viz. the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the fixed stars. The sun and planets were supposed to be firmly set in separate crystalline spheres, inclosed by a concave one, containing the fixed stars, which would of course be all equally distant from the earth. Above this starry sphere were imagined to be the two crystalline spheres, the primum mobile, communicating motion to all the interior spheres; and, finally, the empyrean heaven or heaven of heavens, to which a cubic form was attributed. Beside the above motion, performed in the course of twenty-four hours, the sun and planets were supposed to revolve about the earth in certain stated or periodical times, agreeably to their annual appearances.

The phenomena to be explained by this system are inconsistent with it, and show its absurdity in a very satisfactory manner.

The TYCHONIC or BRAHEAN SYSTEM was invented by Tycho Brahe, a nobleman of Denmark. With Ptolemy he supposed the earth to be at rest in the centre of the universe, and the moon, the sun, planets, and fixed stars, to revolve about it in twenty-four hours. He also supposed that these bodies had an annual motion around the earth; that the moon's orbit was nearest to the earth; then the sun's; and that Mercury, Venus, Mars, Jupiter, and Saturn, revolved about the sun as their centre, and accompanied it as their primary in its annual revolution round the earth. As he denied the earth's diurnal rotation on its axis, he was obliged to admit one of the most gross absurdities of the Ptolemaic hypothesis, that is, the revolution of the whole universe, to its farthest visible limits, about the earth's axis in the space of a day, produced by the primum mobile. Some of his followers, however, varied from his system so far as to ascribe this apparent diurnal motion of the heavens to a real rotation of the earth on its axis, and were therefore called Semi-Tychonics.

The COPERNICAN SYSTEM is so called from Copernicus, a native of Thorn in Royal Prussia, and is the TRUE SOLAR SYSTEM. It had been taught by some of the Pythagorean philosophers, but was nearly lost, when Copernicus undertook to restore it, and published new and demonstrative arguments in its favour. It supposes the sun to be in the centre of the system, and all the planets to move round the sun in the order already mentioned. These, together with the satellites, asteroids, and comets form the constituent parts of the Solar System.

This supposition readily solves all the appearances observable in the motion of the planets, and also agrees with the strictest philosophical and mathematical reasoning.

All the planets are opaque and spherical bodies, and receive their light from the sun. Their orbits are not circular, but elliptical. Hence, in their revolutions, they are sometimes nearer to, and sometimes farther from, that luminary. The influence of the sun is the cause of the motions of the planets; and this influence increases as their distance from the sun decreases. Hence also we see the reason why the planets move faster, as they approach nearer to the sun, and slower as they recede from it.

If a right line, called by some the vector radius, be drawn from the sun through any planet, and supposed to revolve round the sun with the planet, this line will describe, or pass over, every part of the plane of the orbit; so that the vector radius may be said to describe the area of the orbit.

In the solar system are observed two principal laws, which regulate the motions of the planets. These laws are the following: 1" The planets describe equal areas in equal times." That is, the vector radius, in equal portions of time, describes equal areas or portions of the space, contained within the planet's orbit.

2. "The squares of the periodical times of the planets are as the cubes of their mean distances from the sun." That is, as the square of the time, which any planet takes to describe its orbit, is to the square of the time, taken by any other planet to describe its orbit; so is the cube of the mean distance of the former from the sun to the cube of the mean distance of the latter from the

sun.

These laws, together with the facts that the orbits of the planets are elliptical, and that they have the sun in a common focus, were discovered by Kepler, a distinguished astronomer, who flourished about the beginning of the seventeenth century, and who deduced them from a multitude of observations; but the first, who shewed the reason of these laws, was the great Sir Isaac Newton.

By the second law the relative distances of the planets from the sun are known; and were the real distance of any one of them determined, the real distance of all the others would be obtained. By the transits of Venus over the sun in 1761 and 1769, we now know the real distances of the planets from the sun much better than before these, together with other necessary particulars for forming a competent idea of the solar system, are exhibited in Table 1.

The limits to which we are confined will not admit of our introducing the usual proofs to establish the Copernican system.

