84 A CENTURY'S PROGRESS IN ASTRONOMY. 



mination of the rotation period by Schiaparelli 

 and Lowell is now generally accepted, and is 

 confirmed by the theory of tidal friction. It 

 is only right to add that William Frederick 

 Denning (born 1848) in 1881 suspected a 

 rotation period of 25 hours, but this remains 

 unconfirmed. In April 1871 the spectrum of 

 Mercury was examined by Hermann Carl 

 Vogel (born 1842) at Bothkamp. He suspected 

 traces of an atmosphere similar to ours, but 

 was not certain. Of more interest are the 

 photometric observations of Zollner in 1874. 

 These observations indicated that the surface 

 of Mercury is rugged and mountainous, and 

 comparable with the Moon, a conclusion sup- 

 ported by Lowell's observations in 1896. 



Venus, the nearest planet to the Earth, has 

 been attentively studied for three centuries, and 

 still comparatively little is known regarding it. 

 This is due to its remarkable brilliancy, com- 

 bined with its proximity to the Sun. The 

 great problem at the beginning of the nine- 

 teenth century was the rotation of the planet. 

 In 1779 the subject was taken up by Schroter 

 at Lilienthal. Nine years later, from a faint 

 streak visible on the disc, he concluded that 

 rotation was performed in 23 hours 28 minutes, 

 and in 1811 this was reduced by seven minutes ; 



