﻿in Small Orbits. 509 



in accordance with modern doctrines, the ultimate effects of tides 

 on the rotation of a satellite were regarded as the cause of the 

 synchronism of its orbital and rotatory movements ; yet there 

 would seem to be too great a demand on hypotheses in claiming 

 for every minor world the possession of seas and the tempera- 

 rature required for their fluidity. 



A more certain, though perhaps a more slow cause of the 

 arrangement in question, will be revealed on considering how 

 a solid satellite, consigned to a small orbit and rotating in such 

 a manner as to present its opposite sides alternately to the pri- 

 mary, must have its form perpetually changed by the attrac- 

 tion of the latter body; for these changes (even when they 

 proceed from oscillations of the internal matter with unim- 

 paired elasticity) would have the same effect as tides in altering 

 the rotation until it kept pace with the periodical revolution. 

 This I have shown in the Philosophical Magazine for December 

 1861 ; but it may be proved more easily by the aid of the modern 

 doctrine of the definite relation between heat and motion. In 

 the case of the supposed secondary planet, with an incessant 

 change in the gravity at its surface and in the pressure on its 

 internal parts, the resulting oscillations must be accompanied 

 by the production of heat ; and this being necessarily gene- 

 rated at the expense of motion, the rotation would evidently 

 change until it became adapted for keeping the same side ever 

 confronting the primary. The want of coincidence of the planes 

 of orbital and diurnal motion and the eccentricity of the orbits 

 would also be corrected by the oscillations and the calorific 

 action resulting from them ; and the surface of the satellite 

 would thus constantly assume a more tranquil condition, as the 

 disturbance from the central body varied little either in inten- 

 sity or in direction. 



For producing and maintaining the arrangement so favour- 

 able to repose, the causes alluded to are most effective when a 

 large secondary planet is in the closest proximity to its primary, 

 and those extreme cases where stability is scarcely possible 

 may be investigated with the least dependence on hypothesis. 

 Such cases are also recommended for consideration by their 

 connexion with certain celestial phenomena. For maintaining 

 the planetary structure of two large satellites in the region oc- 

 cupied by Saturn's rings, so great a density would be required, 

 that the condition of the mysterious annular appendage must be 

 ascribed to its proximity to the primary. While also the re- 

 sistance of a space-pervading medium may be expected to con- 

 sign to an orbit too small for stability a member of some of the 

 numerous planetary systems of the universe, an exact investi- 

 gation will show that the great dismemberment must take place, 



