THE MAJOR PLANETS. 249 



would give to the whole ring to which it was attached somewhat of the charac- 

 ter of a heavy and sluggish satellite, maintaining itself in an orbit with a cer- 

 tain energy sufficient to overcome minute causes of disturbance, and establish 

 an average bearing on its centre. But even without supposing the existence 

 of any such load of which, after all, we have no proof and granting, there- 

 fore, in its full extent, the general instability of the equilibrium, we think we 

 perceive, in the periodicity of all the causes of disturbance, a sufficient guar- 

 antee of its preservation. However homely be the illustration, we can con- 

 ceive nothing more apt in every way to give a general conception of this main- 

 tenance of equilibrium under a constant tendency to subversion, than the mode 

 in which a practised hand will sustain a long pole in a perpendicular position 

 resting on the finger, by a continual and almost imperceptible variation of the 

 point of support. Be that, however, as it may, the observed oscillation of the 

 centres of the rings about that of the planet is in itself the evidence of a per- 

 petual contest between conservative and destructive powers both extremely 

 feeble, but so antagonizing one another, as to prevent the latter from ever ac- 

 quiring an uncontrollable ascendency, and rushing to a catastrophe. 



Since the plane of the rings coincides with that of Saturn's equator, and since 

 the sun is during one half of Saturn's year north, and during the other half south 

 of his equator, it follows that the northern side of the ring is illuminated, and the 

 southern side dark, during the summer half year of his northern hemisphere, 

 and that the southern side is illuminated and the northern side dark during the 

 winter half year of his northern hemisphere. At his equinoxes the edge of 

 the ring is presented to the sun, and neither side of it is illuminated. Since 

 the half year of Saturn is equal to fifteen terrestrial years, it follows that the 

 northern and southern sides of the rings are alternately illuminated by the sun 

 during intervals of fifteen years. 



It is evident that the rings can only be seen from the earth when the 

 sun and earth are at the same side of Saturn's equator. From the great 

 magnitude of Saturn's orbit, compared with that of the earth, this must be 

 generally the case. In order that the sun and earth should be at opposite 

 sides of the plane of the ring, that plane must be so placed that its edge is di- 

 rected to some point between the sun and earth. This will be the case for a 

 short time before and after it is directed to the sun, that is to say, a little be- 

 fore and after Saturn's equinox. 



If we suppose two lines touching the earth's annual orbit, and parallel to the 

 line of nodes of Saturn's ring, to be drawn and continued in both directions to 

 Saturn, it will be only when Saturn is between these lines that the earth and 

 sun can be at different sides of the ring. These lines will include a length of 

 ( the orbit of Saturn equal to the diameter of the orbit of .the earth, and since 

 ) Saturn will move over such a space in his periodical course round the sun in 

 ( ! one year, it follows that the sun and earth must be always at the same side 

 ( of Saturn's ring, except for six months before and six months after each of 

 Saturn's equinoxes, at which times it may happen that the sun and earth may 

 be on opposite sides of the rings. 



Saturn's rings may become invisible from the earth by any of three causes. 



1 . When the edge of the rings be presented to the sun, the edge being then 

 the only illuminated part, and being too thin to be seen even by telescopes at 

 so great a distance, the ring is invisible. This will happen once every h'iteen 

 years. 



2. When the edge of the ring is presented to the earth, it is invisible be- 

 cause of its minuteness and distance. This will happen once every fifteen years. 



3. When the sun and earth are on opposite sides of the ring. This will 

 ) also happen once every fifteen years. 



