But thefe Things we shall illuftrate hereafter. To be farther infifted upon at prefent would be mifpending of Time, and though you are well verfed in thofe Poets, yet, till you have learn'd their aftronomical Diftinctions of the Seafons, by the Rifing and Setting of the Stars, and by the Globe properly conftructed to fhew them, it will not avail you to have any Examples of this Sort to practife; but when you have that, it will open a fine Field for your Phyfico-Poetical Speculation, and View of diftant Times.

Euphrof. I will make it my Business to profecute that particular Part of Study, as I apprehend there will be fomething very curious:-But what other Problem am I next to practife on the Globe?

Cleon. One other, the most confiderable and entertaining of all, and that is,-to fhew me the Face of the Heavens, or what Stars will be vifible on the Firft Day of October, at IX at Night?

Euphrof. Confidering the Nature of this Problem, I find it is doing but little more than I have done before; but as it is of fo general and curious a Nature, I will proceed to it Step by Step.- First, I rectify the -then the Sun's

Globe to the Latitude of London; Place for that Day at Noon is 7° 30' of Libra, which Point I bring to the Meridian, and fet the Hour-Index to XII. Then I turn the Globe about, till the Index points to X at Night,-and there letting it reft, I see, in one View, all the Stars which then appear above the Horizon; together with all the Conftellations in which they are contained.-This is a noble Problem, Cleonicus, indeed! For, as by this, I fhall be able, on the Evening of any Day, to reprefent the Stars vifible for any Hour of the Night; I can very eafily learn to diftinguifh the Conftellations in the Heavens, by thofe on the Globe; at leaft, I fhall be able to remember, to what particular Afterifm any Clufter, or Affemblage of Stars belong, which I at any Time view in the Heavens. Allo, by this Means, I fhall imprint in my Memory the Idea of all the principal Stars; especially thofe of the first and fecond Magnitude, and fhall foon be able to call them all by their Names,

Cleon. This is the grand Defign and Ufe of the celeftial Globe, which is, as it were, the Primer of Aftronomy, by which our Youth are inftructed in the celeftial Characters, and taught, as it were, to read in the ftarry Heavens.-There are yet other nobler Ufes to be made of the celeftial Globe; one Inftance I fhall give you in Navigation, the mcft beneficial of all Sciences, which is, to find the Hour of the Night, by having the Latitude of the Place, and the observed Altitude of a Star given.

Euphrof. As I am not fo clear in this Problem, as in fome others, I fancy, I had better fee you perform the Thing, than attempt it myself, left I fhould blunder about it.

Cleon. The Method is very eafy. Thus :-Suppofe in the Night, following the Firft of October, a Perfon was to take the Altitude of the Star Bellatrix, and find it just 30° in the Latitude of 51°: 30'. Then to find the Hour, nothing more is requifite, than to rectify the Globe for that Day and Latitude.-Then, I bring the Quadrant of Altitude towards the Star Bellatrix, in the Left Shoulder of Orion, (which is in the Eaftern Hemisphere) and then, moving the Globe and Quadrant fo together, till the faid Star comes exactly under the 30° of the Quadrant.-Here I let the Globe reft; and, if you look at the Index, you will find the Time is thereby fhewn to be Half an Hour after I. in the Morning.

Euphrof. By the Manner of your performing this Problem, I fee it is very eafy, and will fhew the Hour of the Night by Land, as well as at Sea; to which Purpose, I fhall now and then apply it by Way of Amusement.

Cleon, In all thefe Problems of rectifying the Globe for any particular Time, you obferve, among other Things, the Course or Pofition of that remarkable Phænomenon, called the Gallaxy, or Milky-way, among the Stars in the Heavens for that particular Time. Thus, for Inftance, on the First of October, at X at Night, you will obferve the Gallaxy to pafs exactly thro' your Zenith, or over your Head in the Heavens, and to cut the Horizon of Course at right Angles in

the

the E. N. E. and W. S. W. Points of the Horizon; and it is very rare, that ever you fee it in such a Pofition, as to divide the vifible Hemifphere into two equal Parts; as there is but one other Time of the Year in which you can obferve it.

Euphrof. This is a curious Affair, and I fhall diligently attend to fuch Appearances. -One Thing further I have taken Notice of fince I have been inured to the Ufe of this Globe; and that is, that feveral of the Stars never fet in our Latitude, but are vifible the whole Night; and, on the other Hand, that feveral Stars and Conftellations never appear above our Horizon, or are seen by us at all.

