Abbildungen der Seite
PDF
EPUB

mode of education to limit the reading of youth to books on a few particular subjects; so that when young men are ushered into the world, many and even moft of them are unacquainted with even the appropriate terms ufed in other fciences.

But the advantages of a more diffufive fyftem of instruction will not rest here--By reading frequently and repeatedly, passages containing just rules and principles, even above the comprehenfion of young minds, the pupils will learn many of them by heart, and bear the impreffions into future life; bý which means, when their understandings are more matured, they will be enabled to direct, to useful purposes, the principles with which they had ftored their minds in fchool. In this manner, ufeful rules and facts, acquired in youth, like feed fown in a good foil, will produce their fruit in riper years, and increase the harveft of knowledge and improvement, to enrich the community.

In the language, the author has aimed to use a style, which is plain without being vulgar; and technical, without needlefs obfcurity.

In every part of this work, occafions frequently occur of deducing moral and pious reflections from the fubjects treated. On fuch occafions, fpecial care is taken to lead the mind of the reader, from a confideration of the order, beauty and fitnefs of all parts of nature, to contemplate the neceffity and certainty of the exiftence of a Creator, of infinite power, wifdom and goodnefs. This mode of employing natural philofophy in the fervice of religion and piety, has been practiced by the ableft authors and beft men in all ages-it furnishes powerful aids to that firm belief in the being and providence of God, and that pious veneration for his character and attributes, which are the prime ornaments of a wife man and a good citizen.

Elements of Useful Knowledge.

T

SECTION i.

Of the Solar Syftem.

HE folar fyftem confifts of the fun, and the stars or globes which move round it, called planets. Of the Sun. The fun is an immenfe body, placed near the center of the fyftem, diffufing light and heat to all the planets. Its diameter is eight hundred and ninety thousand miles.

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

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 fattellites, and Saturn, feven, which conftantly move round those planets.-The planet Herschel has fix fattellites.

Mercury Mercury is three thousand 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 Herfhel.

B

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

The Earth. This globe which we inhabit, is a planet of nearly eight thousand miles in diameter and twenty five 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.

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. Jupiter. Jupiter is the largeft of the planets, haying 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 less than twelve years.

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.

Herfchel. This planet, which bears the name of its discoverer, who first observed it in 1781, is nearly thirty five thousand miles in diameter; its diftance from the fun, one thoufand and eight hundred millions of miles, and its revolution about the fun is performed in eighty three years and a half.

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 difcovered by Dr. Olbers of Bremen in 1802; its diameter is ninety five miles, and in its revolution it is not confined to the zodiac.

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 twenty four years. It has feven fatellites.

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

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 most diftant, the aphelion. Laws of planetary motion. The nearer a planet is to the fun, or its center, the faster 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 falter in winter, than in fummer; hence the fummer with us is eight days longer than the winter.

Of Attraction and Repulfion. That principle in bodies which difpofes them to unite and cohere, is called attraction. That principle which difposes them to feparate or recede from each other, is called repulfion. The tendency of a planet towards the center of the fystem, is called its centripetal force; the power that impels it to recede from the center, its centrifugal force; and thefe combined are fuppofed to generate the cir cular motion of the planets.

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.

Of 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 comprehends thirty degrees.

[ocr errors]

Names of the Signs. The figns are called Aries

« ZurückWeiter »