The Sun. The Sun is the centre of the system, and is immensely larger than all the other bodies which compose it. Its diameter is 883,246 miles, and its density (that of the earth being 1) is nearly. It weighs 333,928 times as much as the earth, and is 1,380,000 times as large. It appears from calculation, that a body weighing 1 pound on the earth, would weigh 27-7 pounds on the sun. It revolves, on its axis, in 25 days, 14 hours, 8 minutes; and in its orbit, in the same time, around the common centre of gravity of the system. Its revolution in its orbit, as is that of all the planets, is from west to east. The plane of its orbit is not coincident with that of any of the planets; but is nearest to coincidence with the orbit of Venus, The axis of the sun makes an angle of about 824 degrees with the plane of the earth's orbit. The sun, though to the naked eye it appears so extremely bright; yet, with a telescope of but very small powers, is discovered to have dark spots on its surface. These are also very various in their magnitudes. That which appeared in 1779, was more than 31,000 miles in diameter, and was visible to the naked eye. The sun has a revolution

on its axis. It is not ascertained whether it has an atmosphere. Mercury. Mercury is the smallest of the planets. It is 3224 miles in diameter, and 36,583,825 miles from the sun. Its bulk is to that of the earth, nearly as 1 to 15; and its weight, as 0.165 to 1. A body weighing 1 pound on the earth, would weigh 1.03 pounds on Mercury. It is not known whether it revolves on its axis ; yet, as all the other planets do, it is naturally concluded that this does also. It revolves round the sun in 87 days, 23 hours; or little less than 3 months. It emits a very bright, white light. Mercury can be seen only a few days at a time. It is visible in the evening about the eastern elongation. It then disappears about 6 or 7 weeks, after which time it may be seen in the morning, rising before the sun. In about 10 weeks, it reappears in the west, setting after the sun. It has no moon, nor. any spots on its surface. Its hourly motion in its orbit is 111,000 miles. The heat near the poles of Mercury is not probably greater than that of the torrid zone. Near its equator, water would continually boil, and most inflammable substances would be parched up, destroyed, or converted into vapor.

Venus. This is the most beautiful of the celestial luminaries, and the only star that is ever visible in the day time. This happens once in about 8 years; when the planet is at its greatest north latitude, and near its farthest distance from the sun. Venus is 7687 miles in diameter, and its mean distance from the sun is 68, 368,008 miles. Its bulk, compared with that of the earth, is nearly as 8 to 9; and its weight, as 0.89 to 1. A body weighing 1 pound on the earth would weigh 0.98 pounds in Venus Its diurnal rotation on its axis is performed in 23 hours, 22 minutes, and it moves in its orbit 81,000 miles an hour. When Venus appears to the west of the sun, it rises before him in the morning, and is called the morning star; and when it appears to the east of the sun, it shines in the evening, after the sun sets, and is called the evening star; being in each situation, alternately, about 290 days. The axis of Venus is inclined 75 degrees towards the plane of its orbit.

Mercury and Venus are inferior planets. They are called inferior planets in relation to the earth; because they are below the Earth; that is nearer to the sun or centre of the system. Mars, Jupiter, Saturn, and Herschel are called superior planets, because they are above the earth; that is, farther from the centre of the system.

Mercury never appears more than 28 20 from the sun, nor Venus more than 47 48. Of course, they and the sun are never in opposition, i. e. on opposite sides of the earth. They have both, however, an inferior conjunction, when they pass between the earth and the sun; and a superior conjunction, when they pass behind the sun. In their inferior conjunctions, they sometimes pass directly over the sun's disc. This passage is called a transit. In their transits, they appear like small, round, black spots, moving rapidly over the face of the sun. This appearance proves them to be opaque bodies. The transits of Venus are not so frequent as those of Mercury. The last transit of Venus was in 1769; the next will be in 1874,

The last of Mercury was in 1815, and the next will be in 1822. The greatest heat on the planet Venus probably exceeds the heat of the torrid zone about as much, as that exceeds the average heat of 60 degrees north latitude.

The Earth. The Earth is a spherical body. This is obvious from the following considerations: First, From analogy; as all the other planets and heavenly bodies are spherical. Secondly, To people on shore, the mast of a ship appears before the hull; but, were the earth a plane, the hull would appear long before the mast, by reason of the much greater angle which it subtends. Thirdly, The earth has been sailed round by Magellan, Drake, Dampier, Anson, Cook, and many others. Fourthly, The boundary of the earth's shadow upon the moon, in a lunar eclipse, is always circular; and nothing but a spherical body can, in all situations, produce a circular shadow. The unevennesses of the earth's surface have no effect upon its shadow on the moon; for the height of the highest mountain on the globe, is only equal to 1500th part of the earth's diameter.

The earth is not a perfect sphere, but an oblate spheroid; that is, its equatorial diameter is longer than its axis. The difference of these diameters is about 34 miles. The mean diameter of the earth, or the diameter in latitude 45 degrees, is 7928 miles. Of course, the equatorial diameter is 7945 miles, and the length of the earth's axis is 7911. The equatorial circumference of the earth is about 24,970 miles; its mean circumference, in latitude 45 degrees, 24,917; and its meridional circumference, 24,863. The number of square miles on the earth's surface is 197,459,101; and 260,909,292,265 is the number of cubic miles contained in the earth. It performs a rotation on its axis once in 24 hours.

