176 COSMOS. 



the mean distance from the Earth, 9"- 9. His mass, which 

 was determined as yTf-g-yj from the first ohservations of the 

 satelUtes, is, according to Lament's observations, only -ojioj I 

 consequently his density would he between those of Jupiter 

 and Saturn. =^ A flattening of Uranus was already ccnjec- 

 tured by Herschel from his observations with magnifying 

 powers of from 800 to 2400. According to Madler's meas- 

 urements in 1842 and 1843, it would appear to fall between 

 Y^.y and Q^g-.f The original supposition that Uranus had 

 two rings was found to be an optical illusion by the discoverer 

 himself, in all cases so cautious and persevering in confirming 

 his discoveries. 



The Satellites of Uranus. 



" Uranus," says Sir John Herschel, " is attended by satel- 

 lites — four, at least, probably five or six." They present a 

 great and hitherto unparalleled peculiarity, viz., that while 

 all satellites (those of the Earth, of Jupiter, of Saturn), as 

 well as all the principal planets, move from west to east, and 

 with the exception of a few asteroids, in orbits not much in- 

 clined toward the ecliptic, the satellites of Uranus move from 

 east to west in orbits which are nearly circular, and form an 

 angle of 78° 58' with the ecliptic — very nearly perpendicu- 

 lar to it. In the case of the satellites of Uranus, as well as 

 those of Saturn, the arrangement and nomenclature, accord- 

 ing to their distances from the primary, are to be distin- 

 guished from the arrangement according to the epoch of 

 discovery. According to a private communication from Sir 

 John Herschel (November 8th, 1851), Mr. Lassell has dis- 

 tinctly observed on the 24th, 2Sth, and 30th of October, and 

 2d of November of the above vear, two satellites of Uranus 

 which appear to be situated still nearer to the primary than 

 the first satellite observed by Sir William Herschel, to which 

 he ascribed a period of revolution of about 5 days and 21 

 hours, but which was not recognized. The periods of revo- 

 lution of the two satellites now seen by Lassell were near to 

 4 and 2\ days. Of the satellites of Uranus, the second and 

 fourth were first discovered by William Herschel in 1787, 

 then the first and fifth in 1790, and, finally, the sixth and 

 third in 1794. During the fifty-six years which have elapsed 

 Bince the last discovery of a Uranus satellite (the third), the 



* Cosmos, vol. iv., p. 119. 



t Madler, in Schumacher's Asir. Nachr., No. 493. (With resanl to 

 ihe flattouing ofUianus, compare Arago, Annn,i, re fur 18 IQ, p 577 -.779. ? 



