350 SPECIAL RESULTS IN THE URANOLOGICAL 



while Bianchini ( 558 ) at Rome, in 1726, assumed the slow 

 rotation of 24J days. The more exact observations of De 

 Vico, in the years 1840 42, have given from the mean of a 

 great number of " spots of Venus," 23 hours, 21 minutes, 

 21-93 seconds. 



These spots, which appear on the boundary dividing 

 the illuminated from the shaded portion of the planet when 

 Venus appears as a bow, are seen only rarely, and are faint 

 and for the most part variable ; so that both the Herschels, 

 father and son, have believed them to belong, not to the solid 

 surface of the planet, but more probably to an atmosphere sur- 

 rounding it ( 559 ), The variable shape of the horns of the 

 bow, especially of the southern, has been used by La Hire, 

 Schroter, and Madler, partly for estimating the heights of 

 the mountains, and partly and more particularly for deter- 

 mining the rotation. The phenomena are not such as to 

 require for their explanation such elevations as were assumed 

 by Schroter at Lilienthal of 5 German, or 20 English, 

 geographical miles ; but, on the contrary, only such altitudes 

 as the mountains of our own globe present in both conti- 

 nents ( 56 ) . In the little that we know of the aspect of the 

 surfaces or of the physical constitution of the two planets 

 nearest to the Sun (Mercury and Venus), an exceedingly 

 curious enigma is presented by the appearance of an ash- 

 coloured light, or an evolution of light riot derived from 

 any other body, which has been occasionally observed on the 

 dark part of Venus by Christian Mayer, "William Herschel 

 ( 561 ), and Harding. The great distance renders it unlikely 

 that the reflected light of the Earth should be the cause in 

 Venus as it is in the Moon. 



No compression at the poles has yet been observed 



