INDEX. 



515 



SEASONS. 



Seasons, conditions determining the 

 duration of, 74 ; cause of their un- 

 equal periods, 87 ; theory of the 

 tropical dry and rainy, 123. 



Seaweeds, photographic impressions 

 of, 205, 206 ; luxuriance, deep 

 colours of, 253. 



Secchi, Professor, mountains of the 

 moon observed by, 70 ; photo- 

 graphic image of the moon obtained, 

 214 ; temperatures of the sun's 

 surface estimated, 225 ; experi- 

 ments of, in photographing the 

 moon and Jupiter, 226, 227. 



Secular inequalities of planets, 13, 

 14 ; means of discovering, 24, 25 ; 

 effect of, on the mean motion of the 

 moon, 36, 37. 



variations in mean values of the 



magnetic elements, 343. 



Seebeck, point of maximum heat in 

 solar spectrum fixed by, 263 ; dis- 

 covery of, 264 ; relations of heat to 

 electricity discovered by, 332, 333. 



Seed-lobes, proportion in the distri- 

 bution of plants having one or two, 

 252. 



Seleniate of zinc, crystals of, 107. 



Senarmont, M., experiments of, in ex- 

 pansion of crystals, 273. 



Senses, necessarily inaccurate testi- 

 mony of the, 281. 



September, times coinciding in, 84. 



Serpentarius, star in, vanishing, 392. 



Shell-fish, their mode of clinging to 

 rocks, 117. 



Shield, the, clusters of the Milky 

 Way between Ophiuchus and, 387. 



Shooting stars, phenomena of, de- 

 scribed, 421, 422; theories of, 

 423. 



Siberia, Eastern, depression of the 

 barometer observed in, 120. 



Sidereal times, mean, periods of, 83 ; 

 measurement of apparent, ib. 



Sigma Eridani, period of revolution 

 in, 400. 



Silesia, fulgorites from, 293. 



Silver iodized, its sensitiveness to im- 

 pressions, 221. 



Sirius, the Egyptian year estimated 

 from, 85; comet's tail extending 



SOLAR. 



from the Hare to, 373 ; rank of, 

 384; comparative magnitude, 385 ; 

 parallax, 389 ; cause of his irre- 

 gular motion, 392 ; change in 

 colour, 401 ; light, 402 ; extent of 

 surface, 404. 



Smyth, Admiral, his measurement of 

 Etna compared with Sir John Her- 

 schel's, 120; eclipse of a double 

 star observed by, 397 ; its periodic 

 time determined, 398. 



, Piazzi, heat of the moon felt 



by, 227. 



Snow, cause of perpetual, on summits 

 of alpine chains, 119 ; causes modi- 

 fying the height of the line of per- 

 petual, 241 ; protecting vegetation, 

 249 ; radiation of heat by, 257. 



Soda, sulphate of, change of form in 

 its crystals, 107; crystals of the 

 neutral phosphate and the arseniate 

 of, 109. 



Soil, the, dependence of temperature 

 on the nature of its products, 243. 



Solar gravitation, 424, 425. 



magnetism, its connexion with 



terrestrial, 344. 



spectrum, cause of the point of 



maximum heat varying in, 263, 264. 



system, the, gravitation of the 



bodies composing, 5 ; conditions 

 securing the stability of, 11, 12; 

 proof of its stability, 20 ; equi- 

 librium of, underanged by the 

 ethereal medium, 22 ; invariable 

 plane, forming the equator of, 23, 

 24 ; question of its revolution 

 round a common centre, 24; pro- 

 perties of its medium, 32 ; masses 

 of bodies composing, 55, 56 ; their 

 diameters, 56 ; uniform direction 

 of rotation in, 65 ; comparative 

 apparent importance of, in creation, 

 226 ; probably magnetic through- 

 out, 346 ; comets forming part of, 

 365 ; possible ultimate destruction 

 of, 372 ; computations of comets 

 revolving within, 381, 382 ; paths 

 described by heavenly bodies in, 

 382, 383; position of, relatively 

 to the Milky Way, 385 ; direction 

 of its motion, 405. 



