384 A Short History of Astronomy [CH. XHI. 



below the visible surface. The surface markings are in each 

 case definite enough for the rotation periods to be fixed with 

 some accuracy ; though it is clear in the case of Jupiter, 

 and probably also in that of Saturn, that as with the sun 

 ( 298) different parts of the surface move at different rates. 



Laplace had shewn that Saturn's ring (or rings) could not 

 be, as it appeared, a uniform solid body ; he rashly inferred 

 without any complete investigation that it might be 

 an irregularly weighted solid body. The first important 

 advance was made by James Clerk Maxwell (1831-1879), 

 best known as a writer on electricity and other branches 

 of physics. Maxwell shewed (1857) that the rings could 

 neither be continuous solid bodies nor liquid, but that 

 all the important dynamical conditions would be satisfied 

 if they were made up of a very large number of small 

 solid bodies revolving independently round the sun.* The 

 theory thus suggested on mathematical grounds has re- 

 ceived a good deal of support from telescopic evidence. 

 The rings thus bear to Saturn a relation having some 

 analogy to that which the minor planets bear to the sun ; 

 and Kirkwood pointed out in 1867 that Cassini's division 

 between the two main rings can be explained by the 

 perturbations due to certain of the satellites, just as the 

 corresponding gaps in the minor planets can be explained 

 by the action of Jupiter ( 294). 



The great distance of Uranus and Neptune naturally 

 makes the study of them difficult, and next to nothing is 

 known of the appearance or constitution of either ; their 

 rotation periods are wholly uncertain. 



Mercury and Venus, being inferior planets, are never very 

 far from the sun in the sky, and therefore also extremely 

 difficult to observe satisfactorily. Various bright and dark 

 markings on their surfaces have been recorded, but different 

 observers give very different accounts of them. The rotation 

 periods are also very uncertain, though a good many astrono- 

 mers support the view put forward by Sig. Schiaparelli, in 

 1882 and 1890 for Mercury and Venus respectively, that 

 each rotates in a time equal to its period of revolution round 

 the sun, and thus always turns the same face towards the 

 sun. Such a motion which is analogous to that of the 

 * This had been suggested as a possibility by several earlier writers 



