour fooner than at the Observatory; but of this Affair you will be more particularly convinced when we come to the Ufc of the Globes,

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127 16

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i 56 501

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O 1628 16 46 53

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4 29 481 4 20

2

A Table of the Eclipses of Jupiter's first Satellite,

reduced to apparent Time, 1757 Immersions Immersions Immersions Emersions JANUARY FEBRUARY APRIL JUNE d. h. m. 1. d. h. m. f.ld. h. m. 1. d. h. m. f. 2 15 13 41 26 11 42 16 22 8 36 2812 18 31 57 6 10 5324 3

5 14 14 13 6 4 MARCH 25 21 34

116

7 28 33 7 22 36 16

Immersions 9 17 3 49

29 10 31 3319 20 25 3 II II 31 19 0 39 31

21 14 53 27

May 13 5 58 49 3 19 8 10

23 921 46 15 0 26 30 5 13 36 531 Immersions 25 3 50 9 16 18 54 127 8 5 37

26 22 18 33 18 13 21 56.9 2 34 27

5
10 21
7 49 41

3 17

2 23 28 59 30 11 15 13 2 17 28 12 15 32 7 Emersions JULY 23 20 45 1514 10 125 15 13 9

Emersions

5 50 27 9 41

6 14 34 28
17 22 58 39
4

5 43 35 8

6l 29

9 3 4

O11 57 9

19 17 27 28 7

4 30 22 37 1121 11 56 18 3 31 42 5 18 40 21 FEBRUARY

23
6

8 461
25
o 54 4

13 16 28 51 9 7 37 13 Immersions

26 19 22 58195 10 57 23 11 5 41 5 14 28 13 51 53

17

012 20 34 17 3 11 33 1830

18 23 54 3714 15

2 531 5 6

20 18 23 416 9 31 26

APRIL
7
29 37

22 12 51 3218 4 8 18 57 51 Immersions 24 7 19 57 19 22 23 37 10 13 26 6

26 1 48 22 22 16 57 14

2 49 5527 20 16 4523 11 25 56 I 2

7 54 231 14 2 22 41

2 21 18 47 29 14 45 8 25 5 54 38 15 20 51 3

4 15 47 3931 9 13 2927 0 23 22

6 10 16 31 17 15 19 26

JUNE
8
19 9 47 59

4 45 24
Emersions

30 13 20 55
33
923 14 18

AUGUST 22 22 45 611 17 43 12 2

3 41 51

Emersions 4 3 22 10 18 15 6 40 56 5 16 38 46 1 7 49 441 17 9 531 7 DI

7 41 3

2 18 41 18 19 38 50 9 5 35 23 4 20 47 38 20 14 7 3911 3 40 6 15 16 301

IO

II 22

25 11

o 19 7 13

2

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19

20 19 9 12'9 21 27 4118

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A Table of the Eclipses of Jupiter's first Satellite,

reduced to apparent Time, 1757. Einersions Emersions Emersions Immersions AUGUST

SEPTEMBER OCTOBER DECEMBER d. h. m. 1. d. h. m.

f. d. h. m. f. d. h. m. f. 8 945 34

I 42 5 19 50 26

11 23 41

3
9
4 14 35

7 14 19 36

13 18 8 391 11 22 43 371

O 241 9 8 48 42 15 12 36 15 12 19 29 4811

3 17 4817 7 13 17 12 42

3 47 812 21. 46 5119

14 13 59 15 11 41 47

I 31 18 16

8 28 28 14 16 15 53 20 19 58 51 17 6 10 51

18
0 39 56
2 57 4616 10 44 51 22 14 26 23

5 13 4924 8 53 58 22 13 38 2821 15 56 20 19 23 42 48 26 3 21 33

9 38 23 10 25 37 21 18 11 4627 21 49 5 26 2 36

25 4 54 54 23 12 40 37 29 16 16 36 6

26 23 24 1125 7 9 27 31 10 44 5 29 15 35 18 28 17 53 30 27

I 38 11 31 10 4 34

30 12 22 49 28 20 SEPTEMBER

OCTOBER

30 14 35 401 Emersions

Emersions Conjunction 4 33 51

of the Sun and 3 23 3 7 2 6 523 Yupiter, No5 17 32 241 4 I 21' 16.vember 2ift

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

Suppose on the gth Day of October, the Time of the Emersion of Jupiter's first Satellite, be observed by a Telescope at 44 Min. 22 Sec. past 10 at Night; I find by the Table, that the Time of this Emerfion will happen at the British Observatory, 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 Observation, to the East of the British Observatory.

2

1

Euphrof. Well, I suppose by this Time you are tired of talking so much about the Longitude, to one of our Sex, who are so feldom employed in putting in Practice any of the great Discoveries of the Philosophers; but as we have naturally a Curiosity of Enquiry into every Thing that we hear of that is of a public or wonderful Nature, we are oftentimes solicitous to be satisfied about Things that do not so immediately concern us. Of this I shall give you a farther Instance by a Query or two concerning Saturn, his Moons, and Ring. I see they move, after the fame Manner with those of Jupiter, about their Primary; but I observe this System of Moons has not the same Position with respect to the Plane of the Orrery as those of Jupiter have; but as the Time is now far advanced, 'I 'thall beg a more particular Account of what relates to Saturn and his System the next Opportunity we have for Converse on these Subjects.

I

DIALOGUE VIII.
The Use of the Orrery continued.

Cleonicus.
Remember you took Notice, that Saturn's Moons were

not alike posited with those of Jupiter, in regard to the Ecliptic. And it is true, they are not; for those of Jupiter are parallel to that ; but those of Saturn are inclined thereto in an Angle of about 31 Degrees; as is also the Plane of his Ring.

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

Cleon. You rightly observe, they cannot appear fo to. move to an Eye placed upon the Earth ; as you will easily see by the Experiment; for having placed the Pasteboard 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 see on the Pasteboard, that each respective 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,

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Euphrof. It greatly delights me to observe these curious Appearances; and now I see, in fact, how Things are in Nature perforined, and brought about : I see the Reafon why the Moons, while they describe the remote Part of their Orbit, appear direct in Motion, and retrograde, while they describe that Part next to us :- I see 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 must undoubtedly be pleasant to see the wonderful Machinery of Nature thus represented in Epitome, and yet, at the same Time, so perfectly; but there is one Thing which you have not yet adverted to, and that is, that Saturn and his whole system, and the Motion about the Sun, observe a Paralism of Position; or in other Words, the Planet, his Ring, and Satellites, always respect the same Part of the Heavens in every Part of their Orbits; and this you will easily perceive, if you attend to it but a very short Time; but as this is a curious Phænomenon, I shall represent it to you in the Orrery, by Means of the Lamp in the Place of the Sun, and the Pasteboard, connected with the Stem of the Planet ; so as always to be behind it, by which Means the Shadow of the Ring will cast a Shadow upon the Pasteboard, and thereby the several Phases of the Ring will plainly appear, as they are observed through a Telescope in the Heavens during the several Parts of his long Period *.

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

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

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* The Reader will here cast his Eye upon Plate XX, where these Phases re delineated.

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