18. Since the Earth moves round its Axis from Weft to East, 'tis plain a Spectator upon its Surface, together with his Horizon, muft move the fame way; confequently thefe Celeftial Bodies towards the Eaft, that were before inconfpicuous will become vifible, the Horizon being depreffed below them; and these towards the Weft, that were before in view, will become invisible, the Horizon being elevated above them. And hence arifes the apparent Motion of all the Heavenly Bodies, by which they appear to describe Circles round the Poles, parallel to the Celestial Equator, which are greater or lefs according as they are more or lefs diftant from the nearest Pole.

19. When any Celestial Body comes first in view, or when it is on the eastern fide of the Horizon, it is then faid to Rise; and when by its apparent Motion it comes to the Meridian, it is faid to Culminate ; and lastly, when it begins to disappear, or is upon the western fide of the Horizon, it is then said to Set.

20. If through the Center of the Earth there be `drawn a Line perpendicular to the Plain of the Horizon, and produc'd to the Heavens, it will there mark out two Points; the one, which is directly over our Heads, is call'd the Zenith; and the oppofite Point thereto, which is invifible to us, viz. directly under our Feet, is call'd the Nadir.

21. Vertical or Azimuth Circles, are great Circles paffing thro' the Zenith and Nadir, and cutting the Horizon at right Angles. Among the Vertical Circles there are two principal ones, viz. the Meridian, which paffes thro' the Zenith, Nadir, and Poles, and cuts both the Equator and Horizon at right Angles; the points in which it cuts the Horizon are the South and North Points; and the other principal Vertical, call'd the prime Vertical, is that which cuts the Meridian at right Angles, and meets the

L

Hori

Horizon in two oppofite points, call'd the Eaft and West points.

22. Leffer Circles parallel to the Horizon are call'd Almicanthers, or Parallels of Altitude. And thefe continually decrease the nearer they are to the Zenith.

24. The Altitude, or Depreffion of any heavenly Body above or below the Horizon, is an Arch of a Vertical Circle intercepted between the Horizon and Center of the Object.

25. The Zenith Distance of any heavenly Object, is that Arch of the vertical Circle paffing through it, intercepted between the Center of the Object and the Zenith, which is always the Compliment of the Altitude.

26. Let the Circle AHNO reprefent the Earth, projected on the plain of fome Meridian, A fome place upon that Meridian; draw the Diameter HO at a Quadrant, or 90 Degrees, distance from A; then HO will reprefent the Horizon of the Place A (by Art. 16. of this). Let P and p be the two Poles; confequently Pp the Axis of the Earth, and the Diameter EQ at right Angles with that will represent the Equator, (by Art. 5.) make P a equal to P A, and draw the Circle A a parallel to the Equator EQ, and this will be the parallel of Latitude the place A lies on. The Arch AE will be the Latitude of the place A, and AP, the Compliment of it's Latitude (by Art. 8. and 11.) the Point in the Heavens directly above A will be the Zenith, and that directly above N will be the Nadir of the Place A (by Art. 20.) the great Circle ACN will be the prime Vertical (by Art. 21.) and the Points H and O will be the South and North Points, and C will represent the East and West Points in the Horizon of A. Let S be any heavenly Object, and ASN a vertical or azimuth Circle paffing thro' the Cen

ter of the Object; alfo KS it's parallel of Altitude; then SG will be the Altitude and SA the Zenith Distance of the Object S (by Art. 24. and 25.). A.

gain, let any other place upon the Earth be affum'd, as B, and its Meridian will be PBp, and its parallel of Latitude DBd; then the Latitude of B will be BF or DE, and the Compliment of it's Latitude will be BP or DP. Alfo the difference of Latitude between the two places A and B, will be BL or DA (by Art. 10.). If the Meridian paffing thro' A, be fuppos'd the firft Meridian, then the Longitude of B will be EF (by Art. 12.) but if the Meridian of A be not fuppos'd the first Meridian, then the difference of Longitude between the two Places A and B will be E F (by Art. 14.).

27. The Syftem of the Universe according to the latest Aftronomers is as follows, viz. The Sun

is fuppos'd to be in the common Center of Gravity, of fix opake spherical Bodies called Planets, which are at different distances from the Sun, and and perform their feveral Periods round him in different Times; the names of these Planets and the Characters by which they are exprefs'd, are as follows, viz. Mercury, Venus, the Earth e,

B

C

E

A

D

Mars &, Jupiter 4, and Saturn . And they all move round the Sun, from Weft to East, in Orbs very little inclin'd to one another, and the Plains of thefe Orbs cut one another in Lines paffing through the Center of the Sun; confequently a

Spectator plac'd in the Center of the Sun, will be in the Plain of each of their Orbs, and will there view the Planets, performing their feveral Periods round him, from Weft to Eaft, according to the order of the Letters ABCD, (in the annex'd Scheme) and in different Times, viz. Mercury, which is neareft the Sun, moves round his Orb in 87 Days, and 23 Hours, or three Months nearly. Then Venus, which is next to Mercury, performs her Period in 224 Days and 17 Hours, or about 8 Months. The Planet which is third in order from the Sun, is our Earth, which performs its Circuit in 365 Days, 5 Hours, and 49 Minutes, or a Year. Next to the Earth is Mars &, who moves round his Orb in 686 Days and 23 Hours, or a little less than 2 Years. Then Jupiter 4, whose Orb is vastly extended beyond that of Mars, performs his Circuit in 4332 Days, 12 Hours, which is about 12 Years. And laftly Saturn h, who is furtheft diftant from the Sun, compleats his Revolution in 10759 Days, and 7 Hours, which is fomething less than 30 Years. Their distances from the Sun exprefs'd in the Scheme, are nearly proportional to their true distance in the Heavens.

28. Three of the Planets, viz. Mars, Jupiter, and Saturn, whofe Orbs are beyond that of the Earth, are called superior Planets; and the two Planets Venus and Mercury, whofe Orbs are between the Earth's Orb and the Sun, are called the inferior Planets.

29. The three Planets, Jupiter, Saturn, and the Earth, are obferved to have other fmaller ones constantly attending them, called Secondary Planets, or Satellites. These Satellites always attend their respective Primaries in their Revolutions about the Sun, and at the fame Time they are conftantly moving about them; the Earth has one, viz. the

Moon,