of the comet of 1351, was just equal to the earth's mean distance. The perihelion distance of 24 of the others, was greater than this, and of the remaining 72, less. The least distance of the comet of 1680, was only 122,000 miles from the surface of the sun; while its greatest distance was 12,189,000,000 miles. The perihelion distance of the comet of 1759 is about 52,000,000 miles; its aphelion distance 3,342,500,000. These are the only two comets whose periods are known, That of the latter is about 76 years. It appeared in 1759, 1682, 1607, 1531, and 1456; and will probably reappear in 1835. The period of the former is 575 years. It appeared in 1680, 1106, 531, and in 44, before Christ, and probably will not re-appear, till 2255. There is also strong reason to conclude, that the comet of 1264 was the same with that of 1556. If so, its period is 292 years; and it ought to appear again in 1848. Dr. Halley imagined, that the comet of 1661 was the same with that of 1532; and that its period was 129 years; but in 1790, it was found to have violated its engagements. Dr. Halley had the honor first to foretel the return of a comet. It was the comet of 1759. The velocity of a comet increases as it approaches the sun. That of 1680, in its perihelion, moved with the amazing velocity of 880,000 miles an hour. The comet of 1744, had a tail of the length of 23,000,000 of miles; and that of 1759, of more than 40,000,000. The orbits of comets make very different angles with the plane of the ecliptic: 50 out of the 97, whose elements have been calculated, had a direct motion, or from west to east; and 47 from east to west. The comet of 1680, on the 11th November, at 1 hour, 6 minutes, P. M. was only 4000 miles north of the orbit of the earth. If the earth at that time, had been in the part of its orbit nearest to the comet, their mutual gravitation must have caused a change in the plane of the earth's orbit, and in the length of our year.

The following tables, taken, with some alterations, from Clarke's Commentary on the Bible, will present a full and interesting summary of the bodies in our solar system, together with their magnitudes, distances, periods, &c.

:

Names.

lution.

lution.

Meandistances Propor'n inclinai on Periodical Revo-Sidereal Revo-
from the sun. of Light of orbits to
& Heat. the Ecliptic

Satel Diam-Bulk, the Distance from Periodic Revo-Synodic Revo- Grea’stdis htes. eler. Earth be

Jupiter.

lution.

lution.

tance from the Earth.

TABLE III. SATELLITES OF SATURN.

Satellites. Distance from Saturn. Periodic Revolution. Synodic Revolut’n.

Satellites. Distance from Herschel Periodic Revolution. Synodic Revolu❜n.

OF THE FIXED STARS.

Those stars, which, when seen by the naked eye, or through telescopes, keep constantly in the same situation with respect to each other, are called fixed stars. They are easily distinguished from the planets by their twinkling. They appear of various magnitudes. This may arise from their different sizes, or distances, or both. Astronomers have distinguished them, from their apparent magnitudes, into six classes. The first contains those of the largest apparent size, the second those which appear next in bigness; and so on to the sixth, which includes all those that can just be seen without telescopes. Those, which can be seen only by the help of the telescope, are called telescopic stars.

The stars in the preceding table are so numerous, that it would be impossible to furnish names for them all and retain those names in the memory. To remedy this inconvenience the ancients distributed them into constellations, to which they gave the names of birds, beasts, fishes, &c. from an imaginary resemblance between the forms of the constellations, and of those animals. The stars of each constellation are numbered, according to their magnitude, by the letters of the Greek alphabet. a is the largest, 3 the second, the third, &c. This division of the heavens was very ancient; for some of the constellations are mentioned by Homer and Hesiod, by Amos and Job.

The whole number of the constellations is 90. Of these 48 are ancient, and 42 modern; 33 north of the Zodiac, 12 in the Zodiac, and 45 south of it. Those stars, which have never been arranged into constellations, are called unformed stars. Those, whose distance from the nearest pole is less than the latitude of the place, never set below the horizon, and are called circumpolar stars. The circles, which they appear to describe in consequence of the earth's rotation, are called circles of perpetual apparition. Those stars, whose distance from the farthest pole, is less than the latitude of the place, never rise above the horizon. They also receive the same name; and the circles, which they appear to describe, are called circles of perpetual occultation.

