NOTES TO PRECEDING SECTION. 



1 (p. 29.) — The optical considerations on the differences 

 which a single luminous point or a disc of measurable an- 

 gle presents, in which the power of light remains the same 

 at every distance, may be found discussed by Arago, Analyse 

 des travaux de Sir William Herschel (Annuaire du Bu- 

 reau des Long. 1842, p. 410—412. and 441.) 



2 (p. 29.) — "The two Magellanic clouds, Nubecula ma- 

 jor and minor, are highly remarkable objects. The larger 

 cloud is an aggregation of stars, and consists of clusters of 

 •tars of irregular configuration, of globular clusters and 

 nebulous stars of different sizes and densities. Between 

 these lie large nebulae which are not resolvable into stars, 

 which apparently are star dust, and even with the 20-feet 

 telescope present themselves only as a general luminous- 

 ness of the field, as a brilliant back-ground, upon which 

 other objects of very remarkable and incomprehensible con- 

 figuration are scattered. In no other part of the heavens 

 are so many clusters of nebulse and of stars collected to- 

 gether within so small a compass as in this cloud. The 

 Nubecula minor is much less beautiful; it shows a larger 

 quantity of unresolvable nebular light, and the groups of 

 stars it includes are fewer in number and smaller." — Letter 

 from Sir John Herschel, dated Feldhuysen, Cape of Good 

 Hope, June 13. 1836. 



3 (p. 29.) — The fine expression, xd^rof ovpavov, which 

 Hesychius borrows from an unknown poet, I have rendered 

 as above by the phrase " garden of heaven ;" xopfog may, 

 perhaps, rather signify an enclosed place, and would then 

 be better translated, the celestial space. The connection 

 of the word with the German garten, English garden, (Goth- 

 ic gards, according to Jacob Grimm, from gairdan, cingere, 

 to gird,) is, however, not to be overlooked, any more than 

 the affinity with the Sclavonian grad, gorod, and, as re- 

 marked by Pott (Etymol. Forsch. Th. i. S. 144), with the 

 Latin chors, (whence the modern words corte, court, cours), 

 and the Ossetic khart. The northern gard, gard, a fence, 

 an enclosure, and a country-seat ; and the Persian gerd, 

 gird, a circle, and also a princely country-seat, a castle, a 

 town ; as in the old names in Firdusi's Schahnameh : Siya- 

 wakschgird, Daraljgird, <fec. 



■* (p. 30.)— For u Centauri, vide Maclear, in Trans. As- 

 tronom. Soc., vol. xii. p. 370. More probable mean error 

 0"0640: for 61 Cygni, vide Bessel, in Schum. Jahrbuch, 

 1839, S. 47—49, and in Schum. Astronom. Nachr. Bd. 17, 

 S. 401, 402 Mean error 0"0I41. On the relative distan- 

 ces of stars of different orders, as those of the third magni- 

 tude are probably three times more distant, and as to how 

 we are to imagine the strata of stars in their bodily config- 

 uration, 1 find in Kepler's Epitome Astronomiae Copernica- 

 nae, 1618, tom. i. lib. i. p. 34—39, a remarkable passage: 

 " Sol hie noster nil aliud est quam una ex fixis, nobis major 

 et clarior visa, quia proprior quam fixa. Pone terram stare 

 ad latus, una semidiametro viae lacteae, tunc haec via lac- 

 tea apparebit circulus parvus, vel ellipsis parva, tota decli- 

 nans ad latus alterum ; eritque simul uno intuitu conspicua, 

 qu<E nunc non potest nisi dimidia conspici quovis momento. 

 Itaque fixarum sphaera non tantum orbe stellarum, sed etiam 

 circulo lactis versus nos deorsum est terminata." 



5 (p. 31.) — "Si dans les zones abandonn6es par I'atmo- 

 sph^re du soleil il s'est trouv6 des molecules trop volatiles 

 pour s'unir entre elles ou aux planetes ; elles doivent en 

 continuant de circuler autour de cet astre offrir toutes les 

 apparences de la lumiere zodiacale, sans opposer de resis- 

 tance sensible aux divers corps du systeme plan6taire, soit 

 & cause de leur extreme raret6^ soit"^parce que leur mouve- 

 ment est a fort peu pres le m6me que celui des planetes 

 qu'elles rencontrent."— Laplace, Exp. du Syst. du Monde, 

 (5e ed.) p. 415. 



6 (p. 31.)— Laplace, loc. cit. p. 396 and 414. 



7 (p. 31.) — Littrow, Astronomic, 1825, Bd. ii. S. 107 

 Madler, Astr. 1841, S. 212. (Laplace, loc. cit. p. 210.) 



8 (p. 31.)— Kepler on the decreasing density and increas- 

 ing volume of the planets, with their distance from the sun 

 which is described as the most dense of all bodies ; vide his' 

 Epitome Astron. Copern. in vii. libros digesta, 1618 — 1622 

 p. 420. Leibnitz was also of Kepler's and Otto von Gue- 

 ricke's opinion, that the planets increase in volume in pro- 

 portion to their distance from the sun. Vide his Brief an 

 den Magdeburger Burgermeister (Mainz, 1671), in Leibnitz, 

 deulschen Schriften, herausg. von Guhrauer, Th. i. S. 264. 



