The four cardinal points, north and south, east and west. These are marked on the rational horizon.

Upon the celestial globe, are certain circles and points, in addition to those which are common to it and the terrestrial globe.

The zodiac, a belt sixteen degrees broad, which goes round the globe, and in the middle of which runs the ecliptic. Within this circle are the fixed stars, over which the sun passes in his apparent annual course. They are arranged in twelve constellations or signs, which are called the twelve signs of the zodiac. These signs being marked upon the ecliptic, divide it into twelve equal parts, which correspond to the twelve months of the year, as follow:

The sun appears to advance through one of these each month. The first six of these signs are called, in our northern hemisphere, the sammer signs; the other six are called the winter signs.

The two colures are the two meridians which pass through the equinoctial points, and through the solstitial points: the one is called the equinoctial, and the other the solstitial colure.

The poles of the ecliptic are marked upon the celestial globe; and a number of semicircles, springing from them, are called circles of longitude; they serve to measure the distance of heavenly bodies from the ecliptic.

The poles of the horizon; of which that which is immediately over head, is called the zenith; that which is directly under foot, is called the nadir.

Azimuth, or vertical circles, are imaginary circles, meeting in the zenith and nadir, and cutting the horizon at right angles.

The quadrant of altitude is a thin slip of brass, having one of its sides graduated to ninety degrees. It is used for measuring the height of the heavenly bodies above the horizon, and for determining the distances and the bearings of places.

Upon the celestial globe, the latitude of a heavenly body is its distance from the ecliptic, measured upon a circle of longitude; and the longitude of a heavenly body is its distance from the first meridian, measured upon the ecliptic. The distance of the sun or a star from the equinoctial, is called its declination; and distance from the first meridian, measured upon the equator, is called right ascension.

By means of these globes, a number of curious, interesting, and instructive problems may be performed.

QUESTIONS.

What are the globes? Which way must the terrestrial, and which way must the celestial globe be turned, and what motions do those movements imitate? What are the large circles, common to both the globes? What are the smaller circles, common to the terrestrial and celestial globes? What points, and other marks, are common to both the globes? What circles and points are proper to the celestial globe? What is latitude on the celestial globe? What is the longitude of a heavenly body? What is declination? What is right ascension ?

CHAP. XXVIII.

ASTRONOMICAL AND GEOGRAPHICAL
INSTRUMENTS- continued.

THE ORRERY.

THE orrery is a machine contrived to show the motions of the planets round the sun, and of satellites, or secondary bodies, or moons, round their primary bodies, by means of balls of different sizes, moved by machinery. The first orrery made in Great Britain was constructed by Rowley for King George I. The excellent astronomer Ferguson, also, made an orrery which exhibited the movements of the sun, Mercury, Venus, the earth with her moon, and, occasionally, those of the superior planets, and of the moons of Jupiter and Saturn. Since his time these instruments are fabricated with great neatness and ingenuity, of various magnitudes.

The reason of this machine's being called by

the name Orrery, is, that one of the first of them known was made for an earl of Orrery, a very scientific man, and patron of the arts.

THE PLANETARIUM.

The planetarium is a machine, answering the same purposes as the orrery, contrived by an ingenious mathematical instrument-maker in London, of the names of Jones. By various machinery, the planetarium represents all the motions and phenomena of the planetary system. This machine is also made to represent the Ptolemaic system, and to exhibit, at the same time, a manifest confutation of it. It shows all the planets at once in motion about the sun, with the same respective velocities and periods of revolution which they have in, the heavens, with the eclipses which they experience or occasion.

THE COMETARIUM.

The cometarium is a curious machine invented by Desaguliers. It shows the motion of a comet, or eccentric body, moving round the sun, describing equal areas in equal times; and is so contrived as to exhibit such a motion for any degrees of eccentricity.

THE TRAJECTORIUM LUNARE.

The trajectorium lunare is a machine which delineates the paths of the earth and the moon, showing what sort of curves they make in the etherial regions,

THE PENDULUM CLOCK.

The pendulum clock is an instrument contrived to show equal time in hours, minutes, and seconds. By hearing the beats of this pendulum the observer may count time by his ear, while his eye is employed in watching the motion of a heavenly body.

THE ASTRONOMICAL QUADRANT.

Astronomical quadrants are some of the most valuable of astronomical instruments. They are used for observing meridians, and other altitudes of the celestial bodies, and are either mural or portable.

The mural quadrant is in the form of a quarter of a circle, contained between two radii, at right angles to one another, and an arch equal to one fourth part of the circumference of a circle. It is fixed to the side of a wall, and the plane of it erected exactly in the plane of the meridian.

Tycho Brahe was the first person who contrived this mural arch or quadrant.

These instruments are usually made from five to eight feet radius.

The portable astronomical quadrant is an instrument of the greatest use to astronomers, as it may be carried to any part of the world, and put up for observation in an easy and accurate

manner.

THE ASTRONOMICAL OR EQUATORIAL SECTOR, This is an instrument for finding the difference, in right ascension and declination, between two