THE MOON'S HISTORY. 71 



which the earth rotates on its axis was greatly increased ; 

 it is then conceivable that the tension thus arising might 

 be too great for the coherence of the material to with- 

 stand. We believe that the earth could turn round with 

 double the speed that it has at present before this tension 

 approached the point at which disruption would ensue. 

 But supposing the day were to be so much shortened that 

 the period of rotation was only a very few hours instead 

 of twenty-four, there is then good reason to know that 

 the tension in the earth arising from this rapid rotation 

 would be so great that a rupture of the globe would be 

 imminent. 



Provided with this conception, let us think of the initial 

 stage when the moon was quite close to the earth. Our 

 globe was then, as we know, spinning round so rapidly 

 that its materials were almost on the point of breaking 

 up in consequence of the strain produced by the rotation. 

 It is interesting to note that the tidal action of the sun 

 would also conduce to the rupture of our globe in the 

 critical circumstances we have supposed. It seems hardly 

 possible to doubt that such a separation of the glowing 

 mass did actually take place, a small fragment was dis- 

 carded, and gradually drew itself by the mutual attraction 

 of its particles into a globular form and thus became 

 the moon. 



Such is the view of the origin of the moon which is 

 suggested by tidal evolution. Similar deductions from the 

 theory of the tides will enable us to offer a forecast of the 

 future career of the earth-moon system. Let us transfer 

 our attention from the remote past and endeavour to think 

 of the distant future. So long as the solar system shall 



