is the time of perihelion passage; & the longitude of the perihelion; and that of the ascending node for the epoch of the perihelion;, 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 Olber'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,
moving in orbits sensibly elliptic, under 17° degrees 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.

A.

ABERRATION of light explained, 329.
Its uranographical effects, 333. Of an
object in motion, 335. How distin-
guished from parallax, 805. Syste-
matic, 862.
Aboul Wefa, 705.

Acceleration, secular, of moon's mean
motion, 740.
Adams, 506, 767.

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

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

Air, rarefaction of, 33. Law of density,
37. Refractive power affected by mois-
ture, 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.

Aphelion, 368.

Apogee of moon, 406. Period of its re-
volution, 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 excentric
orbits, 697...

Areas, Kepler's law of, 490,
Argelander, his researches on variable
stars, 820..., on sun's proper motion,
854.

Argo, nebulæ in, 887. Irregular star
in constellation, 830.

Ascension, right, 108. (See 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.

Astræa, discovery of, 505.
Astrometer, 783, 784.

-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 nebula, 875.

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

Australia, excessive summer temperature (

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.

Meridian, 174. Reflecting, 197. Re-
peating, 198. Galactic, 793.
Clepsydra, 150.
Clock, 151.
found, 253.

Error and rate of, how

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

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

Collimation, line of, 155.

Coloured stars, 851...
Colures, 307.

Comets, 554. Seen in day-time, 555,
590. Tails of, 556...566, 599. Ex-
treme tenuity of, 558. General de-
scription of, 560. Motions of, and de-
scribed, 561... Parabolic, 564. Elliptic,
567... Hyperbolic, 564. Dimensions
of, 565. Of Halley, 567... Of Cæsar,
573. Of Encke, 576. Of Biela, 579.
Of Faye, 584. Of Lexell, 585. Of
De Vico, 586. Of Brorsen, 587. Of
Peters, 588. Synopsis of elements
(Appendix). Increase of visible di-
mensions in receding from the sun,
571, 580. Great, of 1843, 589... Its
supposed identity with many others,
594... Interest attached to subject,
597. Cometary statistics, and con-
clusions therefrom, 601.
Commensurability (near) of mean mo-
tions; of Saturn's satellites, 550. Of
Uranus and Neptune, 669, and note.
Of Jupiter and Saturn, 720. Earth
and Venus, 726. Effects of, 719.
Compensation of disturbances, how ef
fected, 719, 725.

Compression of terrestrial spheroid, 221.
Configurations, inequalities depending
on, 665...

Conjunctions, superior and inferior, 473.

Perturbations chiefly produced at, 713.
Consciousness of effect when force is ex-
erted, 439.

Constellations, 60, 301. How brought
into view by change of latitude, 52.
Rising and setting of, 58.
Copernican explanation of diurnal mo-
tion, 76. Of apparent motions of sun
and planets, 77.

Correction of astronomical observations,
324... s. Uranographical summary,
view of, 342...
Culminations, 125. Upper and lower,

126.

Cycle, of conjunctions of disturbing and
disturbed planets, 719. Metonic, 926.
Callippic, ib. Solar, 921. Lunar, 922.
Of indictions, 923.

D.

Day, solar, lunar, and sidereal, 143.
Ratio of sidereal to solar, 305, 909,
911. Solar unequal, 146. Mean ditto,
invariable, 908. Civil and astrono-
mical, 147. Intercalary, 916.
Days elapsed between principal chrono-
logical eras, 926. Rules for reckoning

between given dates, 927.
Declination, 105. How obtained, 295.
Definitions, 82...

Degree of meridian, how measured, 210...
Error admissible in, 215. Length of

in various latitudes, 216, 221.
Diameters of the earth, 220, 221. Of
planets, synopsis, Appendix. (See also
each planet.)

Dilatation of comets in receding from
the sun, 578.

Dione, 548.

Discs of stars, 816.

Distance of the moon, 403; the sun, 357;

fixed stars, 807, 812..; polar, 105.
Districts, natural, in heavens, 302.
Disturbing forces, nature of, 609... Ge-
neral estimation of, 611. Numerical
values, 612. Unresolved in direction,
614. Resolution of, in two modes,
615, 618. Effects of each resolved
portion, 616... On moon, expressions
of, 676. Geometrical representations
of, 676, 717.

