satellites reckoning from the primary.

round its
primary.

JUPITER'S SATELLITES.

No. of the Period of re- Mean distance

volution on
its axis and

from its pri

Mean ap

mary in thou

sands of miles.

parent distance.

There is great inequality, in the amount of light emitted by different satellites, of the same primary. Those planets and satellites most remote from the Sun, may, of course, be expected to reflect light, in a less vivid and paler manner than those more near.

CHAPTER VIII.

COMETS.

THE ancients considered comets as supernatural and ominous appearances in the heavens. Modern discoveries in Astronomy, have proved, that comets are bodies governed by kindred laws, with the planets. But unlike planets, all of which revolve from west to east round the Sun, comets revolve in every direction, and in orbits so extremely elliptical, that they continually intersect each other, and the orbits of the planets. Science, while it liberates from the fears of superstition, leads to the conclusion, that it is possible for a comet to approach inconveniently and fatally near, to the habitation of man." The number of comets is supposed to be nearly 500; the elements of only 100 of which have been computed. Commonly they have a small solid nucleus, surrounded with aqueous vapor, and usually accompanied by a luminous transparent train, always in an opposite direction from the Sun.

Newton supposed that the heads of the comets were exceedingly dense matter, that might be raised to a heat 2000 times greater than red hot iron, and the train was vapor or steam

What were the opinions of the ancients respecting comets? By what laws are they governed? How d their orbits and motions differ from those of planets? What event is named as possible? If a comet revolved exactly in the plane of the Ecliptic, how many chances to one against a contact with the Earth? How many comets are supposed to belong to our system? By what are they attended?

What hypotheses have been started respecting their heads? -and their luminous trains?

*If a comet revolved in the plane of the Ecliptic, there would be more than 35,000 chances each revolution against its coming in contact with the Earth, to one chance that it would touch our planet. But as no comet is known to revolve exactly in the plane of the Ecliptic, the danger is incalculably less.

thus produced. These opinions cannot be defended. Others have supposed, that the nucleus of the comet is transparent, and rays of the Sun, tinged by passing through it, become visible. This would account for their tails being always opposite to the

Sun. But a recent and still more probable theory is, that the tails are the electric fluid, passing off like the aurora borealis. THE magnitudes of the comets are various; and the periods of their revolution, are from 40 months to more than 575 years.)

The solid nucleus of the Comet of 1807, was estimated at 583 miles in diameter, while it was surrounded with a mass of aqueous vapor, supposed to be nearly 200 thousand miles in diameter, and attended by a train more than 9 millions of miles in length. The Comet of 1811 had a head very imperfectly defined, and a tail 80 millions of miles in length, as seen in the northern hemisphere, and still more as seen in the southern hemisphere. The tails of some comets have subtended an arc of 60, or 90, or even 104, degrees in the heavens. The splendid Comet of 1744, known in the traditions of New-England as the great blazing star, was supposed to have a head as large as Jupiter. Comets have had an apparent magnitude from that of Venus, to six times that magnitude, and a few have appeared nearly the size of the Moon, though with a more dim and dead light. Of the 98 comets, the elements of which have been computed, 24 have their perihelion, or nearest approach to the Sun, within the orbit of Mercury; 33 between the orbits of Mercury and Venus; 21 between Venus and the Earth; 16 between the Earth and Mars; 3 between Mars and Ceres; and 1 between Ceres and Jupiter. The Comet of 1811 did not approach at its perihelion, nearer than 95 millions of miles from the Sun, and was more than 140 millions distant from the Earth. The Comet of 1680 approaches within 300 thousand miles of the Sun, and recedes 7 times as distant as Herschel. At its perihelion, it moves with the astonishing velocity of more than 800 thousand miles the hour; at its aphelion less than two miles the hour. The correctness, of the estimated elements, of a very few only of the comets, has been proved by the actual appearance of these bodics, when predicted to appear. "The immense distances to which these bodies are carried, and the

What is said of the magnitudes, and periods of revolution of some? What facts are given respecting the comets of 1811? -of 1744 ?-of 1680 ?

comparatively small portions of their orbits which come under our inspection, as well as their imperfect undefined appearance in the telescope, and the great interval of time which elapses between their reappearance, are impediments which nothing but time and the most accurate and diligent observations can remove; and therefore many ages must necessarily clapse before the theory of comets can be brought to perfection. Every thing, however, that ingenuity and industry can accomplish, may be expected from astronomers of the present day, whose diligence and accuracy have led them to many interesting discoveries, and to whose future perseverance we may confidently look for others equally important." Bonnycastle.

CHAPTER IX.*

THE FIXED STARS.

FROM the evening of the sixth day of the creation to the present hour the starry heavens must have been objects of intense interest, to the inquisitive mind of man. (The ancients considered them luminous points in a vast concave sphere which was revolving from east to west round the Earth in 24 hours. A slight observation convinced them that some, which appeared to be stars, changed their positions among the others, or wandered; they were therefore called planets, or wanderers. Those keeping their positions from month to month were called FIXED STARS. The number of these, visible to the naked eye, in one hemisphere, is about (1000. An equal number may be seen in the opposite hemisphere.

How were the starry heavens anciently regarded? Why were some apparent stars called planets? About how many fixed stars are visible to the unaided cye?

* If any Instructer, thinks the subject of the Fixed Stars should be taught later in the course, this chapter can be deferred, and resumed afterwards.