ILLUSTRATED ASTRONOMY.

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LESSON XXVIII.

JUPITER.

THAT is Jupiter?

Jupiter is the largest planet in the solar

ny times larger is Jupiter than the earth? 280 times greater.

the specific gravity of Jupiter?

pout 13 times the weight of water. (1.30.)

is Jupiter from the sun?

illions of miles.

its diameter?

) miles.

diameter is the greater, the polar or equatorial.

equatorial diameter is 6,000 miles greater olar.

SATURN'S RINGS.

(From the American Almanac and Repository of useful Knowledge, 1852.) WITHIN a few months the inquiry has been started with fresh interest, By how many rings is Saturn surrounded, and in what way are these rings sustained? Short saw two or three divisions outside of the centre of breadth. Herschel the first, in 1780, saw a new division near the inner edge. As this appearance was temporary, he thought that observation would not justify the supposition of multiple rings. Lines of demarcation were seen on both rings in 1813 and 1814. Quetelet saw the outer ring divided in 1823. In 1825 and 1826 three divisions were seen on the outer ring by Kater. In 1837 Encke noticed that the outer ring was divided, and that there were several marks near the inner edge of the inner ring. De Vico has given an account of several divisions seen by him on both rings. In 1838 several divisions were seen at Rome, which are described by Decuppis. In 1843 Lassell and Dawes saw a division of the outer ring. Smyth gives an account of the last case, and adds: "After such unquestionable evidence, there can be no reasona ble doubt of the outer ring's being multiple." On the 11th of November, 1850, G. P. Bond saw what he thought at the time a second division of the ring near the inner edge of the inner ring. On the 15th his father thought the new ring was wholly disconnected with the old, though the edge next to the planet was better defined than the outer edge. Micrometric observations gave for the inner diameter of the inner division 25 "3, whereas, according to Encke, the inner diameter of the old inner ring should be at that time 29" 8. From this it was inferred that the large refractor at Cambridge had revealed an entirely unknown and darker ring of Saturn, which was not to be confounded with the division of the old inner ring which had frequently been noticed. The outer edge of the new ring is 1" 5 within the inner edge of any ring hitherto visible. This conclusion was confirmed by observations continued for several weeks. Similar appearances were noticed on the 25th and 26th of November by Mr. Dawes, and afterwards by Lassell of Liverpool, and Schmidt of Bonn.

On the 15th of April, 1851, G. P. Bond communicated to the American Academy of Arts.and Sciences at Boston, a memoir on the rings of Saturn. After rehearsing the facts already detailed in regard to extraordinary divisions of the rings, he draws attention to the circumstance that other observers, as Struve, Bessel, J. F. W. Herschel, and, we may add, Smyth, have seen only the usual' division, even with the best instruments, and under the most favorable circumstances. Moreover, the divisions on both rings are not always seen simultaneously; and the Cambridge teles

uses the equatorial diameter so much to exceed the polar? cope, which has brought to view a ring always before invisible, does not indicate any of the un

uick rotation of the planet upon its axis.

t time does it revolve upon its axis?

out 10 hours. (9h. 55m. 50s.)

at time does it revolve around the sun?

even years, 314 days.

ast does it move in its orbit around the sun? 00 miles an hour.

many moons has Jupiter?

is their magnitude?

are about the size of our Moon.

first discovered them?

leo, the inventor of the telescope in 1610.

are the orbits of these Moons situated? are directly over his equator.

nese moons frequently eclipse the Sun?

y do at each revolution around the Sun.
great discovery was made by observing these eclipses ?
velocity of Light. (Note.)

Jupiter any change of seasons?

as no change of seasons.

- do its seasons not change?

ause its axis is nearly perpendicular to the
its orbit, which causes the sun to be always
t the equator. (See Diagram.)

does Jupiter appear when viewed with a telescope?
ht and dark belts appear to surround it. (See
d 2.)

t are the light belts ?

ey are supposed to be clouds, which are thrown.

usual divisions in the two old rings. It seems a justifiable conclusion from all the facts, that the multiplicity of rings occasionally seen, and the failure to discern more than two at other times, are not referable to the difference of instruments, to the greater or less purity of the air, or to the unequal skill of observers, but to a material fluctuation in the ring itself.

In a lecture publicly delivered in Reading, Pennsylvania, on the 3d of January, 1851, Mr. Kirkwood made the following remarks:-"This gives rise to the interesting question whether the rings of Saturn may not be the most recent cosmical formation within the limits of the solar system, and whether it may not, in the course of future ages, collect about a nucleus and constitute a satellite. The evidence of its solidity is not, I think, by any means conclusive. On the other hand, observations made within the last few years give a degree of plausibility to the presumption that it may be in a state of fluidity. I refer to the occasional appearance of dark lines, chiefly on the outer rings, which have been supposed to indicate a subdivision into several concentric annuli. They do not, however, appear to be permanent; at least they are subject to some change, as they are not always visible even when circumstances would seem most favorable."

