If there be drawn a great circle upon this sphere, equally diftant from its poles, and having the plane of the circle perpendicular to the axis of the fphere, it will represent the celestial equinox, which divides the heavens into two equal parts or hemifpheres; and the two axes of the fphere will represent the two poles of the heavens.

If there be another great circle drawn upon the sphere, cutting the equinoctial at an angle of 23 degrees, in two

oppofite points, this circle will reprefent the ecliptic, or circle of the fun's apparent annual motion; one half of which is on the north fide, and the other half on the south fide of the equinoctial.

If there be made a large ftud to move eastward in this ecliptic, and with fuch a motion as to go quite round it in the time that the fphere is turned round weftward upon its axis 366 times; this ftud will reprefent the fun changing its place every day in the ecliptic, a 365th part; and going round weftward in the fame direction as the stars, but with a motion fo much flower than that of the stars, that they will make 366 revolutions in the time that the fun makes only 365, about the axis of the sphere.

If the terreftrial globe in this machine be about one inch in diameter, and the diameter of the ftarry fphere about five or fix feet, a small infect, placed upon the globe, would fee only a very small portion of its furface; but it would fee one half of the furface of the ftarry fphere, the convexity of the globe hiding the other half from its view. If the fphere be fet in motion as before directed, and the globe also revolving on its own axis, the infect will fee all the phenomena observed by the inhabitants of this world, in the diurnal rotation of the earth round its axis.

The exterior parts of this machine are several brass rings, which represent the principal circles in the heavens: viz. 1. The equinoctial: 2: the ecliptic, divided into the figns and degrees, and alfo into the months and the days of the Q2

year,

year, to fhow in what point of the ecliptic the fun is on any given day in the year 3. the two tropics: 4. the artic and antarЯic circles: 5. the equinoctial colure, which is a great circle paffing through the north and fouth poles of the heavens, and through the equinoctial circle at the points where the equinoctial is cut by the ecliptic: 6. the folftitial colure, which is a great circle paffing through the poles of the heavens, and at right angles to the equinoctial colure. Hence the folftitial colure paffes through the equinoctial at the points where the equinoctial is at the greateft diftance from the ecliptic. These points in the equinoctial are called the fol fitial points.

In the north pole of the ecliptic is a nut, to which is fixed one end of a quadrantal wire, having at the other end a fmall fun, which is carried round the ecliptic, by turning the nut; and in the fouth pole of the ecliptic, another quadrantal wire is fixed, with a finall moon upon it, which may be moved round by the hand. There is alfo a particular contrivance, for causing the moon to move in her own orbit.

On the axis of the fmall globe is fixed a flat celeftial meridian, which may be fet directly over the meridian of any place on the globe; and then turned round with the globe, fo as to keep over the fame meridian. This globe has also a moveable horizon, which turns upon two wires, which proceed from it on the east and weft points of the globe, and entering the globe at the oppofite points in the equator, which is a moveable brafs ring, let into the globe in a groove. whole fabric is fupported on a pedestal, and may be elevated or depreffed to any number of degrees, from 0 to 90,

Defcription of the Terreftrial Globe.

The

On the terreftrial globe are drawn all the principal circles before mentioned, as the equator, ecliptic, tropics, polar circles, and meridians. The ecliptic is divided into twelve figns, and each fign into thirty degrees. Each tropic is 23

degrees

degrees from the equator; and each polar circle 23 degrees from its refpective pole. There are alfo circles drawn parallel to the equator, at every 10 degrees diftance from it, on each fide towards the poles; thefe circles are called parallels f latitude. There are, alfo, feveral other circles, drawn perpendicularly to the equator, and interfecting each other at the poles; these circles are called meridians, and sometimes circles of longitude, or hour circles; and on large globes they are drawn through every tenth degree of the equator; but on globes of less than 12 inches diameter they are drawn through every fifteenth degree.

The globe is hung in a brass ring, called the brazen méridian, turning upon a wire in each pole, funk into one fide of the meridian ring. This meridian is divided into 360 degrees; one half of thefe degrees are numbered from the equator to the poles, to fhow the latitude of places; the other half are numbered from the poles to the equator, to show how to elevate either of the poles above the horizon. This ring divides the globe into two equal parts, called the eaftern and western hemifpheres; as the equator divides it into the northern and Southern hemispheres.

The brazen meridian is let into two notches, made in a broad flat ring, called the wooden horizon; the upper furface of which divides the globe into two equal parts, called the upper and lower hemifpheres. This horizon correfponds to the true rational horizon; and upon it are feveral concentric circles, which contain the months of the year, the signs and degrees anfwering to the fun's place for each month and day, the thirty-two points of the compafs, and the circles of amplitude and azimuth, with fome other circles.

There is a small horary circle, fixed to the north part of the brazen meridian, and having the wire in the north pole of the globe in its centre; on which wine is an index, which goes over all the twenty-four hours of the circle, as the globe is turned round its axis. Sometimes there are two horary circles, one at each pole.

There

There is a thin flip of brass, calledthe quadrant of altitude, divided into 90 degrees. This is occafionally fixed to the uppermost point of the brazen meridian, by a nut and screw, about which the quadrant turns round.

There is alfo, to some globes, a magnetic needle, which moves over a circle, divided into 360 degrees, alfo over the thirty-two points of the compafs. This needle ferves to fix the globe according to the meridian of the place, as the needle makes nearly a conftant certain angle with the meridian, which angle is called the variation of the compass; and which being known, the globe may be rectified to the true meridian of the place: thus, at London, the variation of the needle is 23 degrees northward; therefore, by moving the frame of the globe about, till the needle fettles itself near the twenty-third degree, reckoning weftward from the north point, we then have the brafs meridian of the globe coinciding with the true meridian of the place,

SECT. II.

OF THE SOLUTION OF GEOGRAPHICAL PROBLEMS.

Directions for using the Globe.

IN ufing the globe, the eaft fide of the horizon muft always be kept towards you (except the globe require turning by the Problems), for then the graduated fign of the meridian will be towards you, the quadrant of altitude before you, and the globe divided exactly into two hemifpheres, by the graduated fide of the meridian.

When it is required to turn the globe and frame entirely round, as is the cafe in working fome Problems, the ball of the globe will sometimes be moved from the degree to which it is fet, by fuch motion; to avoid this, the feather end of a quill may be thrust between the ball of the globe and the meridian, which, without hurting the ball, will keep it from turning in the meridian, while you turn the horizon about.

PROBLEM I.

To find the Latitude and Longitude of any given Place upon the Globe.

Turn the globe on its axis, until the given place comes exactly under the graduated fide of the brazen meridian, and obferve what degree of the meridian the place then lies under, which is its latitude, and is either north or fouth.

The globe remaining in this pofition, the degree of the equator, which is under the graduated fide of the brazen. meridian, is the longitude of the place; and is either east or weft, as the place lies on the east or weft fide of that which is accounted the firft meridian. Thus, all the Atlantic Ocean, and the continent of America, is on the weft fide of the meridian of London; and the greater part of Europe and of Africa, together with all Afia, is on the eaft fide of the meridian of London, which is reckoned the first meridian of the globe, by British geographers and aftronomers; though the exact meridian for England is that of Greenwich.

PROBLEM II.

To find any Place on the Globe, having the Latitude and Longitude.

This is the reverse of the former Problem, and is found by bringing the point of longitude in the equator to the brazen meridian: then count from the equator on the brazen

meridian