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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 references 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.

692. Of planetary orbits, 694. Li-

bration of, 694. Motion in orbits
Abberation of light explained, 329. very near to circles, 696. In excen-

Its uranographical effects, 333. Of tric orbits, 697...
an object in motion, 335. How dis- | Areas, Kepler's law of, 490.
tinguished from parallax, 805. Sys- Argelander, his researches on variable
tematic, 862.

stars, 820..., on sun's proper mo-
Aboul Wefa, 705.

tion, 854.
Acceleration, secular, of moon's mean Argo, nebulæ in, 887. Irregular star n
motion, 740.

in constellation, 830.
Adams, 506. 767.

Ascension, right, 108. (See Right ascen-
Adjustment, errors of, in instruments, sion.)

136. Of particular instruments. (See Asteroids, their existence suspected pre-
those instruments.)

vious to their discovery, 505. Ap-
Ætna, portion of earth visible from, 32. pearance in telescopes, 525. Gravity
Height of, 32. note.

on surface of, 525. Elements, Appen-
Air, rarefaction of, 33. Law of den- dix, Synoptic Table.

sity, 37. Refractive power affected Astræa, discovery of, 505.
by moisture, 41.

Astrometer, 783, 784.
Airy, G. B. Esq., his results respecting Astronomy. Etymology, 11. General

figure of the earth, 220. Researches notions, 11.
on perturbations of the earth by Atmosphere, constitution of, 33... Pos-
Venus, 726. Rectification of the mass sible limit of, 36. Its waves, 37.
of Jupiter, 757.

Strata, 37. Causes refraction, 38.
Algol, 821.

Twilight, 44. Total mass of, 148.
Altitude and azimuth instrument, 187. Of Jupiter, 513.
-s. Equal, method of, 188.

Attraction of a sphere, 445–450. (See
Andromeda, nebula in, 874.

Gravitation.)
Angle of position, 204. Of situation, 311. Augmentation of moon's apparent dia-
Angles, measurement of, 163. 167. meter, 404.
Hour, 107.

Augustus, his reformation of mistakes
Angular velocity, law of, variation of, in the Julian calendar, (919.) Era
350.

of, 926.
Anomalistic year, 384.

Australia, excessive summer tempera-
Anomaly of a planet, 499.

ture of, 369.
Annular nebulæ, 875.

Axis of the earth, 82. Rotation per-
Apex of aberration, 343. Of parallax, manent, 56. Major of the earth's

343. Of refraction, 343. Solar, 854. orbit, 373. Of sun's rotation, 392.
Of shooting stars, 902. 904.

Axis of a planetary orbit. Momentary
Aphelion, 368.

variation of, caused by the tangential
Apogee of moon, 406. Period of its force only, 658. 660.' Its variations
revolution, 687.

periodical, 661... Invariability of,
Apsides, 406. Motion of investigated, and how understood, 668.

675. Application to lunar, 676... | Azimuth, 103.- and altitude instru.
Motion of, illustrated by experiment, ment, 187.

treme tenuity of, 558. General de.

scription of, 560. Motions of, and
Barometer, nature of its indication, 33.

described, 561... Parabolic, 564.
Use in calculating refraction, 43. In

Elliptic, 567... Hyperbolic, 564. Di-
determining heights, 287.
Belts of Jupiter, 512. Of Satum, 514.

mensions of, 565. Of Halley, 567...
Benzenberg's principle of collimation,

Of Cæsar, 573. Of Encke, 576. Of
179.

Biela, 579. Of Faye, 584. Of

Lexell, 585. Of De Vico, 586. Of
Bessel, his results respecting the figure
of the earth, 220. Discovers parallax

Brorsen, 587. Of Peters, 588. Sy-
of 61 Cygni, 812.

nopsis of elements (Appendix). In-

crease of visible dimensions in re-
Biela's comet, 579...
Biot, his aëronautic ascent, 32.

ceding from the sun, 571.580. Great,
Bode, his (so called) law of planetary

of 1843, 589... Its supposed identity
distances, 505. Violated in the case

with many others, 594... Interest at-
of Neptune, 507.

tached to subject, 597. Cometary

statistics, and conclusions therefrom,
Borda, his principle of repetition, 198.

601.
Bouvard, his suspicion of extraneous
influence on Uranus, 760.

Commensurability (near) of mean mo-

tions; of Saturn's satellites, 550. Of
Uranus and Neptune, 669. and note.

Of Jupiter and Saturn, 720. Earth
Cæsar, his reform of the Roman calen and Venus, 726. Effects of, 719.
dar, 917.

Compensation of disturbances, how ef-
Calendar, Julian, 917. Gregorian fected, 719. 725.
914...

Compression of terrestrial spheroid, 221.
Cause and effect, 439, and note. Configurations, inequalities depending
Center of the earth, 80. Of the sun, 462. on, 655...

Of gravity, 360. Revolution about, Conjunctions, superior and inferior, 473.
452.

Perturbations chiefly produced at, 713.
Centrifugal force. Elliptic form of earth Consciousness of effect when force is

produced by, 224. Illustrated, 225. exerted, 439.
Compared with gravity, 229. Of a Constellations, 60. 301. How brought
body revolving on the earth's sur into view by change of latitude, 52.
face, 452.

Rising and setting of, 58.
Ceres, discovery of, 505.

Copernican explanation of diurnal mo-
Challis, Prof., 506, note.

tion, 76. Of apparent motions of
Charts, celestial, 111. Construction sun and plancts, 77.

of, 291... Bremiker's, 506, and note. Correction of astronomical observations,
Chinese records of connets, 574. Of 324... S. Uranographical summary,
irregular stars, 831.

view of, 342...
Chronometers, how used for determining Culminations, 125. Upper and lower,
differences of longitude, 255.

126.
Circle, arctic and antarctic, 94. Verti-Cycle, of conjunctions of disturbing

cal, 100. Hour, 106. Divided, 163. and disturbed planets, 719. Meto-
Meridian, 174. Reflecting, 197. Re- nic, 926. Callippic, ib. Solar, 921
peating, 198. Galactic, 793.

Lunar, 922. Of indictions, 923.
Clepsydra, 150.
Clock, 151. Error and rate of, how

found, 253.
Clouds, greatest height of, 34. Magel- Day, solar, lunar, and sidereal, 143.
lanic, 892...

Ratio of sidereal to solar, 305. 909.
Clusters of stars, 864... Globular, 867. 911. Solar unequal, 146. Mean
Irregular, 869.

ditto, invariable, 908. Civil and
Collimation, line of, 155.

astronomical, 147. Intercalary, 916.
Collimator, 178...

Days elapsed between principal chro-
Coloured stars, 851...

nological eras, 926. Rules for reckon-
Colures, 307.

ing between given dates, 927.
Comets, 554. Seen in day-time, 555. Declination, 105. How obtained, 295.

590.' Tails of, 556...566. 599. Ex- | Definitions, 82...

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