those images, are called constellations. Twelve of these are represented on the ecliptic circle, and ex- › tend both northward and southward from it. So many of those stars as fall within the limits of 8 degrees on both sides the ecliptic circle, together with such parts of their images as are contained within the aforesaid bounds, constitute a kind of broad hoop, belt, or girdle, which is called the zodiac.

The names and the respective characters of the twelve signs of the ecliptic, may be learned by in-spection on the surface of the broad paper circle, and the constellations from the globe itself.

The zodiac is represented by eight circles parallel to the ecliptic, on each side thereof; these circles are one degree distant from each other, so that the whole breadth of the zodiac is sixteen degrees.

Amongst those parallels, the latitude of the planets is reckoned; and in their apparent motion they never exceed the limits of the zodiac.

On each side of the zodiac, as we observed, other constellations are distinguished; those on the north side are called northern, and those on the south side of it, southern constellations.

OF THE PRECESSION OF THE EQUINOXES.

All those stars which compose these constellations, are supposed to increase their longitude continually; upon which supposition, the whole starry firmament has a slow motion from west to east; insomuch that the first star in the constellation of Aries, which ap

peared in the vernal intersection of the equator and ecliptic in the time of Meton, the Athenian, upwards of 1900 years ago, is now removed about 30 degrees from it.

This change of the stars in longitude, which has now become sufficiently apparent, is owing to a small retrograde motion of the equinoctial points, of about 50 seconds in a year, which is occasioned by the attraction of the sun and moon upon the protuberant matter about the equator. The same cause also occasions a small deviation in the parallelism of the earth's axis, by which it is continually directed' towards different points in the heavens, and makes a complete revolution round the ecliptic, in about 25,920 years. The former of these motions is called the precession of the equinoxes, the latter the nutation of the earth's axis. In consequence of this shifting of the equinoctial points, an alteration has taken place in the signs of the ecliptic; those stars, which, in the infancy of astronomy were in Aries, being now got into Taurus, those of Taurus into Gemini, &c. so that the stars which rose and set at any particular season of the year, in the times of Hesiod, Eudoxes, and Virgil, will not at present answer the description given of them by those writers.

PROBLEM I. To represent the motion of the equinoctial points backwards, or in antecedentia, upon the celestial globe.

Elevate the north pole, so that its axis may be

perpendicular to the plane of the broad

paper circle, and the equator will then be in the same plane; let these represent the ecliptic, and then the poles of the globe will also represent those of the ecliptic; the ecliptic line upon the globe will, at the same time represent the equator, inclined in an angle of 23 degrees, to the broad paper circle now called the ecliptic, and cutting it in two points, which are called the equinoctial intersections.

Now if you turn the globe slowly round upon its axis, from east to west, while it is in this position, these points of intersection will move round the same way; and the inclination of the circle, which in shewing this motion represents the equinoctial, will not be altered by such a revolution of the intersecting or equinoctial points. This motion is called the precession of equinoxes, because it carries the equinoctial points backwards amongst the fixed stars.

The poles of the world seem to describe a circle from east to west, round the poles of the ecliptic, arising from the precession of the equinox. It is a very slow motion, for the equinoctial points take up 72 years to move one degree, and therefore they are 25,920 years in describing 360 degrees, or completing a revolution.

This motion of the poles is easily represented by the above described position of the globe, in which, if the reader remembers the broad paper circle represents the ecliptic, and the axis of the globe being perpendicular thereto, represents the axis of the ecliptic; and the two points where the circular lines

t

meet will represent the poles of the world, whence, as the globe is slowly turned from east to west, these points will revolve the same way about the poles of the globe, which are here supposed to represent the poles of the ecliptic. The axis of the world may revolve as above, although its situation, with respect to the ecliptic, be not altered; for the points here supposed to represent the poles of the world, will always keep the same distance from the broad paper circle, which represents the ecliptic in this situation of the globe.*

From the different degrees of brightness in the stars, some appear to be greater than others, or nearer to us: on our celestial globe they are distinguished into seven different magnitudes.

* A globe is sometimes mounted with an apparatus peculiarly for this purpose, and is the best adapted to convey a sufficient idea of this curious phenomenon, and which I have already more clearly explained in page 174.

OF

THE USE

OF THE

CELESTIAL GLOBE,

IN THE SOLUTION OF

PROBLEMS RELATIVE TO THE SUN.

EVERY thing that relates to the sun is of such importance to man, that in all things he claims a natural pre-eminence. The sun is at once the most beautiful emblem of the Supreme Being, and under his influence, the fostering parent of worlds; being present to them by his rays, cheering them by his countenance, cherishing them by his heat, adorning them at each returning spring with the gayest and richest attire, illuminating them with his light, and feeding the lamp of life.

To the ancients, he was known under a variety of names, each characteristic of his different effects; he was their Hercules, the great deliverer, the restorer of light out of darkness, the dispenser of good, continually labouring for the happiness of a depraved He was the Mithra of the Persians, a word derived from love, or mercy, because the whole world is cherished by him, and feels, as it were, the effects of his love.

race.