NOTES. XV11 



Jupiter's Satellites (the third) its ratios of volume and mass to the centra 

 planet are, of volume T?ir?r f mass nJUfr' On the Ellipticity of Uranus, 

 see Schumacher's Astron. Nachr. 1844, No. 493. 



( 41 ) p. 90. Beer und Madler, der Mond, 185, S. 208, and 347, S. 332; 

 and Phys. Kenntniss der himml. Korper. S. 4 und 69, Tab. I. 



( 42 ) p. 92. The paths of the four first comets which it has been possible 

 to investigate, have been computed from Chinese observations. They are 

 those of 240 (in the reign of Gordian III.), of 539 (in that of Justinian), of 

 565, and of 837. According to Du Sejour, the last of these must have been 

 for four-and-twenty hours 2000000 geographical miles only from the earth. 

 It appeared during the reign of Louis-le-Debonnaire (the son of Charlemagne), 

 and was thought to prognosticate disasters which that monarch sought to avert 

 by founding conventual establishments, whilst the Chinese astronomers were 

 scientifically engaged in tracing its path. Its tail was 60 degrees in length, 

 and appeared sometimes simple and sometimes divided. The first comet 

 computed exclusively from European observations, is that of 1456, or Halley's 

 comet, so named on the occasion which was long, though erroneously, sup- 

 posed to have been its first well ascertained appearance. Arago, Annuaire, 

 1836, p. 204 ; Laugier, Comptes-rendus des Seances de 1'Academie, 1843, 

 T. xvi. p. 1006. 



() p. 93. Arago, Annuaire, 1842, p. 209211. As the tail of the 

 comet of 1402 was seen in bright sunshine, so also in the recent great comet 

 of 1843 the nucleus and tail were visible on the 28th of February, in North 

 America, between one and three o'clock in the afternoon. (J. G. Clarke, of 

 "Portland, in the State of Maine.) The distance of the very dense nucleus 

 from the sun's limb could be measured with great exactness. The nucleus and 

 tail appeared as a very pure white cloud ; between the tail and the nucleus 

 there was one darker part. (American Journal of Science, Vol. xlv. No. 1, 

 p. 229 ; Schum. Astr. Nachr. 1843, N. 491, S. 175.) 



() p. 93. Phil. Trans, for 1808, Pt. ii. p. 155 ; and for 1812, Pt. i, 

 p. 118. Herschel found the diameters of the nuclei 538 and 428 English 

 miles. For the dimensions of the comets of 1798 and 1805, vide Arago, in 

 the Annuaire for 1832, p. 203. 



( 45 p. 95. Arago, des Changemens physiques de la Comete de Halley du 

 1523 Oct. 1835, in the Annuaire for 1836, p. 218221. The usual 

 direction of the emanations had been remarked in the time of Nero : " Com 

 radios solis etl'ugiunt." Seneca, Nat. Queest. vii. 20. 



( 46 ) p. 95. Bessel, in Schumacher's Astronomische Nachrichten, 183fl 



