76 IN STARRY REALMS. 



for our earth- moon system when sufficient ages have 

 elapsed. Mars rotates on its axis in about half an hour 

 longer than our present day. The interior satellite to 

 which we have referred courses around the planet so 

 quickly that it completes a circuit in about seven hours 

 and a half, so that the Martial moon revolves three times 

 round its primary in the same time that Mars requires for 

 a single rotation. In the case of this planet and his 

 satellite the dimensions and the masses involved are much 

 smaller than are those in the case of the earth-moon sys- 

 tem. It seems, therefore, that we have in the adjustment 

 of Mars and his moon a sort of miniature representation 

 of the state of things to which the earth- moon system 

 tends in the excessively remote future. 



