portioned to their distance from the sun: the curve which they describe in their revolution is what is termed their orbit.

GENERAL EXPLANATION OF THE SOLAR SYSTEM BY THE PLANETARIUM.

In the course of the system is the sun, placed in the heavens by that Almighty Power who said, "Let there be light, and there was light,” to be the fountain of light and heat to all the planets revolving round him.

"His rapid rays,

"Themselves unmeasured, measure all our days:
"A thousand worlds confess his quick'ning heat,
"And all he cheers are fruitful, fair, and sweet."

The situation of this glorious body, in the system, is pointed out in this machine by the brass ball in

the centre.

Mercury is the nearest planet to the sun, and moves round him in about 88 days. To observe this by the planetarium, observe the parts of the ecliptic where Mercury and Venus are situated, or set them to any two given places therein, and then turn the handle; and when Mercury is returned to the place from whence he set out, the earth will have gone over 88 days of the ecliptic. In the same manner you will find the periods of the other planets corresponding to their respective periods in the heavens.

As Mercury moves round him in rather less than three months, that, consequently, is the length of his

year; the year in each planet being the space of time which it occupies in going round the sun. Mercury is seldom seen, on account of his being so near to the sun as to be generally concealed by his rays; and the time of his rotation on his axis, or the length of his days and nights, has not yet been dis

covered.

Venus, the next planet to Mercury, distinguished in the heavens by her superior lustre and brightness, completes her annual or yearly revolution round the sun in about 225 days; and her diurnal or daily rotation upon her own axis, in about 23 hours. When this planet appears to the west of the sun, she rises before him in the morning, and is called the morning star; and when she appears to the east of the sun, she shines in the evening after he sets, and is then called the evening star; being in each situation, alternately, for about 7 months.

The next planet above Venus is the Earth, whose annual revolution is performed in 365 days, 5 hours, and 49 minutes, or rather more than 12 months (the brazen ecliptic is, however, only divided into 365 days), and its diurnal rotation in about 24 hours. Every fourth year, one day is added at the end of February, to recover the time which the earth spends in her annual course above the 365 days, which compose a common year. This fourth year, therefore, consists of 366 days, and is called bissextile, and also leap-year.

Next above the earth's orbit is that of Mars, who completes his revolution round the sun in somewhat

less than two of our years, and his rotation upon his axis is rather more than 24 hours.

Jupiter, the largest of all the planets, holds the next place to Mars in distance from the sun. He performs his annual revolution in rather less than 12 years, and his diurnal rotation in about ten hours. Jupiter, as well as Venus, is sometimes called a morning, and sometimes an evening star.

Next to the orbit of Jupiter is that of Saturn, who completes his annual revolution round the sun in about 29 years. The time of his diurnal rotation is unknown.

Saturn was generally considered as the remotest planet of our system, till, on the 13th of March, 1781, Dr. Herschel discovered another, still further, distant from the sun, round which it revolves in an orbit nearly circular, in about 82 years. To this planet Dr. Herschel has given the name of the Georgium Sidus.

Besides these seven primary planets, there are fourteen others, called secondary planets, or satellites, which move round their primaries in the same manner as those move round the sun.

The first of these is the moon, represented by the small ball annexed to the earth. While it accompanies the earth in its annual progress through its orbit, it is continually revolving round it; as you will see in that part of the instrument that is particularly designed to illustrate the phenomena of the moon.

Jupiter has four satellites, Saturn seven, and the Georgium Sidus six: they are all invisible to the

naked eye, and are only to be seen by the assistance of telescopes. Saturn, besides his seven satellites, has a bright shining ring, which encompasses him: it is at such a distance from his body, that the fixed stars may frequently be seen between the inner edge of the ring and the planet itself. Dr. Herschel has lately discovered that this ring is divided into two parts, an inner and an outer ring, which are separated from each other by a space of one thousand miles.

To explain, by the planetarium, why the sun, being a fixed body, appears to pass through all the signs of the zodiac in twelve months, or one year. It will shew that this phenomenon is occasioned by the annual motion of the earth.

As the general phenomena of the planetary system will be best understood by an induction of particulars, I should advise the tutor to remove all the planets but those whose motion he is going to explain; for instance, let him now leave only the earth and sun; place the earth over Libra, and it is plain that the sun will then be transferred, by the eye of the spectator to Aries, in which sign it will appear at the latter end of March: move the earth on its orbit to Capricornus, and the sun will appear at Cancer in June, seeming to have moved from Y to, though it has not stirred, the real motion of the earth having caused the spectator to transfer the sun to all the intermediate points in the heavens,

and thus given it apparent motion. Continue to move the earth till it arrives at Aries, and the sun will be seen in Libra, in the month of September: moving the earth on to Cancer, the visual ray of the spectator refers the sun to Capricorn, as it appears in the month of December, Lastly, continue moving the earth, and it will arrive at Aries, where we set out. Thus we have shewn that it is the motion of the earth which causes the sun to appear in all the different signs of the zodiac. Custom, indeed, has taught us to say the sun is in Aries, when it is between us and Aries, and so of any other sign; whereas it would have been more proper to say, that the earth is in Libra.

To shew why, at different times of the year, we see the heavens decorated with an entire different collection of stars.

This phenomenon is occasioned by the earth's progressive or annual motion; while the earth is traversing his course under the vast concave of fixed stars, we are gradually carried under the different constellations. From hence it is evident, that at night, when the earth is turned from the sun, we shall, in succession, have the opportunity of viewing, from time to time, all the stars in the zodiac, and consequently a different constellation will present itself every month.

Thus, the Pleiades are not visible in the summer; but in the winter the earth is got between the sun