PLANETARY SYSTEMS. 83 



as a fusion of agglomerated satellites) moving from west to 

 east, the satellites of Uranus are almost perpendicular to the 

 ecliptic, and move retrogressively from east to west, as Sir 

 John Herschel has proved by observations continued during 

 many years. If the primary and secondary planets have been 

 formed by the condensation of rotating rings of solar and 

 planetary atmospheric vapour, there must have existed singular 

 causes of retardation or impediment in the vaporous rings 

 revolving round Uranus, by which, under relations with which 

 we are unacquainted, the revolution of the second and fourth 

 of its satellites was made to assume a direction opposite to 

 that of the rotation of the central planet. 



It seems highly probable that the period of rotation of all 

 secondary planets is equal to that of their revolution round 

 the main planet, and therefore that they always present to the 

 latter the same side. Inequalities, occasioned by slight vari- 

 ations in the revolution, give rise to fluctuations of from 

 6 to 8, or to an apparent libration in longitude as well as in 

 latitude. Thus, in the case of our moon, we sometimes observe 

 more than the half of its surface, the eastern and northern 

 edges being more visible at one time, and the western or 

 southern at another. By means of this libration* we are 

 enabled to see the annular mountain Malapert (which occa- 

 sionally conceals the Moon's south pole), the arctic landscape 

 round the crater of Gioja, and the large gray plane near 

 Endymion, which exetf<is in superficial extent the Mare 

 Vaporum. Three-sevenths of the Moon's surface are entirely 

 concealed from our observation, and must always remain so, 

 unless new and unexpected disturbing causes come into play. 

 These cosmical relations involuntarily remind us of nearly 

 similar conditions in the intellectual world, where, in the 

 domain of deep research into the mysteries and? the primeval 

 creative forces of nature, there are regions similarly turned 

 away from us, and apparently unattainable, of which only a 

 narrow margin has revealed itself, for thousands of years, to 

 the human mind, appearing, from time to time, either glim- 

 mering in true or delusive light. We have hitherto con- 

 sidered the primary planets, their satellites, and the concentric 



* Beer and Miidler, op. cit., 185, s. 208, and 347, s. 332 ; and in 

 their Pkys. Kenntniss der himml Korper. s. 4 und 69, Tab. 1. (Phy- 

 sical History of the Heavenly Bodies.) 