Such are the considerations which led Mr. Bond to reject the idea of solid rings, and to suppose these appendages of Saturn to be fluid or semi-fluid. If this is the material, it is unnecessary to suppose that the inner and outer surfaces move round in the same time. The velocity at every point may be such, that the centrifugal and other forces balance each other. "And even should an accumulation of disturbances, of which the absence of inequalities lessens the probability, bring the rings together, the velocities at the point of contact will be very nearly equal, and the two will coalesce without disastrous consequences." "If in its normal condition the ring has but one division, as is commonly seen, under peculiar circumstances it might be anticipated that the preservation of their equilibrium would require a separation in some regions of either the inner or outer ring; this would explain the fact of occasional subdivisions being seen. Their being visible but for a short time, and then disappearing to the most powerful telescopes, is accounted for by the removal of the sources of disturbance, when the parts thrown off would reunite." "Finally, a fluid ring, symmetrical in its dimensions, is not of necessity in a state of unstable equilibrium, with reference either to Saturn or the other rings."

At the meeting at Cincinnati of the American Association for the Advancement of Science, Professor B. Peirce read a memoir on the constitution of Saturn's ring, containing the same general views which he submitted to the American Academy of Arts and Sciences at Boston, on the 15th of April, 1851. Mr. Peirce arrives at the same results by analysis as those which Mr. Bond had derived from observations, illustrated and combined by his own ingenious computations. Mr. Peirce differs in opinion from Laplace, in regard to the efficacy of an irregular figure in sus. taining Saturn's ring. He considers this statement of Laplace, which his successors have blindly adopted, as a careless suggestion, and not the ripened fruit of his usual rigid examination. “I maintain," he says, "unconditionally, that there is no conceivable form of irregularity, consistent with an actual ring, which would serve to retain it permanently about the primary, if it were solid." The stability of the ring does not depend on the attraction exerted on it by the planet. In the circulation of the fluid annulus around Saturn, the velocity is least at the greatest distance. Hence the matter accumulates at the most remote point of the ring, and to such an extent that the quantity of matter balances the distance, and the attraction exerted by the ring and the planet on each other is the same in every direction. The ring is held together by the attraction of the primary; but it is not sustained as a whole by the primary. It is sustained by the satellites. The satellites disturb it, and sustain it by a delicate equipoise of disturbances. Something like this restorative action had been hinted at by Sir J. F. W. Herschel. But the remedial power is insufficient to sustain a solid ring. It follows that no planet can have a ring unless richly provided with menials to hold it. Saturn alone of all the planets seems competent to preserve a ring when once bestowed.

Mr. Peirce concludes his memoir with the following weighty paragraph: "Were the ring, however, supposed to be a large gaseous mass of a circular figure, the condensation which would occur at the point of aphelion might easily lead to chemical action. Precipitation might ensue, and the necessary consequence would seem to be a continually accelerated accumulation at this point, which would terminate in the formation of a planet. Under this modification, the nebular hypothesis may possibly be free from some of the objections with which it has been justly as sailed. But in approaching the forbidden limits of human knowledge, it is becoming to tread with caution and circumspection. Man's speculations should be subdued from all rashness and extrayagance in the immediate presence of the Creator."

[The rings of Saturn have been considered by the most celebrated astronomers, Laplace, Struve,

allel lines by the quick rotation of the planet Bessel, Sir William Herschel, J. F. W. Herschel, Smyth and others, as being solid bodies surround

axis.

at are the dark belts?

ey are probably the body of the planet, seen the clouds.

hese belts always appear the same?

ey change frequently, and sometimes the clouds pieces. (See Fig. 3.)

at is the velocity of its equatorial parts, in turning on its axis? 000 miles an hour.

at amount of light and heat has Jupiter?

as 27 times less than the earth.

ing the planet and of the same materials and density; but Mr. G. P. Bond of Cambridge, and Professor Peirce, have advanced a new theory in regard to these rings, that they are a fluid or semiliquid matter. In regard to this novel Theory it is not considered by astronomers as fully established. We have not adopted this new theory in this work, preferring to wait for a further de monstration of it. We are however inclined to believe that the various phenomena are more easily explained upon this hypothesis.—AUTHOR.] ́

NOTE.

In 1675 it was observed by Roemer, a Danish Astronomer, that when the earth was nearest to Jupiter the eclipses of his satellites took place 8 minutes 13 seconds sooner than the time specified in the astronomical tables; but when the earth was farthest from Jupiter, the eclipses took place 8 minutes 13 seconds too late, the difference being 16m. 26s. From this it appears that it takes light 16m. 26s. to pass across the earth's orbit, which is 190 millions of miles in diameter 190 millions of miles divided by 986 the number of seconds in 16m. 26s. gives 192,697 miles as the velocity of light per second.