COXTIXEXTS AND SEAS. 



an axis, while they make their periodical revolutions round 

 the sun, and that the diameter in which they so rotate has 

 been selected in such a manner as to secure to each of them 

 regular alternations of light and darkness in every part of their 

 surfaces ; in fact, they, like the earth, have days and nights. 

 But are those days and nights regulated by the same intervals 

 as ours ? for that is an important 

 question ; such intervals being as 

 we have shown, a key to the 

 organisations and functions of the 

 creatures upon them respectively. 



20. When a telescope of ade- 

 quate power is directed to the 

 planet Mars, it is observed that 

 the surface of his disc is diversi- 

 fied by certain features of light 

 and shade like that of the moon. 

 Some of these lights and shadows 

 are shifting and variable, but 

 most of them are permanent and 

 unalterable. In fig. 6, a view of 



these permanent lineaments as they are presented in a certain 

 aspect is given, taken from a telescopic drawing of the planet, 

 made by M. Madler, the celebrated Prussian observer. 



Now if these outlines of light and shade be watched for some 

 hours, they will be observed to be carried slowly from one side 

 of the disc to the other. Each of these will in succession dis- 

 appear at one side, others coming into view at the other, and 

 after an interval of about twelve hours, the marks which disap- 

 peared at one side will be found to re-appear at the other, and 

 this goes on continually. 



It is scarcely necessary to say that those are the effects of the 

 rotation of the planet on its axis, and since the same features 

 after disappearing at one side always return to the same precise 

 position on the disc after an interval of 24h. 37m. 22s., it follows 

 that the planet turns upon its axis in that interval. 



21. By means very nearly similar strong reasons have been 

 found for concluding that the globe of Venus turns on its 

 axis in 23h. 21m. 21s., and that of Mercury in 24h. 5m. 



22. Thus it appears that these three planets, not only have 

 days and nights, but that these days and nights are for all practical 

 purposes similar to those of the Earth. They are regulated by 

 the same average duration ; and He that gave them those alter- 

 nations has seen it good to " divide the light from the darkness " 

 after the same fashion. 



13 



