ON THE SOLAR SYSTEM. 407 



undergo, that, like our moon, they always present the same face to their 

 primary planet. 



The ring of Saturn is inclined 31 degrees to our ecliptic ; of his seven 



satellites, six are nearly in the same plane with the ring ; hut the plane 



of Jhe seventh or outermost satellite is hut half as much inclined to the 



i^ ^'j/ecliptic. The ring has heen observed by Dr. Herschel to revolve in 10 1 



hours, which is considerably less than the time that would be occupied by 



.the revolution of a satellite at the same distance. The planes of the six 



s^ellites of the Georgian planet are nearly perpendicular to the ecliptic ; 



anil some of their revolutions are supposed to be rather retrograde than 



**iet.* 



\ -sides the bodies which revolve completely round the sun, within the 

 limits of our observation, there are others, of which we only conclude from 

 analogy, that they perform such revolutions. These are the comets ; they 

 generally appear attended by a nebulous light, either surrounding them as 

 a coma, or stretched out to a considerable length as a tail ; and they some- 

 times seem to consist of such light only. Their orbits are so eccentric, 

 that in their remoter situations the comets are no longer visible to us, 

 although at other times they approach much nearer to the sun than any 

 of the planets : for the comet of 1680, when in its perihelion, was at the 

 distance of only one sixth of the sun's diameter from his surface. Their 

 tails are often of great extent, appearing as a faint light, directed always 

 towards a point nearly opposite to the sun : it is quite uncertain of what 

 substance they consist ; and it is difficult to determine which of the con- 

 jectures respecting them can be considered as the least improbable ; it is 

 possible that, on account of the intense cold, to which the comets are sub- 

 jected in the greatest part of their revolutions, some substances, more light 

 than any thing we can imagine on the earth, may be retained by them in 

 a liquid, or even in a solid form, until they are disengaged by the effect 

 of the sun's heat : but we are still equally at a loss to explain the rapidity 

 of their ascent : for the buoyancy of the sun's atmosphere cannot possibly 

 be supposed to be adequate to the effect ; and on the whole there is, per- 

 haps, reason to believe that the appearances are derived from some cause, 

 bearing a considerable analogy to the fluid, supposed to be concerned in 

 the effects of electricity. It is probable that the density of the nucleus, 

 or the body of the comet itself, is comparatively small, and its attraction 

 for the tail consequently weak, so that it has little tendency to reduce 

 the tail, even if it consists of a material substance, to a spherical form : 

 for since some comets have no visible nucleus at all, there is no difficulty 

 in supposing the nucleus, when present, to be of very moderate density, 

 and perhaps to consist of the same kind of substance as constitutes the tail 

 or coma, in a state of somewhat greater condensation. If, therefore, it 

 should ever happen to a planet to fall exactly in the way of a comet, of 

 which there is but very little probability, it is to be supposed that the 

 inconvenience suffered by the inhabitants of the planet might be merely 

 .temporary and local : the chances are, however, much greater, that a comet 



* J. Herschel on the Satellites of Uranus, Mem. of the Ast. Soc. vol. viii. 