Cleon. It is rightly obferved, my Euphrofyne; and the Reafon is evident from the very Pofition of the Globe itfelf; fince all thofe Stars, which are at a lefs Distance from the North Pole than the Latitude of the Place, muft neceffarily circulate about the Pole above the Horizon, when on the North Part of the Meridian; and, on the other Hand, thofe Stars, whose Southern Declination exceeds the Complement of the Latitude, can never appear above the Horizon, which you well remember in the Cafe of the Comet at our last Interview.

DIALOGUE IX.

Of the PRECESSION of the EQUINOX, MUTATIONS of SEASONS, and the Celestial Phænomena, resulting from the retrograde MoTION of the EARTH'S AXIS about that of the ECLIPTIC, illuftrated by a NEW CONSTRUCTION and Apparatus of the CELESTIAL GLOBE.

Euphrofyne.

Suppofe in the preceding Dialogue you pretty well confidered the principal Ufes of the ftellated Globe;

not

not that I am weary of thefe celeftial Studies; if any Thing farther remains, I fhall be all Attention to understand it.

Cleon. There is only one Particular more of Conse, quence to be confidered, which is a Point of a very curious and fublime Nature. I have formerly mentioned to you fomething of this Matter; when I told you, that the North Star, whofe Motion is now altogether infenfible, will, in other Ages of the World, have a Motion like the rest of the Stars, extremely obvious, round other Points in the Heavens.In fhort, the whole Affair is this; the Globe of our Earth has another Motion befides thofe already mentioned, when we difcourfed of the Orrery, which is properly a conical Motion of its Axis about the Axis of the Ecliptic, by which Means, the Poles of the World have a Motion about thofe of the Ecliptic. This Motion is retrograde, or contrary to the Order of the Signs, and, in Confequence of this, not only the celeftial Poles, but the Stars that are near them, and, indeed, all that we fee in the Heavens, muft neceffarily appear, in Courfe of Time, to move forward. This Motion of the Earth's Axis backward is, indeed, very flow, being only at the Rate of about 50' in a Year, or 1° in 72 Years; therefore to move through 360°, or a whole Circle, will require no less than 25920 Years; and in that Space of Time, the Stars will all of them have an apparent Motion quite round the Heavens; from whence you will understand, that thofe, who live in the different Parts, or Ages of this great Period, will neceffarily fee the Stars in different Parts of the Heavens; for fince the Place of each Star moves forward one whole Degree in 72 Years, the Change of their apparent Places will be very evident in the Space of one Century, and therefore, from one Age to another, the Longitude of the Stars will be continually increafing, till they have compleated one entire Revolution through the Heavens.

Euphrof. This to me is a new Doctrine, and as it conveys fuch wonderful Ideas, I fhall think myfelf extremely happy, if I can underftand it by any Means, or mechanical Contrivance, applied to the Globe; for I obferve, you have made a Provifion of that Kind, as I

here fee a Globe with fuch Appurtenances as are not ufual or common; by which, I prefume, you are to explain this Phænomenon to me.

Cleon. That is the Defign of this new Conftruction, my Euphrofyne.-There are feveral Ways, by which this Motion may be eafily reprefented to you; one is, by this new Difpofition of the celestial Globe; others you will fee hereafter.This Globe is fo contrived, that the Poles of the Equinoctial are made to move at Pleasure round the Poles of the Ecliptic, and fo to reprefent the Face of the Heavens for any Time of that long Period above-mentioned, namely, twenty-fix Thoufand Years.-The late Mr. Senex, among many other Improvements of the Globe, contrived this principal one By Means of these Pieces of Brafs, which you fee on either Side of the Globe; in one End of each is a Piece of Iron, fixed and applied to the Brafs Meridian as usual, to give the Globe a Motion on the Axis, or Poles of the World.-The other End of each Brafs Arm is applied to the Poles of the Ecliptic, in fuch-a Manner, as that it may be fixed, or moveable at Pleasure. By this Means, the Poles of the World, or of the Equinoctial, may be placed in any Pofition about the Poles of the Ecliptic; and as they revolve, they will defcribe a Circle of 47° in Diameter, about the faid Poles in the Ecliptic.-And to make the Idea still more easy and familiar to you, I have added to this Invention a moveable equinoctial Circle with its Colures, and alfo a moveable Ecliptic; whence, by a few Examples of the Ufe of the Globe, thus furnished and conftructed, you cannot but understand-how all the Stars must have an apparent Motion forward in the Heavens ; — how their Longitudes and Declinations must be conftantly altering;-how various the Times of their Rifing and Setting muft be;-how those which are visible, in one Age, become invifible in another ;-the Change of Seafons thro' all the Months of the Calendar ;-and what is ufually called the Preceffion of the Equinoxes by Aftronomers; I fay, how all thefe great Phænomena are produced by this one fimple Motion, you will now fee explained by Example.

Euphrof. This will give me the greatest Pleasure, Pray, proceed,