Tis the time of perihelion passage; the longitude of the perihelion; and

that of the ascending node for the epoch of the perihelion;

4, the inclination to the ecliptic; a, the semi-axis; e, 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 state-
ment 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 170 inclination, O 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.

ABBERATION of light explained, 329.
Its uranographical effects, 333. Of
an object in motion, 335. How dis-
tinguished from parallax, 805. Sys-
tematic, 862.
Aboul Wefa, 705.

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

tric orbits, 697...
Areas, Kepler's law of, 490.
Argelander, his researches on variable
stars, 820..., on sun's proper mo-
tion, 854.

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.

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.

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 Halley, 567...
Of Cæsar, 573. Of Encke, 576. Of
Biela, 579. Of Faye, 584.

Lexell, 585. Of De Vico, 586. Of
Brorsen, 587. Of Peters, 588. Sy-
nopsis of elements (Appendix). In-
crease of visible dimensions in re-
ceding from the sun, 571.580. Great,
of 1843, 589... Its supposed identity
with many others, 594... Interest at-
tached to subject, 597. Cometary
statistics, and conclusions 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, 655...
Conjunctions, superior and inferior, 473.
Perturbations chiefly produced at, 713.
Consciousness of effect when force is
exerted, 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. Meto-
nic, 926. Callippic, ib. Solar, 921.
Lunar, 922. Of indictions, 923.

D.

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 direc-
tion, 614. Resolution of, in two
modes, 615. 618. Effects of each
resolved portion, 616... On moon,
expressions of, 676. Geometrical re-
presentations of, 676. 717.
Diurnal motion explained, 58. Paral-
lax, 339. Rotation, 144.
Double refraction, 202. Image micro-
meter, a new, described, 203. Co-
met, 580. Nebulæ, 878.
Double Stars, 833... Specimens of each
class, 835. Orbitual motion of, 839.
Subject to Newtonian attraction, 843.
Orbits of particular, 843. Dimen-
sions of these orbits, 844. 848. Co-
loured, 851... Apparent periods af-
fected by motion of light, 863.
Dove, his law of temperature, 370.

E.

Earth. Its motion admissible, 15.
Spherical form of, 18. 22... Optical |
effect of its curvature, 25. Diurnal
rotation of, 52. Uniform, 56. Per-
manence of its axis, 57. Figure sphe-
roidal, 219... Dimensions of, 220.
Elliptic figure a result of theory, 229.
Temperature of surface, how main-
tained, 366. Appearance as seen
from moon, 436. Velocity in its
orbit, 474. Disturbance by Venus,
726.

Eclipses, 411... Solar, 420. Lunar, 421

Annular, 425. Periodic return
of, 426. Number possible in a year,
426. Of Jupiter's satellites, 538. Of
Saturn's, 549.

Ecliptic, 305... Its plane slowly vari-
able, 306. Cause of this variation
explained, 640. Poles of, 307. Li-
mits, solar, 412. Lunar, 427.

Egyptians, ancient, their chronology,

912.

Elements of a planet's orbit, 493. Va-
riations of, 652... Of double star or-
bits, 843. Synoptic table of planet-
ary, &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.
Enceladus, 548., note.

Encke, comet of, 576. His hypothesis
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 center,
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
taking means, 137. Of clock, how
obtained, 293.

Establishment of a port, 754.
Ether, resistance of, 577.
Evection of moon, 748.
Excentricities, stability of Lagrange's
theorem respecting, 701.
Excentricity of earth's orbit, 354. How
ascertained, 377. Of the moon's, 405.
Momentary perturbation of, investi-
gated, 670. Application to lunar
theory, 688.

Variations of, in orbits

nearly circular, 696. In excentric
orbits, 697. Permanent inequalities
depending on, 719.