72 IN STARRY REALMS. 



last without the intervention of other agents than those 

 known to us by the law of gravitation, it seems certain 

 that the progress of the earth-moon system must be in 

 substantial accordance with the principles here laid down. 

 We have seen that at the present moment the day is 

 becoming gradually longer and the moon is steadily 

 receding farther and farther from the earth. At present 

 these changes take place with extreme slowness, but in the 

 primitive periods of which we have already spoken, the 

 changes in the length of the day, and the changes in the 

 distance of the moon, proceeded at a rate far more rapid 

 than at present. As the moon has receded farther from 

 the earth its efficiency as a tide-producer has declined, and 

 consequently the rate at which the consequences of tidal 

 action have proceeded is continually lessening. It must 

 therefore be expected that the progress of tidal evolution 

 in the future will be ever getting slower and slower, so 

 that the periods of time required for the further develop- 

 ment of the phenomena far exceed those which have 

 elapsed in the course of the history already given. We 

 can, however, foreshadow what is to happen in the folldV- 

 ing manner. The length of the day will slowly increase ; 

 and we can indicate a state of things in the excessively 

 remote future towards which it may be said the system 

 is tending. The day will grow until it becomes not merely 

 twenty- five or twenty-six hours, but until it becomes 

 as long as two or throe of our present days. In fact, as 

 we stretch our imagination through ages so inconceivable 

 that I forbear to specify any figures which might charac- 

 terize them, we seem to discern that the length of the day 

 may go on ever getting longer and longer until at last a 



