and is a Kind of general Azimuth, as this, a general Meridian. But, enough of this Circle, till we come to the practical Part on the Globes; and fhall leave it with the poetical Defcription, which Manilius has given of it, as follows:

This from the Bear

Defcrib'd, furrounds the Middle of the Sphere;
Divides the Day, and marks exactly Noon,
Betwixt the rifing and the fetting Sun:
The Signs it changes as we move below,
Run Eaft or Weft, it varies as you go;
For 'tis that Line, which Way foe'er we tread,
That cuts the Heav'n exactly o'er our Head,
And marks the Vertex; which doth plainly proves
That it must change as often as we move.
Not one Meridian can the World fuffice,
It paffes thro' each Portion of the Skies;
Thus, when the Sun is dawning o'er the Eaft,
'Tis their fixth Hour, and fets their fixth at Weft;
Tho' thofe two Hours we count our Day's Extremes,
Which feebly warm us with their diftant Beams.

DIALOGUE

IV.

Of the EQUINOCTIAL, and the DEGREES of

TH

LONGITUDE.

Euphrofyne.

HE next Circle then, Cleonicus, which you are to inftruct me in the Knowledge of, is the Equinoctial: I fhall here, as before, defire to be informed of the Reafon of the Name.

Cleon. To understand this well, you must remember, as I told you, that this great Circle furrounds the Sphere, exactly in the Middle, or at equal Diftance from the Poles of the World; and, fecondly, you must observe, that the Ecliptic, which reprefents the Sun's Way, cuts this Circle in two oppofite Points, and therefore of Courfe, the Sun is twice in the Year upon this Circle; and fince one Half of this Circle is always above the Horizon,

Horizon, and the other Half below it, it comes to pass, that, when the Sun is in those two Points, the Days and Nights will then be equal, which is implied by the Name Equinoctial. For the fame Reafon alfo, thofe two Points are called the Equinoxes, or Equinoxial Points.

Euphrof. Very good: Pray, has this Circle any other

Name?

Cleon. Yes; upon this Sphere, and the celeftial Globe, it is called the Equinoctial; but on the terreftrial Globe, it is called the Equator; because it equates, or divides the Globe of the Earth into two equal Parts, or Hemispheres, as I faid: It is alfo called the Equator in all Maps, and Sea Charts.

Euphrof. I am glad I know that; for I would not (though a Woman) fpeak improperly in naming this, or any other Part of the Sphere, or Globes. But what is the proper Ufe of this Circle?

Cleon. It is, you fee, divided into 360 Degrees, beginning and ending in the Equinox Point, and numbered from Weft to Eaft at every tenth Degree. These Degrees are called the Longitude, on the terreftrial Globe; and on the celeftial Globe and Sphere, they are called Degrees of Right Afcenfion.

Euphrof. I have heard much Talk of the Longitude, and have as little Knowledge of it, after all, as though I had never heard it mentioned: But as we are now come to the Circle of Longitude, I prefume you can give me a better Idea of that Matter.

Cleon. The Longitude of a Place is the Distance thereof, reckoned in Degrees of the Equator, from that Point of the Equator where the Degrees begin; and the Meridian, which paffes through this Beginning of Longitude, is called by the Geographers, the first Meridian. Now, fuppofe the firft Meridian be that of the City of London, as it is upon the beft Globes, then the Arch of the Equator, contained between this and the Meridian of any other Place, is faid to be the Longitude of that Place from London; which you will learn with Eafe to find, when we come to the Problems of the Globe.

Euphrof. Then I understand, that as the Latitude of a Place is its Diftance, North or South of the Equator, reckoned on the general Meridian; fo the Longitude of

N 3

any

any Place is its Distance, Eaft or Weft, from the first Meridian, reckoned on the Equator.

Cleon. You apprehend it very well, Euphrofyne; it is fo; and the fame Thing which on the terrestrial Globe and Maps is called Longitude, is, on the celestial Globe and Sphere, called Right Afcenfion; for with refpect to the Sun, Planets, and Stars, their Distance. Eastward is also counted on the Equinoctial, from a first, or fixed Meridian, and it is that which paffes through the Equinox (Q) where the Graduation begins; fo that the Meridian, paffing through the Sun or any Star, fhews on the Equinoctial the Degrees of its Right Afcenfion, or Distance from the Equinoctial Point: But in the Ecliptic, their Distance is called Longitude, and is reckoned from a very large, bright Star in the Constellation, Aries, which Star I fhall fhew you hereafter, both on the Globe, and in the Heavens.

Euphrof. But why is their Longitude reckoned from that Star, and not from the Beginning of the Ecliptic, as their Right Afcenfion is?

