18 A CENTURY'S PROGRESS IN ASTRONOMY. 



period of eleven years, the discovery of which 

 was afterwards made by Schwabe. 



Herschel closely scrutinised the surfaces of 

 the planets. Mercury alone was neglected by 

 him. From 1777 to 1793 he observed Venus, 

 with the object of determining the rotation 

 period, but he was unable to observe any 

 markings on the surface of the planet. He 

 did not place reliance on Schroter's value of 

 the rotation period (about twenty-three hours). 

 Meanwhile, Schroter announced the existence 

 on Venus of mountains which rose to five or 

 six times the height of Chimborazo. As to 

 these, said Herschel, " I may venture to say 

 that no eye which is not considerably better 

 than mine, or assisted by much better instru- 

 ments, will ever get a sight of them." Herschel 

 demonstrated the existence of an extensive 

 atmosphere round Venus. 



" The analogy between Mars and the Earth," 

 Herschel wrote in 1783, "is perhaps by far the 

 greatest in the whole Solar System." In 1777 

 he began, in his house at Bath, a series of 

 observations on the red planet, which yielded 

 results of the utmost importance. Fixing his 

 attention on the white spots at the north and 

 south poles, discovered by Maraldi, nephew of 

 Cassini, he soon ascertained the fact that they 



