THE MOON'S &ISTOXY. 65 



gether when the subject of our satellite is under discus- 

 sion. 



The significance of the gradual elongation of the day by 

 the tides arises from the circumstance that the change 

 always takes place in one direction. In this form of effect 

 the tide differs from other more familiar astronomical 

 phenomena which sometimes advance in one direction and 

 then after the lapse of suitable periods return in the 

 opposite direction, and thus restore again the initial state 

 of things. But the alteration of the length of the day is 

 not of this character, it is not periodic, its motion is never 

 reversed, is never even arrested. Only one condition is 

 therefore necessary to enable it to obtain tremendous dimen 

 aions, and that is sufficient time in which it can operate. 



There are many lines of reasoning which show the ex- 

 treme antiquity of our globe : the disclosures of geology are 

 specially instructive on this head. Think, for instance, of 

 that mighty reptile the Atlantosaurus, which once roamed 

 over the regions now known as Colorado. The bones of 

 this vast creature indicate an animal surpassing in propor- 

 tions the greatest elephant ever known. No one can count 

 the SBons of years that have elapsed since the Atlantosaurus, 

 whose bones are now to be seen in the museum at Yale 

 College, breathed its last. A still more striking concep- 

 tion of time than even the antiquity of this creature 

 affords is derived from the consideration that his mighty 

 form was itself the product of a long and immeasurable line 

 of ancestry, extending to a depth in the remote past far 

 beyond the limits of computation. I have mentioned this 

 illustration of the antiquity of the earth for the purpose of 

 showing the ample allowance of time that is available for 



