A GLIMPSE THROUGH THE CORRIDORS OF TIME. 497 



I mentioned that one of the satellites of Mars presented a phenomenon 

 unparalleled in the solar system. The satellite revolved around Mars 

 in a period of seven hours, while Mars himself rotated on his axis in 

 a period of twenty-four hours. We here actually find the moon of 

 Mars rotating around Mars in much less than one of Mars's own days. 

 This was a most curious and unexpected circumstance, but the obser- 

 vations of the discoverer, Asaph Hall, placed the great fact beyond 

 any doubt. The mystery has now been explained. It is due to the 

 action of the solar tides on Mars. Nay, more, we can actually foresee 

 that at some incredibly remote future time our earth and moon are 

 destined to present the same movements which have seemed so anoma- 

 lous in Mars. 



Left to themselves the earth and the moon would have remained for 

 ever in the condition of compromise. The moon would have revolved 

 round the earth in 1,400 hours. The earth would have rotated on its 

 axis in 1,400 hours also. But now the solar tides intervene. They 

 have little effect upon the moon ; it revolves as before, but the solar 

 tides begin to retard the earth still further. Instead of a period of 

 1,400 hours, the earth will have a still longer day, so that finally the 

 moon revolves more rapidly around the earth than the earth rotates on 

 its axis. 



It seems to me that the episode I have mentioned is one of the 

 most interesting in the whole of modern astronomy. We have first a 

 most delicate telescopic discovery of the tiny satellite of Mars and of 

 its anomalous movements. We then have a beautiful explanation of 

 how this anomalous motion has arisen from the action of solar tides. 

 Finally, we have in this miniature system of Mars a foreshadowing of 

 the ultimate destiny of our earth and our moon. 



Do I say the ultimate destiny ? Nothing is ultimate in nature. 

 The moon and the earth would have come to an amicable and a final 

 agreement had they been let alone. But now the sun has intervened 

 and disturbed the earth's rotation. The truce once broken, the moon 

 again produces tides on the earth, the earth reacts on the moon, and a 

 whole chain of complicated movements are the consequence. I shall 

 not now attempt to trace the further progress of events. 



I have dealt with very large figures in this lecture, and perhaps I 

 have taxed your imagination by my demands that you should conceive 

 of periods of tens of millions of years. Yet, after all, let us look at the 

 results in their true proportion, compared with the universe in which 

 our lot has been cast. 



Truly we have been engaged with a very trifling matter. Is not 

 our earth one of the most insignificant bodies in the universe ? And 

 our moon is much smaller still. Nor is it even the life-history of our 

 earth that we have been considering, it is merely a brief episode in 

 that history. What are the periods of time we have been discussing 

 when compared with those infinitely longer periods during which the 

 vol. xx. 32 



