204 FROM NEBULA TO NEBULA 



least theoretically ought to be longer) than it was a mil- 

 lenium ago, there was probably a time millions of years 

 ago when the earth rotated on her axis much more swiftly 

 than she does now. Granting this supposition, there is 

 no reason why we may not suppose, further, that she even 

 rotated in from three to four hours ; at least there is no 

 one in a position to disprove it. However, this supposi- 

 tion is orthodoxically permissible ; for, according to La- 

 place, the earth-moon system was originally a spheroidal 

 nebula rotating on its axis, and, by common acceptation, 

 it rotated faster and faster in the course of its shrinking. 

 Suppose, then, that when the nebular earth had shrunk to 

 a girth not much larger than it has at present, it had al- 

 ready formed a weak crust and, incidentally, was rotating 

 in the brief period mentioned. In that case it is possible 

 that the earth may have flung off the moon with sufficient 

 violence to detach her permanently from the planet and 

 make a satellite of her. It is not necessary to stipulate 

 just how far away the moon was thus hurled, but for the 

 sake of illustration we may suppose she was flung to a 

 height of some 4,000 miles, and we may further assume 

 that both parent and offspring were in the molten, or a 

 quasi-molten, state. Under these circumstances the two 

 bodies would doubtless raise great tides upon each other's 

 surfaces and these protuberances would, by their eccen- 

 tric attractions upon each other, retard the rotational ve- 

 locities of their respective globes. This would bring in- 

 to play the mysterious law of conservation of moment of 

 momentum, and the moon would consequently go out 

 further and further by way of compensating for her 

 slowly expiring axial rotating energy, until, finally, that 

 rotation would become reduced to a minimum and the 

 satellite would turn the same face constantly earthward 

 as it does now. 



Careful computations have demonstrated that on ac- 

 count of the relatively small mass of the moon, her tem- 

 perature under the impact theory could nothave exceeded, 

 at the very beginning, more than Farenheit. This 

 finding is plainly incompatible with the observed volcanic 

 rugosity of her surface. It is also to be noted that where- 



