Of Useful Knowledge.

SOLAR SYSTEM.

SECT. 1. Of the fun, and the stars or globes HE folar fyftem confifts of, the Solar Syftem.

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which move round it, called planets.

2. Sun. The fun is an immenfe body, placed near the centre of the system, diffusing light and heat to all the planets. Its diameter is eight hundred and ninety thoufand miles.

3. Planets. The primary planets, exclufive of those which have been lately difcovered, are feven; Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Herfchel.* To thefe may be added, Ceres, Pallas and Hercules, which have been lately difcovered. All the planets revolve about the fun, and receive their light and heat from that refplendent luminary.

4. Secondary Planets.-The fecondary planets are smaller orbs which revolve round the primary planets, as the moon, which moves round the earth in about twenty nine days. Jupiter has four moons, or fatelites, and Saturn, feven, which conftantly move round those planets.-The planet Herschel has fix fatelites.

5. Mercury. Mercury is three thoufand two hundred and twenty miles in diameter, and revolves round the fun in about eighty eight days, at the distance of nearly thirty feven millions of miles.

* Pronounced Hershel,

SCHOOLS

FOR THE USE OF SCHOOL

Ralph & Shollige

By NOAH WEBSTER, Esq.

"Father of light and life! thou GOOD SUPREME !
"O teach me what is good! teach me thyself!
"Save me from folly, vanity and vice;

"From every low pursuit! and feed my soul

"With knowledge, conscious peace, and virtue pure ;
"Sacred, substantial, never fading bliss." THOMSON.

6. Venus. Venus is about feven thousand, seven hundred miles in diameter, and revolves round the fun in about feven months and a half, at the diftance of nearly fixty eight millions of miles.

7. The Earth. This globe which we inhabit, is a planet of nearly eight thousand miles in diameter and twentyfive thousand miles in circumference. Its diftance from the fun is about ninety-five millions of miles, and its revolution round the fun makes our year, or three hundred and fixty-five days, and nearly fix hours.

8. Mars. Mars is in diameter four thousand two hundred miles. Its revolution about the fun is performed in one year and three hundred and twenty one days, at the distance of one hundred and forty four millions of miles.

9. Jupiter. Jupiter is one of the largest of the planets, having a diameter of ninety four thousand miles. Its diftance from the fun is calculated to be nearly four hundred and ninety five millions of miles, and its revolution round the fun is performed in a little lefs than twelve years.

10. Saturn. Saturn has a diameter of feventy nine thousand miles. Its diftance from the fun is calculated to be nearly nine hundred millions of miles, and its revolution round the fun is performed in twenty nine years and a hundred and fixty feven days. Saturn has two rings and a belt of spots.

II Herfchel. This planet, which bears the name of its discoverer, who first observed it in 1781, is nearly thir ty five thousand miles in diameter; its distance from the fun, one thousand and eight hundred millions of miles, and its revolution about the fun is performed in eighty "three year and a half.

12. Ceres and Pallas. Ceres was difcovered by Mr. Piazzi, at Palermo in Sicily, in the year 1801. Its diameter is one hundred and fixty two miles. Pallas was discovered by Dr. Olbers of Bremen in 1802; its diameter is ninety five miles, and in its revolution it is not confined to the Zodiac.

13. Hercules. In 1804, Dr. Olbers difcovered another planet, which is three times as large as Jupiter, and which for its magnitude, he calls Hercules. Its diftance from the fun is three thousand and forty feven millions of miles, and its revolution is calculated to be two hundred and eleven years. It has seven fatellites.

14.

Orbits of the Planets. The planets move round their centers, not in a true circle, but in a figure called an ellipfis, whch fomewhat resembles the form of an egg.

15. Perihelion and Aphelion. As the planets do not revolve round the fun in a circle, they approach nearer to the fun in one part of their orbits, than in other parts. That part of the orbit nearest the fun, is called the perihelion ; and the part moft diftant, the aphelion.

16. Lars of planetary motion. The nearer a planet is to the fun, or its center, the fafter it moves. Thus Mercury moves much fafter than Saturn. So a planet moves more rapidly in its perihelion, than in its aphelion. The earth is nearer the fun and moves faster in winter, than in fummer; hence the fummer with us is eight days longer than the winter.

17. Of Attraction and Repulfion. That principle in bodies which disposes them to unite and cohere, is called attraction. That principle which difpofes them to separate or recede from each other, is called repulfion. The tendency of a planet towards the centre of the fyftem, is called its centripetal force; the power that impels it to recede from the centre, its centrifugal force; and thefe combined are supposed to generate the circular motion of the planets.

18. Divifion of a Circle. Every circle is divided into. three hundred and fixty degrees, a quarter of which or a quadrant is ninety degrees. Every degree is divided into fixty minutes, and each minute into fixty feconds.

19. The Zodiac. The Zodiac is a broad circle or belt in the heavens, containing the twelve figns, or conftellations, most of which are reprefented by certain animals, by whofe names they are called. Each fign compehends thirty degrees.