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The excentricities of the 1st and 2d Satellites are insensible, those of the 3d and 4th small, but variable, in consequence of their mutual perturbation.

3. SATELLLITES OF SATURN.

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The longitudes are reckoned in the plane of the ring from its descending node with the ecliptic. The first seven satellites move in, or very nearly in, its plane ; that of the 8th is inclined to it at an angle about half way intermediate between the planes of the ring and of the planet's orbit. The apsides of Titan have a direct motion of 30%2811 per annum in longitude (on the ecliptic). .

The discovery of Hyperion is quite recent, having been made on the same night (Sept. 19. 1848), by Mr. Lassell, of Liverpool, and Prof. Bond, of Cambridge, U. S. Its distance and period are as yet hardly more than conjecture. Messrs. Kater, Encke, and Lassell agree in representing the ring of Saturn as subdivided by several narrow dark lines, besides the broad black divisions which ordinary telescopes show.

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5. SATELLITES OF NEPTUNE. One only bas been certainly observed, -its approximate period being 54 205 50m 45', - distance about 12 radii of the planet

IV. ELEMENTS OF PERIODICAL COMETS AT THEIR LAST APPEARANCE.

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T is the time of perihelion passage ; w the longitude of the perihelion; and that of the ascending node for the epoch of the perihelion ; i, the inclination to the ecliptic; a, the semi-axis ; €, the excentricity; P, the period in days.

N. B. The reader will find a complete list of elements of all known comets up to June, 1847, by all their several computors, in Prof. Encke's edition of Olbers's “ Abhandlung über die leichteste und bequemste Methode die Bahn eines Cometen zu berechnen.” The list is compiled by Dr. Galle. It contains orbits of 178 distinct comets. From an examination of these orbits we collect the following, as a more correct statement of cometary statistics than that in art. 601. viz. :— Retrograde comets under 10° inclination, 3 out of 15; under 20°, 9 out of 29. Retrograde comets, inoving in orbits sensibly elliptic, under 17° inclination, 0 out of 9. In such orbits, of all inclinations from 0 to 90°, 11 out of 37. Thus we see that the induction of that article is materially strengthened by the enlarged field of comparison.

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INDEX.

N. B. The reference! are to the articles, not to the pages.

... attached to a reference number indicates that the reference extends to the article cited, and
several subsequent in succession.

Abberatios of light explained, 329.
Its uranographical effects, 333. Of
an object in motion, 335. IIow dis-
tinguished from parallax, 805. Sys-
tematic, 862.

About We/a, 705.

Acceleration, secular, of moon's mean
motion, 740.

Adams, 506. 767.

Adjustment, errors of, in instruments,
136. Of particular instruments. (See
those instruments.)

JEtna, portion of earth visible from, 32.
Height of, 32. note.

Air, rarefaction of, 33. Law of den-
sity, 37. Refractive power affected
by moisture, 41.

Airy, G. B. Esq., his results respecting
figure of the earth, 220. Researches
on perturbations of the earth by
Venus, 726. Rectification of the mass
of Jupiter, 757.

Algol, 821.

Altitude and azimuth instrument, 187.
—s. Equal, method of, 188.

Andromeda, nebula in, 874.

Angle of position, 204. Of situation, 311.

Angles, measurement of, 163. 167.
Hour, 107.

Angular velocity, law of, variation of,
350.

Anomalistic year, 384.

Anomaly of a planet, 499.

Annular nebulae, 875.

Apex of aberration, 343. Of parallax,
343. Of refraction, 343. Solar, 854.
Of shooting stars, 902. 904.
Aphelion, 368.

Apogee of moon, 406. Period of its
revolution, 687.

Apsides, 406. Motion of investigated,
675. Application to lunar, 676...
Motion of, illustrated by experiment,

692. Of planetary orbits, 694. Li-
bration of, 694. Motion in orbits
very near to circles, 696. In execu-
tric orbits, 697...

Areas, Kepler's law of, 490.

Argelandcr, his researches on variable
stars, 820..., on Bun's proper mo-
tion, 854.

Argo, nebulas in, 887. Irregular star Tj
in constellation, 830.

Ascension, right, 108. (Sec Right ascen-
sion.)

Asteroids, their existence suspected pre-
vious to their discovery, 505. Ap-
pearance in telescopes, 525. Gravity
on surface of, 525. Elements, Appen-
dix, Synoptic Table.

Astraa, discovery of, 505.

Astromeler, 783, 784.

Astronomy. Etymology, 11. General
notions, 11.

Atmosphere, constitution of, 33... Pos-
sible limit of, 36. Its waves, 37.
Strata, 37. Causes refraction, 38.
Twilight, 44. Total mass of, 148.
Of Jupiter, 513.

