234 THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 



Another basis upon which the address urges the limitation of the 

 earth's history is found in tidal friction. The limitations assigned on 

 this basis are not, however, very restrictive. The argument is closed 

 as follows: 



"Taking into account all uncertainties, whether in respect to Adams's 

 estimate of the ratio of frictional retardation of the earth's rotary 

 speed, or to the conditions as to the rigidity of the earth once consoli- 

 dated, we may safely conclude that the earth was certainly not solid 

 five thousand million years ago, and was probably not solid one thou- 

 sand million years ago." (P. 670). 



And in a footnote it is added: 



"It is probable that the date of consolidation is considerably more 

 recent than one thousand million years ago." 



The foundations of any argument involving the relations of the 

 moon to the earth are very infirm. In the first place, no hypothesis 

 respecting the moon's mode of origin, or of the time in the history of 

 the earth when it became aggregated and came into effective possession 

 of its tidal function, can claim even a remote approach to substantia- 

 tion. There is not only no substantiated theory of the origin of the 

 moon, but there can scarcely be said to be even a good working hypo- 

 thesis, for the radical reason that the hypotheses offered will not work. 

 George Darwin, who has probably studied the subject more assiduously 

 and more profoundly than any other investigator, ancient or recent, 

 strongly expresses the situation when he says, in his recent work on The 

 Tides (p. 360): 



"The origin and earliest history of the moon must always remain 

 highly speculative, and it seems fruitless to formulate exact theories 

 on the subject." 



The annular theory of Laplace encounters in their maximum intensity 

 the objections which arise from the application of the modern doctrine 

 of molecular velocities. The gravitative control of an attenuated ring 

 having the mass of the moon over its constituent material must have 

 been exceedingly low, while the high temperature necessary to sustain 

 the refractory material of the moon in a gaseous condition must have 

 rendered the molecular velocities very high, so that no material except 

 that of very high atomic weight and consequent low molecular velocity 

 could be presumed to have been retained. But the specific gravity of 

 the moon (3.4) seems a fatal objection to the assumption that it is com- 

 posed wholly of material of very high atomic weight. Besides, it is 

 diflBcult to understand how the high temperature of a ring of such 

 attenuation could have been maintained during the time necessary for 

 its concentration. This was less difficult when it was assumed, as for- 

 merly, that the temperature of the sun at that time was excessively 

 high, as was also that of the earth. But modern inquiry seems decid- 



