MARS AS A WORLD. 17* 



first Professor Hall was greatly puzzled by the extra- 

 ordinary behaviour of one of these little moons. It first 

 appeared at one side of the planet, and then at the other, 

 in a manner so incomprehensible that Professor Hall 

 began to think there must be two or three bodies instead 

 of a single one. 



Nor was such a supposition unnatural. It was, indeed, 

 the only alternative to an explanation which at first 

 seemed wildly improbable. This latter supposition, which 

 ultimately turned out to be the correct one, showed that 

 the inner of the two moons accomplished a feat entirely 

 without parallel in the solar system. It actually com- 

 pleted three revolutions around Mars in less time than 

 Mars required for a single rotation on his own axis. To 

 appreciate the significance of this statement, let us refer 

 to the condition of our own earth-moon system. The 

 moon requires about twenty-seven days to complete its 

 circuit of our earth, so that the revolution of the moon is 

 twenty-seven times as long as the rotation of the earth. 

 In a similar way, though not always to the same extent, 

 the periods of revolution of all the satellites that had been 

 previously discovered were each much larger than the 

 period of rotation of their primaries. 



Contrary, however, to the invariable precedent exhibited 

 in the case of every other satellite of our system, the 

 inner satellite of Mars exhibits, to our astonishment, the 

 spectacle of a little moon galloping round the planet 

 three times for every single rotation that the planet itself 

 accomplishes. The outer of the two Martial satellites 

 follows the analogy of the other similar bodies in our 

 system. Its period of revolution is, however, only about 