9 (p. 31.)— For a co ordination of the masses, see Encke, 

 in Schum. Astr. Nachr. 1843, Nr. 488, S. 114. 



f.^R ^I'^^'^T}^ ^}^ semidiameter of the moon, according 

 to Burckhardt's determination, be 02725, and Its volum! 

 Too 9' ''^ density would then come out 0-5596 • nearly 4. 



wie Madler, Astr. S. 157. The actual contents of the moon 



that 



acording to Hansen, equal -^^, according to Madler, 

 of the material contents of the earth ; its mass _' 

 of the earth. In the case of the largest of alfjuniter'^s 

 satellites, the third, the relations to the primary in volume 

 fl** fT3To ; '!» »he mass, yy jjnr- ^n ^he oblateness of 

 Uranus, vide Schum. Astron. Nachr. 1844, Nr. 493 



11 (p. 33.)— Beer und Madler, 1. c. i, 185, S. 208, n. ^ 347 

 S. 332 ; also their Phys. Kenntniss der himml. Korner. S* 

 4 und 69, Tab. i. ' 



li (p. 34.)— The four oldest comets whose orbits have 

 been calculated — and this from Chinese observarions— are 

 those of the years 24 (under Gordian III.), 539 (under Jus- 

 tinian), 565 and 837. Whilst the last of these comets was 

 less than 500,000 miles from the earth, and Louis the Pious, 

 greatly alarmed, was seeking to avert the presumed danger 

 by founding various monasteries, the Chinese astronomers 

 were following quite scientifically the course of the star 

 whose tail, 60° in length, appeared first simple and then 

 divided. The first comet which could be calculated from 

 European observations alone, is that of 1456, (those of Hal- 

 ley, which were long but incorrectly regarded as the first 

 accurate elements). Vide Arago, in Annuaire, 1836, p. 204. 

 See, also, under Note 26. 



13 (p. 34.)— Arago, in Annuaire, 1832, p. 209—211. In 

 the same way as the tail of the comet of 1402 was seen in 

 bright sunshine, so was the last great comet of 1843 seen 

 both in Its nucleus and tail between 1 and 3 o'clock on the 

 28th of February by J. G. Clarke, of Portland, Maine, U. S. 

 Distances of the extremely dense nucleus from the sun's 

 edge could be measured with great accuracy. The nucleus 

 and tail presented themselves as a very pure white cloud ; 

 only between the tail and the nucleus there was a darker 

 space. — American .lournal of Science, vol. xlv. No. l,p 229 

 (Schum. Astr. Nachr. 1843, Nr. 491, S. 175.) 

 .Q^it ^P- 34.) -Phil. Trans, for 1808, pt. ii. p. 155, and for 

 1812, pt. 1. p. 118. The diameters of the nuclei found by 

 Herschel were 538 and 428 English miles. For the dimen- 

 sions of the comets of 179S and 1085, vide Arago, in An- 

 nuaire pour 1832, p. 203. 



15 (p. 35.)— Arago des changemens physiques de la Co- 

 mete de Halley du 15—23 Oct. 1835, in Ann. 1836, p. 218— 

 221. The more usual direction of the tail or emanation 

 was observed in Nero's time : " Comte radios solis effu- 

 giunt." Seneca, Nat. Quaest. vii. 20. 



lt> (p. 35.)— Bessel, in Schum. Astr. Nachr. 1836, Nr. 

 300-302, S. 188, 192, 197, 200, 202, und 230. Also in 

 Schum. Jahrb. 1837, S. 149—168. W. Herschel also be- 

 lieved that he had observed the rotation of the nucleus and 

 tail lu his observations on the beautiful comet of 1811 

 (Philos. Trans. 1812, pt. i. p. 140.) So, also, Dunlop in the 

 third comet of 1825 at Paramatta. 



17 (p. 35.)-Bessel, in Astr. Nachr. 1836, Nr. 302, S. 231, 

 (Schum. Jahrb. 1837, S. 175). Vide, also. Lehmann uber 

 Cometenschweife in Bode's Astron. Jahrb. fiir 1826, S. 168. 



18 (p. 35.)— Aristot. Meteor, i. 8, 11—14 und 19—21 (ed' 

 Ideler t. i. p. 32—34). Biese, Phil, des Aristoteles, Bd. ii. 

 S. 86. In the influence which Aristotle exerted on the 

 whole of the middle ages, it is infinitely to be lamented that 

 he showed himself so inimicallj disposed to the grand views 

 of the structure of the universe espoused by the old Pytha- 

 goreans, and which approached the truth so closely. He de- 

 clares the comets to be transient meteors belonging to our 

 atmosphere, in the same book in which he quotes the opin- 

 ion of the Pythagoreans to the effect that comets were 

 planets with long periods of revolution. This doctrine of 

 the Pythagoreans, which, however, from the testimony of 

 Apollonius Myndius, appears to be much older, and to have 

 been that of the Chaldeans, passed over to the imitative . 

 Romans. The Myndian describes the orbits of comets as 

 passing far into the upper celestial spaces. Whence Seneca 

 (Nat. Quaest. vii. 17) : " Cometes non est species falsa, sed 

 proprium sidus sicut solis et lunse: altiora mundi secat et 

 tunc demum apparet quum in imum cursum sui venit ;" and 

 (vii. 27): " Cometas seternos esse et sortis ejusdem, cuius 

 catera (sidera), etiamsi faciem illis non habent similem.'* 