Cleon. Because this Star was formerly near the Meridian of that Point, yea, almoft in the Equinox itself, at the Time of Hipparchus, about 2000 Years ago; fince when, it has moved forward (with the flow Motion of the Stars I formerly mentioned to you) about 29 Degrees; or rather the Equinoxes have moved fo much backward; and this Motion of the Equinoxes backward, makes the Sun enter thofe Points fooner every Year by nearly 20 Minutes, and this makes what you will find in Books of Aftronomy is called the Preceffion of the Equinoxes.

Euphrof. Well, I think I understand now what you call the Longitude, and Right Afcenfion, of the Planets and Stars: Pray, to what other Purpose does this Circle ferve?

Cleon. I have already told you, that the Distance of the Sun, Planet, or Star, from the Equinoctial, North or South, is called its Declination: Befides, the Degrees of the Equinoctial are convertible into Time, and thew in what Time any Part of a Revolution of the Sphere, or Globe, is performed.

Euphrof. This, I fuppofe, is done by means of the Hour-Circle, and Hand, which is fixed about the North Pole.

Cleon. You judge right; for that Circle being fixed about the Axis, and the Hand, or Index, upon the Axis, and the Axis itfelf being moveable in the Meridian at each Pole, it must follow, that, if the Sphere be moved, or revolved, the Axis will also be revolved, and fo carry the Index round upon the Hours engraved on the Circle.

Euphrof. I obferve, that on the Circle the 12 Hours are twice engraved, pray, why is that?

Cleon. The firft 12 Hours fhew the Time from Noon till Midnight; the laft from Midnight to Noon the next Day; for you must know the Aftronomers begin the Day at Noon; therefore the two Hours of XII. ftand exactly upon the graduated Edge of the general Meridian. Euphrof. Well, now, Cleonicus, tell me how the Time in which any Motion is perform'd.

you find

Cleon. I will; and firft, you obferve, I bring the Beginning of the Degrees, or Equinoctial Point, to the Meridian. Secondly, I then (holding the Sphere in that Pofition) turn the Index about to the Hour of XII. upon the Edge of the Meridian next to us. Thirdly, then you obferve, I turn the Sphere once round, till the faid Point comes again to the Meridian, and the Index paffes once round the Hour-Circle, which fhews, that one Revolution of the Sphere is performed in twenty-four Hours Time.

Euphrof. That is extremely eafy to understand; and

how then?

Cleon. If one Revolution, or 360 Degrees, be made. in 24 Hours, then half a Revolution, or 180 Degrees, is made in 12 Hours; a Quarter, or 90 Degrees, in fix; and in that Proportion 15 Degrees of the Equinoctial paffes the Meridian in one Hour; and 15 Minutes, or one-fourth of a Degree, in one Minute of Time; which Notions of Motion and Time it will be neceffary for you to have a clear Idea of, inafmuch as they are the Grounds of many Problems on the Globes, which you willthink very curious and diverting,

Euphrof. I am obliged to you thus to prepare me with fuch previous Knowledge for a right underftanding of the Globes, towards which my Inclinations are very strong.

Cleon. We'll make the Introduction to them as short as can be; and to that End we fhall dwell no longer on the Equinoctial here; which Circle, Manilius thus defcribes.

The Equinoctial in the Midft divides

The Sphere, and fees the Pole on both its Sides.

And there, when Phoebus drives, he fpreads his Light
On all alike, and equals Day and Night:

For in the Midft he does the Heav'ns divide,

And cheers the Spring, and warms the Autumn's Pride, And this large Circle drawn from Cancer's Flame,

Twice twelve Degrees divides the ftarry Frame.

Creech's Man. Lib. I.

DIALOGUE V.

Of the ECLIPTIC, and the Longitude and Latitude of the Heavenly Bodies,

WE

Cleonicus.

E are now come to the great Circle, called the Ecliptic; because (as I formerly told you) all the Eclipfes happen in, or very near it; for this great Circle reprefents the Sun's apparent annual Path, or Tract, through the Heavens; and therefore, whenever the Moon obfcures the Sun, or is eclipfed by him, it must be when the is in, or near this Line.

Euphrof. This I understand; as alfo I remember you told me, this Circle was divided into 12 Parts, or Signs, whofe Names and Characters you explained, and are fuch as I fee here engraved: But, pray, what was the Reafon of fuch a Divifion of the Ecliptic?

Cleon. For the greater Eafe and Readinefs in judging of, and exprefling the Places of the Sun, Moon, and Planets, at any Time; for we can form a more diftinct