Page images
PDF
EPUB

is near four Feet in Diameter; the Outfide thereof is adorned with 12 Pilafters, curiously wrought and gilt: Between these, the twelve Signs of the Zodiac are neatly painted, with gilded Frames. Above the Frame is a broad Ring, fupported with twelve Pillars; this Ring represents the Plane of the Ecliptic, upon which there are two Circles of Degrees, and between these the Names and Characters of the twelve Signs. Near the Outside is a Circle of Months and Days, exactly correfponding to the Sun's Place at Noon, each Day throughout the Year.

Above the Ecliptic ftand fome of the principal Circles of the Sphere, according to their refpective Situations in the Heavens, viz. (No. 10.) are the two Colures, divided into Degrees and half Degrees. No. 11. is one Half the Equinoctial Circle, making an Angle of 23 Degrees. The Tropic of Cancer and the Arctic Circle are each fixed parallel, and at their proper Distance from the Equinoctial. On the Northern Half of the Ecliptic, is a Brafs Semicircle, moveable upon two Points fixed in and ≈: This Semi-circle ferves as a moveable Horizon, to be put to any Degree of Latitude upon the North Part of the Meridian. The whole Machine is alfo fo contrived, as to be set to any Latitude, without affecting any of the Infide Motions: For this Purpose, there are two ftrong Hinges (No. 13.) fixed to the Bottom-Frame, upon which the Inftrument moves; and a ftrong Brafs Arch, having Holes at every Degree, through which a strong Pin is to be put, according to the Elevation. This Arch and the two Hinges fupport the whole Machine, when it is lifted up, according to any Latitude; and the Arch, at other Times, lies conveniently under the BottomFrame.

When the Machine is fet to any Latitude, (which is eafily done by two Men, each taking hold of two Handles, conveniently fixed for that Purpose) fet the moveable Horizon to the fame Degree upon the Meridian, and you may form an Idea of the refpective Altitude, or Depreffion of the Planets, both Primary and Secondary. The Sun (No. 1.) ftands in the Middle of the whole Syftem upon a Wire, making an Angle with the Plane of the Ecliptic of about 82 Degrees, which is the Inclination of the Sun's Axis, to the Axis of the Ecliptic.

Next the Sun is a fmall Ball (No. 2.) reprefenting Mercury: Next to Mercury is Venus, (No. 3.) reprefented by a large Ball, and both thefe ftand upon Wires, fo that the Balls themselves may be more vifibly perceived by the Eye. The Earth is reprefented (No. 4.) by an Ivory Ball, having fome of the principal Meridians and Parallels, and a little Sketch of a Map defcribed upon it. The Wire, which fupports the Earth, makes an Angle with the Plane of the Ecliptic of 66 Degrees, which is the Inclination of the Earth's Axis to that of the Ecliptic. Near the Bottom of the Earth's Axis is a Dial-plate, (No. 9.) having an Index pointing to the Hours of the Day as the Earth turns round its Axis.

Round the Earth is a Ring, fupported by two small Pillars; which Ring represents the Orbit of the Moon, and the Divifions upon it anfwer to the Moon's Latitude: The Motion of this Ring reprefents the Motion of the Moon's Orbit, according to that of the Nodes. Within this Ring is the Moon, (No. 5.) having a black Cap, or Cafe, which by its Motion reprefents the Phafes of the Moon, according to her Age.-Without the Orbits of the Earth and Moon is Mars (No. 6.) The next in Order to Mars is Jupiter, and his four Moons (No. 7.) Each of thefe Moons is fupported by a crooked Wire, fixed in a Socket, which turns about the Pillar that fupports Jupiter: Thele Satellites may be turned by the Hand to any Pofition; and yet when the Machine is put into Motion, they will all move in their proper Times. The outermost of all is Saturn, and his five Moons, and Ring, (No. 8.) Thefe Moons are fupported and contrived after the fame Manner with thofe of Jupiter. The whole Machine is put into Motion by turning a small Winch, like the Key of a Clock, (No. 14.) and all the Infide Work is fo truly wrought, that it requires but very small Force to put the whole in Motion.

Euphrof. How are thefe planetary Bodies to be put in Motion? Could I fee, or be made fenfible of it?

Cleon. Yes: Above the Handle there is a cylindrical Pin, which may be drawn a little out, or pushed in at Pleasure When it is pufhed in, all the Planets, both Primary and Secondary, will move according to their refpective Periods, by turning the Handle: When it is drawn

O 3

drawn out, the Motion of the Satellites of Jupiter and Saturn will be ftopped, while all the reft move without Interruption. This is a very good Contrivance to preferve the Inftrument from being clogged by the fwift Motions of the Wheels belonging to the Satellites of Jupiter and Saturn, when the Motion of the rest of the Planets are only confidered.

There is alfo a Brafs Lamp, having two Convex Glaffes, to be put in the Room of the Sun; and alfo a fmaller Earth and Moon, made fomewhat in Proportion to their Distance from each other, which may be put on at Pleasure.

The Lamp turns round at the fame Time with the Earth, and by Means of the Glaffes cafts a ftrong Light upon her And when the fmaller Earth and Moon are placed on, it will be easy to fhew when either of them will be eclipsed.

