INDEX. 



491 



ELLIPSOID. 



turbance, 15 ; cause and measures 

 of variation in, 17 ; described by 

 comets, 363, 366. 



Ellipsoid, an, of revolution, mass as- 

 suming the form of, 45 ; its equa- 

 torial and its polar radius, 48 ; 

 permanent axes of rotation, 76. 



Elliptic motion, ratio of forces pro- 

 curing, 382. 



Elliptical polarization of light, 192, 

 193; of heat, 267. 



nebulae, 409 ; their connec- 

 tion with double stars, '411 ; fre- 

 quency, 413 ; difficult of resolution, 

 415. 



Encke, Professor, sun's parallax found 

 by, 53 ; his comet, 169 ; aspects, 

 period of his comet, 365, 366 ; 

 cause of acceleration in its revolu- 

 tion, 366, 367 ; crossing the ter- 

 restrial orbit, 368 ; prospective and 

 present planetary influence on, 369 ; 

 disappearance of its tail and nucleus, 

 369 ; referred to, 377 ; contraction 

 of diameter, ib. 



England, arcs of the meridian mea- 

 sured in, 48 ; course of the tidal 

 wave towards its west coast, 94; 

 peculiarities of photography in, 213; 

 meteors falling in, 421. 



Engravings copied by photography, 

 204 ; impressions taken by contact 

 with iodized silver, 221 ; impres- 

 sions taken from, by galvanism, 

 309. 



Epipolic light, 197. 



Epsilon Orionis, zone of stars passing 

 through, 385. 



Equation of the centre, defined, 9 ; 

 lunar, 35. 



Equator, the, forces compelling the 

 wider circle of, 6 ; inclination of 

 the terrestrial to the plane of the 

 ecliptic, 23 ; of the solar system, 

 24 ; measure of the centrifugal force 

 at, 49 ; calculation from lunar ac- 

 tion on the terrestrial, 55 ; effects 

 produced by external attraction in- 

 flucing the direction of its plane, 

 79, 80 ; inequality in its polar 

 motion, 81 ; cause of the calms at, 



' 122; depth of the underground 



ETHEREAL. 



stratum of constant temperature at, 

 228 ; maximum of solar heating 

 influence, 238 ; superficial extent 

 of land, 244 ; mean annual tem- 

 perature, 245. 



Equator of the sun, maximum of solar 

 heat attained in, 225. 



, dynamic, surrounding the ter- 

 restrial globe, 343. 



, magnetic, of the earth, 343. 



Equinoctial circle, the, defined, 9. 



points, effects of solar and lunar 



attraction on, 79 ; period of their 

 revolution, 80 ; measuring time, 

 83. 



Equinoxes, the, defined, 9; vernal, a 

 point whence planetary motions are 

 estimated, ib. ; of the planets, cause 

 of a precession in, 66 ; causes pre- 

 venting their invariable correspond- 

 ence with points of the ecliptic, 79 ; 

 precession affecting the seasons, 80 ; 

 secular motion of, periodic varia- 

 tions, 80, 81 ; eras depending on the 

 precession of, 86, 87 ; tides aug- 

 mented in, 97. 



Eras, astronomical, determined by the 

 position of the major axis of the 

 solar ellipse, 86, 87, 



Eratosthenes, the earth's circumfer- 

 ence measured by, 49. 



Eridanus, nebulous patches crossing, 

 417. 



Erman, M., depression of the baro- 

 meter observed by, 120. 



Eruptions, volcanic, recorded, 234. 



Eta Aquilae, a variable star, 391 . 



Argus, zone stretching from, 



390; nebula round, 418, 419. 



Coron.se, periodic time of, 398. 



Etna, measurements of, 120. 



Ethereal medium, undulations of, 

 propagating heat, 267 ; permeable 

 to lines of magnetic force, 344 ; its 

 density, 356 ; transmitting gravity, 

 16. ; magnetic, 356, 357 ; offices 

 discharged by, 357 ; pervading the 

 visible creation, 35.8 ; influence of, 

 on comet motion, 365 ; astral 

 revolutions accelerated by, 366 ; 

 probable increase in density of, 

 367. 