Attraction of a sphere, 445—450. (See
Gravitation.)

Augmentation of moon's apparent dia-
meter, 404.

Augustus, his reformation of mistakes
in the Julian calendar, (919.) Era
of, 926.

Australia, excessive summer tempera-
ture of, 369.

Axis of the earth, 82. Rotation per-
manent , 56. Major of the earth's
orbit, 373. Of sun's rotation, 392.

Axis of a planetary orbit. Momentary
variation of, caused by the tangential
force only, 658. 660. Its variations
periodical, 661... Invariability of,
and how understood, 668.

Azimuth, 103.—and altitude instru-
ment, 187.

B.

Barometer, nature of its indication, 33.

Use in calculating refraction, 43. In

determining heights, 287.
Belts of Jupiter, 512. Of Saturn, 514.
Benzenberg's principle of collimation,

179.
Bcssel, his results respecting the figure

of the earth, 220. Discovers parallax

of 61 Cygni,812.
Biela's comet, 579...
Biot, his aeronautic ascent, 32.
Bode, his (so called) law of planetary

distances, 505. Violated in the case

of Neptune, 507.
Borda, his principle of repetition, 198.
Bouvard, his suspicion of extraneous

influence on Uranus, 760.

C.

Casar, his reform of the Roman calen-
dar, 917.

Calendar, Julian, 917. Gregorian'
914...

Cause and effect, 439. and note.

Center of the earth, 80. Of the sun, 462.
Of gravity, 360. Revolution about,
452.

Centrifugal force. Elliptic form of earth
produced by, 224. Illustrated, 225.
Compared with gravity, 229. Of a
body revolving on the earth's sur-
face, 452.

Ceres, discovery of, 505.

Challis, Prof., 506, note.

Charts, celestial, 111. Construction
of, 291... Bremiker's, 506, and note.

Chinese records of comets, 574. Of
irregular stars, 831.

Chronometers, how used for determining
differences of longitude, 255.

Circle, arctic and antarctic, 94. Verti-
cal, 100. Hour, 106. Divided, 163.
Meridian, 174. Reflecting, 197. Re-
peating, 198. Galactic, 793.

Clepsydra, 150.

Clock, 151. Error and rate of, how
found, 253.

Clouds, greatest height of, 34. Magel-
lanic, 892...

Clusters of stars, 864... Globular, 867.
Irregular, 869.

Collimation, line of, 155.

Collimator, 178...

Coloured stars, 851...

Colures, 307.

Comets, 554. Seen in day-time, 555.
59a Tails of, 556...506. 599. Ex-

treme tenuity of, 558. General de-
scription of, 560. Motions of, and
described, 561... Parabolic, 564.
Elliptic 567... Hyperbolic, 564. Di-
mensions of, 565. Of HaUey, 567...
Of Cassar, 573. Of Encke, 576. Of
Biela, 579. Of Faye, 584. Of
Lexell, 585. Of De Vice, 586. Of
Brorsen, 587. Of Peters, 688. Sy-
nopsis of elements (Appendix). In-
crease of visible dimensions in re-
ceding from the sun, 571.580. Great,
of 1843, 589... Its supposed idenutr
with many others, 594... Interest at-
tached to subject, 59". Cometa.7
statistics, and conclusions therefrom,
601.

Commenswability (near) of mean mo-
tions; of Saturn's satellites, 550. Of
Uranus and Neptune, 669. and note.
Of Jupiter and Saturn, 72a Earth
and Venus, 726. Effects of, 719.

Compensation of disturbances, how ef-
fected, 719. 725.

Compression of terrestrial spheroid, 221.

Configurations, inequalities depending
on, 655...

Conjunctions, superior and inferior, 47J.
Perturbations chiefly produced at, 7 IS.

Consciousness of effect when force a
exerted, 439.

Constellations, 60. 301. How brought
into view by change of latitude, Si-
Rising and setting of, 58.

Copernican explanation of diurnal mo-
tion, 76. Of apparent motioni of
sun and planets, 77.

Correction of astronomical observations,
324... s. Uranographical sammarr,
view of, 342...

Culminations, 125. Upper and lower,
126.

Cycle, of conjunctions of disturbing
and disturbed planets, 719. Meto-
nic, 926. Callippic, ib. Sour, 921
Lunar, 922. Of indictions, 923.

D.

Day, solar, lunar, and sidereal, Itt
Ratio of sidereal to solar, 303. W-
911. Solar unequal, 146. Mean
ditto, invariable, 908. Civil and
astronomical, 147. Intercalary, 916

Days elapsed between principal chro-
nological eras, 926. Rules for reckon-
ing between given dates, 927.

Declination, 105. How obtained, 295.

Definitions, 82...

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