Euphrof. Well! I think every Thing admirably adapted to answer the Purpose of fuch a Machine; and now I prefume you are ready to put it in Metion, and gratify my Sight with a View of the Secondary Planets as well as the Primary, in the various Revolutions, which I have not yet yet feen in any Inftrument. Cleon, I fhall do it this Inftant: You see I put on the Handle, and pufh in the Pin juft above it; and place a black Patch (or Bit of Wafer) upon the Middle of the Sun, right against the first Degree of Aries (v). You may alfo place Patches upon Venus, Mars, and Jupiter, right against fome noted Point in the Ecliptic. If you lay a Thread from the Sun to the first Degree of Aries (r), you may fet a Mark where it interfects the Orbit of each Planet; and that will be a Help to note the Times of their Revolutions.

Euphrof. Why am I to place Patches upon Venus, Mars, and Jupiter, and not upon Mercury and Saturn?

Cleon. Because these Planets are known to have a Motion about their Axis by a conftant Obfervation of the Motion of Spots, feen on their Surfaces, when obferved through good Telescopes. Thefe Spots, like those of the Sun, are found to be regular, and to defcribe fuch Lines on the Surfaces of the Planets as they really would do, were the Planets to be moved about their Axis, with

thofe

those Spots on their Surfaces in the Times and Manner you here fee reprefented in the Orrery, fo that they plainly prove the diurnal Rotation of those Planets. But as for Mercury and Saturn, no Spots have been obferved on their Surface fufficient to answer such a Purpose.

Euphrof. In what Times do those Planets revòlve about

their Axis?

Cleon. You will immediately fee; for obferve, one entire Turn of the Handle answers to the diurnal Motion of the Earth round her Axis, as may be feen by the Motion of the Hour-Index, which is placed at the Foot of the Wire, on which the Terella is fixed. When the

Index has moved the Space of Ten Hours, you may obferve, that Jupiter has made one Revolution compleat round its Axis: The Handle, being turned until the Hour-Index has paffed over 23 Hours, will bring the Patch upon Venus to its former Situation with refpect to the Ecliptic, which fhews that Venus (?) has made one entire Revolution round her Axis. Mars makes one compleat Revolution round his Axis in 24 Hours, and about 40 Minutes. When the Handle is turned 25 Times round, the Spot upon the Sun will point to the fame Degree of the Ecliptic, as it did when the Inftrument was firft put in Motion. And, thus by obferving the Motion of the Spots upon the Surface of the Sun, and of the Planets in the Heavens, their diurnal Motion was difcovered; after the fame Manner as we do here obferve the Motion of their Representatives, by that of the Marks placed upon them.

Euphrof. Thefe Things delight me very much, as they are quite new to me; I obferve alfo, that the Earth moves upon its Axis; but as this is a more important Subject, it will require a particular Confideration, which I fhall trouble you with fome other Time; at present I fhall be glad to attend you on the Subject of the Secondary Planets, and their Motion round the Primary; and firft, if you pleafe, let me obferve the Motion of the Moon about the Earth at the fame Time as the Earth keeps moving on in her Orbit about the Sun.

Cleon. This you fhall do, and perform it with your own Hands: Take the Winch, and while you turn the

0 4

Handle

Handle you will obferve the Planets perform their Motions, in the fame relative Times as they really do in the Heavens then about the Earth you will obferve, that the Lunar Orb will depart from a Point in the Ecliptic about the Earth, and return to the fame again in 27 Turns and of the Handle, which is called the periodical Month; in Contradiftinction to the fynodical Month.

44

Euphrof. Pray, what is the Difference then between the periodical and fynodical Month? You have formerly mentioned it to me by Way of Theory, and I fhould be glad to fee it exemplified, as it were, in Fact.

Cleon. I will fhew you wherein the Difference confifts: I have already obferved to you, that the periodical Month is the Time, which is taken up by the Moon in departing from any one Point of the Ecliptic, and returning to it the fame again; but more particularly thus: I ftop the Motion of the Earth, and place the Moon juft between the Earth and the Sun; which you know is what we then call the New Moon; then, putting the Machine in Motion, you obferve, that in 27 and Turns of the Handle, the Moon will make one Revolution, and come to the fame Point between the Earth and the Sun again; therefore were the Earth to stand still in the Heavens, the Space of Time between the new Moons would then be 27 and Days, and the periodical and fynodical Months would then be the fame. But, as the Cafe now ftands, fince the Earth really has a Motion about the Sun, at the fame Time that the Moon moves about it, we must represent both in Motion in the Orrery: Therefore, bringing the Moon in the Situation between the Earth and the Sun as before, you will obferve, by turning the Winch, that while the Earth moves progreffively on, you will make not lefs than 29 of the Winch, before the Moon will come exactly between the Earth and the Sun again; and this fhews the Space of Time in what we ufually call the fynodical Month, viz. two Days and longer than the periodical Month.

Turns

Euphrof. You have made the Thing very clear and eafy to be underftood; and I fancy I fhall be able to read a Lecture on the Orrery myself ere long: The Moon, you fee, always fhews the fame Face to the Earth in

the

« PreviousContinue »