The earth is surrounded with a thin, invisible, elastic fluid, called air, the whole body of which forms what is called the atmosphere. The density of the air is not always the same, it being subject to be expanded by heat and contracted by cold. In its mean state it is found to be about 850 times lighter than water.

Notwithstanding the seeming inequality in the distribution of light and darkness, it is certain, that, throughout the whole world, there is nearly an equal proportion of light diffused on every part, if we disregard what is absorbed by clouds, vapors, and the atmosphere itself. The equatorial regions have indeed the most intense light during the day, but the nights are long and dark; while on the other hand, in the northerly and southerly parts, though the sun shines less powerfully, yet the length of time that it appears above the horizon, with the longer duration of twilight, makes up for the seeming deficiency.

Mars. The diameter of Mars is 4189 miles, and its mean distance from the sun is 144,000,023 miles. Its annual revolution occupies 1 year, 321 days, 23 hours, 31 minutes, and its rotation on its axis 24 hours, 39 minutes, 22 seconds. It moves in its orbit at the rate of 56,000 miles an hour. Its bulk, compared with that of the earth, is as 7 to 24; and its density, as 7 to 10. One pound on the earth would weigh 0.34 in this planet. Mars is of a fiery

red colour. By the telescope, dark spots are discoverable on it surface; but round its poles, particularly the southern, an intense and permanent brightness. Mars is an oblate spheroid. Its axis is to its equatorial diameter, as 98 to 103. It has an atmosphere of considerable extent.

Jupiter. Jupiter, the largest of the planets, is 89,170 miles in diameter, and 491,702,301 miles from the sun. Its bulk, compared with that of the earth is nearly as 1400 to 1; its density as 5 to 22; and its weight as 312 to 1. One pound on the earth would weigh 2.33lbs. in Jupiter. Its shape is that of an oblate spheriod. Its polar diameter is to that of its equatorial, as 12 to 13; and the difference of their lengths, is upwards of 6000 miles. Its ecliptic and equator are nearly coincident; that is, its axis is nearly perpendicular to the plane of its orbit. Hence this planet has no sensible change of seasons. If its axis were inclined any considerable number of degrees towards the plane of its orbit, just so many degrees round each pole would, in their turn, be almost six years together in total darkness. It revolves on its axis in 9 hours, 55 minutes; and round the sun in 11 years, 314 days, 18 hours, 45 minutes. Its hourly motion in its orbit is 30,000 miles. From a comparison of the most ancient, with the modern observations, there is some reason to conclude, that the period of its revolution is decreasing. Jupiter is surrounded by faint substances, called belts. These were discovered in 1665. They are parallel to each other, and to the equator of the planet. The quantity of light and heat enjoyed by Jupiter, is to that enjoyed by the Earth, as 37 to 1000.

Saturn. The diameter of Saturn is 79,042 miles, and its distance from the sun is 901,668,908 miles. Its bulk is proportioned to that of the earth nearly, as 1000 to 1. Its density, as 26 to 288, and its weight, as 98 to 1. A body weighing 1lb. on the earth, would weigh 1.02 on this planet. It is an oblate spheriod, its axis being to its equatorial diameter, as 10 to 11. It revolves on its axis in 10 hours, 16 minutes, 2 seconds, and round the sun in 29 years, 164 days, 7 hours, 21 minutes. Its hourly motion in its orbit is about 22,000 miles. The intensity of the sun's light and heat, is about 95 times greater at the Earth, than at Saturn. This planet has belts discoverable on its disc; but they are not so large or numerous as the belts of Jupiter. The most remarkable appearance, however, is a large ring, entirely separated from the planet itself, and yet completely surrounding it. The plane of the ring coincides with the plane of Saturn's equator, so that the axis of the planet makes a right angle with it. When the outer edge of the ring is turned towards the earth, it is invisible, except with telescopes of very great powers; either on account of its thinness, or of its almost total incapacity to reflect light. The ring is double, or is com-, posed of two rings, having the same plane and the same centre. The outside diameter of the larger ring is 204,883 miles, and its inner diameter 190,248 miles; so that the breadth is 7318 miles. The outside diameter of the smaller ring is 184,393 miles, its inner diameter 146,345, and its breadth 19,024. The space between the rings is 2,977 miles. There is no visible connection