An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

:xlV NALYSIS.
Art. Art.
How found by the spaces fallen Figure of the orbit of Halley's comet. 393
throungh, Fig. 75................ 379 Source of the light.................. 394
How f6tnd in planets which have no Direction of the tails............... 394
satellites 380 Quantity of matter in comets........ 895
Densities, how found............... 381 How the orbit of a comet mlay be
Specific gravities of the sun and plan- cllanged...... 396
ets respectively.................. 31 Exam'ple in the coinet of 1770..... 396
Comparative densities.......... 881 ORBITS AND MOTIONS OF COMETS..... 397
How thley differ from those of planets; 397
Chapter XII.-PERTURBETIONS OF PLAN ETS Elements enumerated............... 398
-STABILITY OF THE SYSTEM-NUMERI- Their investigation, why diflicult.398-399
CAL LATIONS-ROBLMS.. How the return of a comet is predicted. 400
PERTURIBATIONS —Numnerous causes... 382 Exemplified in llalley's coret...... 400
Periodical and secular perturbations Its return in 1759 and 1835.......400-401
disitinguished..................... 382 Wilhy an astronomical event of great
Case whiere the only bodies are a cen- interest................401
tral and a revolving body.......... 383 Encke's comet-its period........... 402
How these irregularities have been Question of a resisting medium...... 402
discovered....................... 383 Comlet of 1843-its remarkable pecouMinuteness of some perturbations... 383 liarities....................... 403
Whether tile perturbations accuml- Physical nature of comets......... 404
late indefinitely................3.. 84 Possibility of tlleir striking the earth. 405
Stability of thle system-how Illm1!.- METEromc SHowERs-great shower of
tained............. 384 November, 1833.................. 406
Nature of the evidence to prove theu Point of apparent radiation.......... 406
stability....................... 84 Extent and duration............... 406
luvariability of the major axes...... 384 Periods of its recurrence............ 407
Litnits to the variation of the eccen- Why an astronomical or costilical
tricity...................... 385 phenomenon.............. 407
Also to that of the inclination....... 385 Of the periods of meteoric showers.. 407
What kind of perturbations are cu- Conclusions respecting the meteors,
nmllative and what are oscillatory.. 385 as to their origin, nature, velocity,
Conditions essential to this stability. 386 size, light, and heat.............. 408
Long inequality of Jupiter and Sat- Reasons for these conclusions........ 409
urn.............................. 386
Also of the Earth and Venus........ 386
NUMERICAL RELATIONS OF THE PLANETARY SYSTEM................ 387 Part III. —OF THE FIXED STARS
Change of velocity necessary on in- AND SYSTEM OF THE WORLD.
creasing the mass................. 387
Acretlsoiig o tle ieasg...t....... 387 Chapter I.-FIXED STARS-CONSTELLAAlso ol inclreasinlg the distance...... 387 TIONS
Members of the solar system, how adjusted.. 87 Why calledfixed stars......................... 410
Relation between the rate of motion Classification...................... 410
distance, periodic time, and force of Number in each class............... 410
gravity.................... 387 Antiquity of the constellations...... 411
Demlonstration of the rules.......... 387 Their names-how individual stars
The rules stated................... 388 are denoted...................... 411
Given, tile velocity, to lind the other Catalogues of the stars............ 412
termlsl2......... 388 Number in the catalogue of HipparGiven, the distan, ce. 388 chus 412
(;ixre-n,~ ~ 8 t lie.da.......................... 412
Given, the jperiodic time............. 3S8 Number in Lalande's............... 412
Given, thleforce of' gravitation...... 388 Utility of learning the constellations. 413
Required, the rate of motion, dis- Cornstellations of the Zodiac-Aries
tance, period, and force of gravita- Taurus........................... 413
tion respectively................. 388 Seven stars in Pleiades.............. 413
Problelims..... 389 Gemini, Cancer........... 413
SHOWERS. Leo, Virgo, Libra.413
Scorpio, Sagittarilusj Capricornus,
COMETS-their several parts..........- 390 Aquarius, Pisces................. 413
Number belonging to thie system...... 391 Northern CGonstellations.... 414
The six most remarkable.............., 91 Ursa Minor, Ursa Major............ 414
Variations in magnitude and bright- Drac o............... 414
iness.................,.. 392 Cepheus, Cassiopeia, Camelopard,
To what owing............... 392 Andromeda............. 415
Periods of revolution.... 93....... Perseus, Auriga, Leo Minor, Canes
Distances from the sun............... 1 393 Venatici, Coma Berenices, Bootes.. 415