898 



SCIENCE. 



[N. S. Vol. IX. No. 235, 



But I think I do not err in assuming that 

 mathematical computations, so far as they 

 can approach a solution of the exceedingly 

 complex problem, are at least quite as fa- 

 vorable to a slow as to a rapid aggregation. 

 If this be so, the problem of internal tem- 

 perature must be attacked on the lines of 

 this hypothesis as well as those of the com- 

 mon hypothesis before any safe conclusion 

 can be drawn from it respecting the age of 

 the earth. 



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 un- 

 certainties, whether in respect to Adams' 

 estimate of the ratio of frictional retarda- 

 tion of the earth's i-otary speed, or to the 

 conditions as to the rigidity of the earth 

 once consolidated, we may safely conclude 

 that the earth was certainly not solid 5,000 

 million years ago, and was probably not solid 

 1,000 million years ago " (p. 670) and in a 

 foot-note it is added : " It is probable that 

 the date of consolidation is considerably 

 more recent than 1,000 million years ago." 



The foundations of any argument involv- 

 ing 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 sub- 

 stantiation. There is not only no substan- 

 tiated theory of the origin of the moon, 

 but there can scarcely be said to be even a 

 good working hypothesis, for the radical 

 reason that the hypotheses offered will not 

 viorh. George Darwin, who has probably 

 studied the subject more assiduously and 

 more profoundly than any other investi- 

 gator, 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 in- 

 tensity the objections which arise from the 

 application of the modern doctrine of 

 molecular velocities. The gravitative con- 

 trol 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 ex- 

 cept 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 

 composed wholly of material of very high 

 atomic weight. Besides, it is difficult 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 dilEcult 

 when it was assumed, as formerly, that the 

 temperature of the sun at that time was 

 excessively high, as was also that of the 

 earth. But modern inquiry seems decidedly 

 opposed to the assumption of excessivel}' 

 high temperatures at that stage. On the 

 contrary, it has recently been urged from 

 diiferent quarters that the early tempera- 

 ture of the sun's surface must have been 

 much lower than at present, and this is also 

 implied in certain statements of the address 

 (p. 711, Sec. 43). There are also grounds 

 for grave question as to the high tempera- 

 ture of the earth, as has already been indi- 

 cated. Under the revised forms of the 

 nebular hypothesis there seems no sub- 

 stantial reason for supposing that if the 

 matter of the moon was once distributed in 

 a ring about the earth, it could maintain 



