116 



NOTES TO PRECEDING SECTION. 



hhulers " that it become entirely night." Still more uncer- 

 tain, or rather more erroneous, is the assertion that the 

 "trabes quas Sokovs vocant'' (Plin. ii. 22 and 27) was ap- 

 plied to the ascending tongue-shaped zodiacal light as Cas- 

 Kini (p. 231) and Mairan (p. 15) will have it. Everywhere 

 with the ancients the word "trabes" is taken as synony- 

 mous with fire-balls and fiery-meteors generally, and even 

 occasionally with streaming comets. On Sokos, SoKias, So- 

 KiTVi, vide Schafer, Schol. Par. ad Apoll. Rhod. 1813, t.-ii. 

 p. 206 ; Pseudo-Aristot. de Mundo, 2, 9 ; Comment. Alex., 

 Joh. Philop. et Olymp. in Aristot. Meteor, lib. i. cap. vii. 3, 

 p. 195, Ideler; Seneca, Nat. Quaest. i. 1. 



63 (p. 44.) — Humboldt, Monumens des peuples indigenes 

 de l'Am6rique, t. ii. p. 301. The rare MS. which belonged 

 to the Archbishop of Rheims, Le Tellier, contains very va- 

 rious extracts from an Aztekian ritual book, from an astro- 

 logical calendar, and from historical annals from 1197 to 

 1549. The last include natural phenomena — dates of earth- 

 quakes, comets — as of 1490, 1592, and, for Mexican chronol- 

 ogy, important eclipses of the sun. In the MS. Historia de 

 Tlascala of Camargo, the light which appeared in the east 

 and rose almost to the zenith is spoken of as "sparkling, 

 and as if thickly sown over with stars." The account of 

 the 40 days' phenomenon (Prescott, Hist, of the Conquest 

 of Mexico, vol. i. p. 284) will by no means apply to an erup- 

 tion of Popocatepetl which rises close by in the south-east. 

 Later commentators have confounded this phenomenon, 

 which Montezuma regarded as one foreboding him misfor- 

 tune, to the " estrella que humeava." (properly which spar- 

 kled ; Mexican choloa, to leap, to sparkle). On the con- 

 nection of this vapour with the star CiUal Choloha (Venus) 

 and the staiTy mountain (Citaltepetl, the volcano of Oriza- 

 ba), see my Monumens, t. ii. p. 303. 



64 (p. 44.) — Laplace, Expos, du Syst. du Monde, p. 270; 

 M6canique c61este, t. ii. p. 169 and 171. Schubert, Astr. 

 Bd. iii. I) 206. 



65 (p. 44.) — Arago, in Annuaire, 1842, p. 408. See Sir 

 John Herschel's Considerations on the Volume and Light 

 of the Planetary Nebulae, in Mary Somerville's Connexion 

 of the Physical Sciences, 1835, p. 108. The opinion that 

 the sun is a nebulous star, whose atmosphere has the ap- 

 pearance of the zotliacal light, was not advanced by Cassiui, 

 but by Mairan, 1731. — {Vide Trait6 de I'Aurore bor. p. 47 

 and 263. Arago, in Annuaire, 1842, p. 412.) It was a re- 

 vival of the views of Kepler. 



66 (p. 45.) — Cassini, as well as Laplace, Schubert and 

 Poisson after him, adopted the hypothesis of a detached 

 ring as an explanation of the figure of the zodiacal light. 

 He says expressly: " Si les orbites-de Mercure et de Venus 

 6toient visibles (mat6riellement dans toute I'fetendue de 

 leur surface), nous les verrions habituellement de la m^me 

 figure et dans la nifenie disposition A regard du Soleil et 

 aux mfimes terns de l'ann6e que la lumiere zodiacale." — 

 (M^m. de I'Acad. t. viii. 1730, p. 218, and Biot, in the 

 Comptes rendus, 1836, t. iii. p. 666.) Cassini believed that 

 the vaporiform ring of the zodiacal light was composed of 

 an innumerable host of small planetary bodies, which re- 

 volve about the sun. He was himself not indisposed to be- 

 lieve that the fall of fire-balls might be connected with the 

 passage of the earth through the zodiacal nebulous ring. 