Diurnal motion explained, 58. Parallax,

339. Rotation, 144.

Double refraction, 202. Image micro-
meter, a new, described, 202. Comet,
580. Nebulæ, 878.
Double Stars, 833... Specimens of each
class, 835. Orbitual motion of, 839.
Subject to Newtonian attraction, 843.
Orbits of particular, 843. Dimensions
of these orbits, 844, 848. Coloured,
851... Apparent periods affected by
motion of light, 863.

Dove, his law of temperature, 370.

E.

Earth. Its motion admissible, 15. Sphe-
rical form of, 18, 22... Optical effect
of its curvature, 25. Diurnal rotation

Elements of a planet's orbit, 493. Va-
riations of, 652... Of double star orbits,
843. Synoptic table of planetary, &c.,
Appendix.

Ellipse, variable, of a planet, 653. Mo-
mentary or osculating, 654.
Elliptic motion a consequence of gravi

tation, 446. Laws of, 489... Their
theoretical explanation, 491.
Ellipticity of the Earth, 221.
Elongation, 341. Greatest, of Mercury
and Venus, 467.
Encke, comet of, 576. His hypothesis
Enceladus, 548, note.
of the resistance of the ether, 577.
Epoch, one of the elements of a planet's
orbit, 496. Its variation not inde-
pendent, 730. Variations incident on,
731, 744.

Equation of light, 335. Of the centre,
375. Of time, 379. Lunar, 452.
Annual, of the moon, 738.
Equator, 84.
Equatorial, 185.
Equilibrium, figure of, in a rotating body,

224.

Equinoctial, 97. Time, 935.
Equinox, 293, 303.
Equinoxes, precession of, 312. Its ef-
fects, 313. In what consisting, 314...
Its physical cause explained, 642...
Eras, chronological list of, 926.
Errors, classification of, 133. Instru-
mental, 135... Their detection, 140.
Destruction of accidental ones by tak-
ing means, 137. Of clock, how ob-
tained, 293.

Its

Gallactic circle, 793. Polar distance, ib.
Galaxy composed of stars, 302. Sir W.
Herschel's conception of its form and
structure, 786. Distribution of stars
generally referable to it, 786.
course among the constellations, 787...
Difficulty of conceiving its real form,
792. Telescopic analysis of, 797. In
some directions unfathomable, in others
not, 798.

Galle, Dr., 506. Finds Neptune in place
indicated by theory, 768.
Galloway, his researches on the sun's
proper motion, 855.
Gauging the heavens, 793.

Gay Lussac, his aëronautic ascent, 32.
Geocentric longitude, 503. Place, 371,
497.

Golden number, 922.

Goodricke, his discovery of variable stars,
821...

Gravitation, how deduced from pheno-
mena, 454... Elliptic motion a con-
sequence of, 490...

Gravity, centre of, see Centre of gravity.
Gravity diminished by centrifugal force,
231. Measures of, statical, 234. Dy-
namical, 235. Force of, on the moon,
433... On bodies at surface of the
sun, 440. Of other planets, see their

names.

Gregorian reform of calendar, 915...

H.

Halley. His comet, 567. First notices

Hansen. His detection of long inequali-
proper motions of the stars, 852.

ties in the moon's motions, 745...
Heat, supply of, from sun alike in sum-
Harding discovers Juno, 505.
mer and winter, 368. How kept up,
400. Sun's expenditure of estimated,
397. Received from the sun by dif-
ferent planets, 508. Endured by
Hebe, discovery of, 505.
comets in perihelio, 592.

Heights above the sea, how measured,
286. Mean, of the continents, 289.
Heliocentric place, 500.
Heliometer, 201.

Hemispheres, terrestrial and aqueous,

284.

793.

Herschel, Sir Wm., discovers Uranus,
505, and two satellites of Saturn, 548.
His method of gauging the heavens,
Views of the structure of the
Milky Way, 786. Of nebular subsi-
dence, and sidereal aggregation, 869,
874. His catalogues of double stars,
835. Discovery of their binary con-
nexion, 839.
Of the sun's proper
motion, 854. Classifications of nebulæ,
868, 879, note.

Horizon, 22. Dip of, 23, 195. Rational
and sensible, 74. Celestial, 98. Arti-
ficial, 163.

247...

Geography, 111, 205...

Globular clusters, 865. Their dynami-

determined, 175...

867.