The real number of the fixed stars cannot be ascertained. Before the invention of the telescope, it was not supposed to surpass 3000. But since that event it has been found, that the greater the

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perfection of that instrument, the greater, in a very high proportion, is the number of the stars, which may be observed. Galileo found 80 stars in the belt of Orion's sword. De Rheita counted 188 in the Pleiades, and more than 2000 in the constellation of Orion, of which only 78 are visible to the naked eye. The fixed stars, as seen through a telescope, are found to be collected in clusters. When a small magnifying power is used, these clusters appear like small light clouds, and hence have been called nebula. Dr. Herschel has given a catalogue of more than 2000 nebulæ, which he has discovered. When these nebulæ are examined with a telescope of great magnifying power, they are found to consist of immense multitudes of stars. Dr. Herschel is of opinion, that the starry heaven is replete with these nebulæ : that each nebula is a distinct and separate system of stars; and that each star is the sun or centre of its own system of planets. That bright, irregular zone, which we call the Milky Way, he has very carefully examined, and concludes that it is the particular nebula to which our sun belongs. In examining it, in the space of a quarter of an hour, he has seen the astonishing number of 116,000 stars pass through the field of view of a telescope of only 15' aperture; and, in 41 minutes, he saw 258,000 stars pass through the field of his telescope. It is probable that each nebula in the heavens is as extensive, and as well furnished with stars, as the milky way; that many nebulæ, within the reach of the telescope, have not yet been discovered; and that very many more lie beyond its reach, in the remote regions of the universe. If this be true, the number of 75,000,000, which La Lande assigned, as the whole number of the fixed stars, will be seen to fall far short of the truth.

The distance of the fixed stars, however, is so great, that their number will, probably, never be calculated with certainty. The diameter of the earth's orbit is 190,000,000 miles. Of course, when the eye is placed at one end of this diameter, it is so much nearer given stars, than when at the opposite end. Yet this immense distance makes no apparent difference in the size of any of them,nor any difference in their relative situations. The distance of the nearest fixed star is estimated to be more than 5,000,000,000,000 miles from us, a distance which a cannon ball, moving at the rate of 480 miles an hour, would not pass over in less than 1,180,000 years. Astronomers generally, however, have calculated the distance of the nearest fixed star, at 400,000 times the diameter of the earth's orbit.

The real magnitudes of the fixed stars are not known. In astronomical calculations they are generally supposed to be equal to that of the sun.

With regard to their nature we can make nearer approaches to certainty. We know that they shine by their own light, because if they borrowed their light from any large luminous body which was pear them, that body would itself be visible. They resemble the sun in several other particulars. Many of them are observed to revolve on an axis; to have spots on their surface, and changeable spots, too, like those of the sun. Hence they are very

fairly concluded to be suns, each one a centre of light, and warmth, and motion for its own system of planets.

THE GLOBES, AND THEIR USE.

A globe is a round body, whose surface is every where equally remote from the centre. But by the globes, sometimes called artificial globes, is here meant two spherical bodies, whose convex surfaces are supposed to give a true representation of the earth and the apparent heavens. One of these is called the terrestrial, the other the celestial globe. On the convex surface of the terrestrial globe, all the parts of the earth and sea are delineated in their relative size, form, and situation.

On the surface of the celestial globe, the images of the several constellations and the unformed stars are delineated; and the relative magnitude and position, which the stars are observed to have in the heavens, are carefully preserved.

In order to render these globes more useful, they are fitted up with certain appurtenances, whereby a great variety of useful problems are solved in a very easy and expeditious manner.

The brazen meridian is that ring in which the globe hangs on its axis, represented by two wires passing through its poles. The circle is divided into four quarters of 90 degrees each; in one semicircle the divisions begin at each pole, and end at 90 degrees on the equator, where they meet. In the other semicircle, the divisions begin at the equator, and proceed thence toward each pole, where they end at 90 degrees. The graduated side of this brazen circle serves as a meridian for any point on the surface of the earth, the globe being turned about till that point come under it.

The hour circle is a small circle of brass, divided into 24 hours, the quarters and half quarters. It is fixed on the brazen meridian, with its centre over the north pole; to the axis is fixed an index, that points out the divisions of the hour circles as the globe is turned round its axis. Sometimes the hour circle, with its divisions, is described or marked about the north pole on the surface of the globe, and is made to pass under the index. In some of Adams's globes, the equator is used as an hour circle, over which is placed a semicircular wire, carrying two indices, one on the east side of the brazen meridian, and the other on the west.

The horizon is represented by the upper surface of the wooden circular frame encompassing the globe about its middle. On this wooden frame there is a kind of perpetual calendar, contained in several concentric circles. The inner one is divided into four quarters of 90 degrees each; the next circle is divided into the 12 months, with the days in each according to the new style; the next contains the 12 equal signs of the zodiac or ecliptic, each being divided into 30 degrees; the next the 12 months and days according to the old style; and there is another circle, containing the 32 points of the compass, with their halves and quarters. Although these circles are on most horizons, yet they are not always placed in the same order.