 Olmsted, and especially Biot (1. c. p. 673), have endeav- 

 oured to demonstrate this connection with the Noveniber 

 phenomenon ; any such connection, however, is doubted by 

 Olbers (Schum. Jahrb. 1837, p. 281). On the question 

 whether the plane of the zodiacal light perfectly agrees 

 with the plane of the sun's equator, wide Houzeau, in Schum. 

 Astr, Nachr. 1843, No. 492, p. 190. 



67 (p. 45.)— Sir John Herschel, Astron. ^ 487. 



68 (p. 45.) — Arago im Annuaire, 1832, p. 246. — Many 

 physical facts appear to indicate that, with a mechanical 

 division of matter into its minutest particles, when the mass 

 becomes extremely small in comparison with the surface, 

 the electrical tension may arise to the point of producing 

 luminous and calorific rays. Experiments with a large 

 concave mirror have not yet given any decisive indications 

 of the existence of radiating heat in the zodiacal light.— 

 (Lettre de Mr. Matthiessen A Mr. Arago, in the Comptes 

 rendus, t. xvi. 1843, Avril, p. 687.) 



69 (p. 45.)— "What you tell me of the variations in the 

 light of the zodiacal light, and their causes within the trop- 

 ics, has interested me by so much the more, as I have for a 

 long time every spring given particular attention to the 

 phenomenon in our northern latitudes. I have myself al- 

 ways believed that the zodiacal light rotates, but I conclu- 

 ded that it extended with constantly increasing intensity of 

 lustre quite to the sun (in opposition to Poisson's view, 

 which you communicate to me). The luminous ring, which 

 shows itself about the sun under a total eclipse, I have re- 

 garded as constituted by this most brilliant portion of the 

 zodiacal light. I have persuaded myself that this light is 

 very different in different years ; that for several years in 

 succession it is extremely bright and extensive ; in others, 

 again, that it is not even to be seen. I fancy I can see the first 

 indications of a recognition of the zodiacal light in a letter 



j from Rothmann to Tycho, when he says, that in the fsptingj 

 i he had found the sun 24° below the horizon at the end of 

 the evening twilight. Rothmann must certainly have coiv 

 founded the disappearance of the sinking zodiacal light, in 

 the vapours of the evening horizon, with the actual end of 

 the evening twilight. I have not myself observed any ri- 

 sings and fallings, probably by reason of the weakness with 

 which the zodiacal light appears in our latitudes. But you 

 are assuredly correct when you ascribe such sudden altera- 

 tions in the light of the heavenly bodies, which you ob- 

 served within the tropics, to our atmosphere, especially to 

 changes in its higher regions. This is especially shown in 

 the tails of great comets. One frequently sees, especially 

 in clear weather, pulsations in these tails, which begin from 

 the head of the comet as the lowest point, and tremble 

 through the entire length of the tail in 1 or 2 seconds, when 

 the tail appears to be lengthened and immediately after- 

 wards to be shortened by several degrees. That those 

 pulsations, to which Hooke and Schrocter and Chladni paid 

 particular attention, do not take place in the comets' tails 

 themselves, but are produced by our atmosphere, becomes 

 obvious when we reflect that the several portions of the tail 

 (several millions of miles in length) lie at very difl^erent dis- 

 tances from us, and tliat its light can only reach us at in- 

 tervals of time, several minutes apart from one another. 

 Whether what you observed on the Orinoco, not at intervals 

 of seconds, but of minutes, were true corruscations of the 

 zodiacal light, or belonged wholly and solely to the upper 

 strata of our light-circle, I will not pretend to determine. 

 Neither do I know how the remarkable luminousness of en- 

 tire nights, and the anomalous increase and protraction of 

 the twilight in the year 1831, are to be explained, espe- 

 cially as it was observed that the brightest parts in these 

 extraordinary twilights did not correspond with the sun's 

 place below the horizon." — From a letter of Dr. Olbers to 

 me, dated Bremen, 26th March, 1833. 



■''O (p. 45.) — Biot, Traite d'Astron. physique, 3me 6d., 

 1841, t. i. p. 171, 238, and 312. 



71 (p. 45.) — Bessel, in Schum. Jahrb. fiir 1839, S. 51 ; 

 probably one million of mil^s daily ; in relative velocity, 

 834,000 miles ; and therefore more than twice the velocity 

 of the earth in its orbit round the sun. 



72 (p. 46.) — On the Motion of the Solar System, after 

 Bradley, Tobias, Mayer, Lambert, Lalande, and William 

 HerscWl, see Arago, in Annuaire, 1842. p. 388—399; Ar- 

 gelander, in Schum. Astron. Nachr. Nr. 363, 364, 398; and 

 in the treatise : Von der eigenen Bewegung des Sonnensys- 

 tems, 1&37, S. 43, on Perseus as the central constellation 

 of the entire stratum of stars. See also Otho Struve, in 

 Bull, de I'Acad. de St. Petersb. 1842, t. x. No. 9, p. 137— 

 139 ; according- to whom, from a subsequent combination, 

 the direction of the sun's motion was found to be 261° 23' 

 R. A. -|- 37° 36' Decl. ; and, as a mean from Argelander's 

 and his own laboun, from a combination of 797 stars, 259° 

 9' R. A. -f 340 36' L^clination. 



73 (p. 46.)-Aristot. C« Ccelo, iii. 2, p. 301 ; Bekker, Phys. 

 viii. 6, p. 256. 



74 (p. 46.)— Savary, in th» Connaissance des Tems, 1830, 

 p. 56 and 163 ; Encke, Berl. Jahrb. 1832, S. 253 ; Arago. in 

 Annuaire, 1834, p. 260, 295 ; JcHn Herschel, m Mem. of tho 

 .^stronom. Soc. vol. v. p. 171. 



75 (p. 46.)— Bessel, Untersuchun^desTheils der plane ta- 

 rischen Storungen, weJche aus der Bewegung der Sonno 

 entstehen, in Abh. der Berl. Akad. aer Wissensch. 1824 

 (Mathem. Classe), S. 2 — 6. The question was opened up 

 by Johann Tobias Mayer, in Comment. Soc. Reg. Gutting. 

 1804—1808, vol. xvi. p. 31—68. 



76 (p. 47.) — Philos. Transact, for 1803, p. 5.25 ; Arago, in 

 Annuaire, 1842, p. 375. If the reader Avould Ibrm a mora 

 tangible idea of the distance of the fixed stars referred to 

 some short way before in the text, let him suppose the 

 earth to be at the distance of one foot from the sun, Uranus 

 would then be 19 feet, and Vega in Lyra 34^ German geo- 

 graphical (1586 English) viiles from that luminary. 



77 (p. 47.)— Bessel, in Schum. Jahrbuche, 1839, S. 5a. 



78 (p. 47.) — -MSdler, Astr. S, 476; Derselbe, in Schum. 

 Jahrb. 1839, S. 95. 



79 (p. 47.) — Sir William Herschel, in the Philos. Trans- 

 act, for 1817, Pt. ii. p. 328. 



80 (p. 47.)— Arago, in Annuaire, 1842, p. 459. 



81 (p. 47.)— Sir John Herschel, in a letter from Feldhuy- 

 sen of the 13th Jan. 1836 ; Nicholl, Archit. of the Heavens, 

 1838, p. 22. See also some observations of Sir William 

 Herschel on the great starless space which, at a vast dis- 

 tance, separates us from the milky way, in the Philos. 

 Trans, for 1817, Part ii. p. 328. 



83 (p. 47.) — Sir John Herschel, Astron. I) 624 ; and farther 

 in Observations on Nebulae and Clusters of Stars (Phi). 

 Trans. 1833, Pt. ii. p. 479, fig. 25) : " we have here a broth- 

 er system, bearing a real physical resemblance and strong 

 analogy of structure to our own." 



83 (p. 48.)— Sir William Herschel, in the Transact, for 

 1785, Pt. i. p. 257 ; Sir John Herschel, Astr. I) 616 (" the 

 nebulous region of the heavens forms a nebulous milky way 



