June 30, 1899.] 



SCIENCE. 



899 



the gaseous condition throughout the stages 

 of its condensation. The hj'pothesis there- 

 fore rests upon exceedingly doubtful prem- 

 ises and upon exceedingl}' questionable 

 deductions from these doubtful premises. 



The fission hj'pothesis of George Darwin 

 has recently replaced it in favor, but the 

 above quotation implies that even its 

 founder does not now rest much confidence 

 in it. The objections to the theory are 

 several and grave. In the first place, 

 the theory of the fission of a celestial 

 body by high rotation, as worked out inde- 

 pendently by Darwin and Poincare, re- 

 quires that the separated bodies should not 

 be very greatly different in mass, i. e., the 

 smaller body should not be less than one- 

 third the mass of the larger. But the mass 

 of the moon is but -^^ of that of the earth, 

 and hence it lies far outside the computed 

 limits of applicability of the fission process. 



Another difficulty lies in the effect of 

 tidal strain itself. George Darwin, in his 

 recent work on 'The Tides' (p. 259), as- 

 signs 11,000 miles from the center of the 

 earth as Roche's limit. This leaves a tract 

 of 7,000 miles above the terrestrial surface 

 within which the earth's tidal force would 

 be so great as to tear the moon to fragments, 

 and, perhaps, scatter these into the form of 

 a ring. The rings of Satux-n are supposed 

 to illustrate this form of intense tidal ac- 

 tion. The escape of the moon, even pre- 

 suming it to have been separated from the 

 earth would, therefore, have been jeopard- 

 ized by its transformation into a meteoroidal 

 ring or swarm. If the fragments, after 

 having been torn apart, were still suf- 

 ficiently affected by a minute tide to be 

 carried away from the earth in a slow 

 spiral, the time occupied in passing out- 

 ward beyond Eoche's limit must have been 

 protracted ; and, after their escape from it 

 into a zone where conditions not hostile to 

 aggi-egation might, perhaps, have been af- 

 forded, there must probably have been 



another protracted period before the aggre- 

 gation of the moon would have been suf- 

 ficiently advanced to give it appreciable 

 tidal effect upon the earth. It remains, 

 therefore, to be determined, if this hy- 

 pothesis is followed, at what stage in the 

 evolution of the moon it was sufficiently 

 concentrated to assume effective tidal func- 

 tions. This is a question also applicable to 

 the aggregation of the moon under the 

 Laplacean hypothesis, if it be modified so 

 as to conform to the demands of modern 

 scientific probability. It also applies to 

 any hypothesis which postulates aggrega- 

 tion from a dispersed condition. In any 

 case, it seems necessary to determine when 

 the moon became full grown before it is 

 possible to assign a positive date for the 

 commencement of effective tidal action. It 

 would appear that such action might be de- 

 veloped gradually as the material of the 

 moon became aggregated. During such 

 gradual assumption of the tidal function 

 the reaction between the moon and the 

 earth must have been of a feeble sort, and a 

 recomputation of its amount based on a se- 

 ries of hypotheses which shall cover the 

 whole ground of legitimate speculation 

 would seem necessary before any satisfac- 

 tory conclusions can be reached. 



It may be, urged that the computations 

 of George Darwin following, in backward 

 steps, by the masterly application of mathe- 

 matical analysis, the stages of the earth- 

 moon relationship give a firmer ground for 

 conclusions. In a qualified degree this must 

 be conceded. But it is to be remarked, in 

 the first place, that the mathematics be- 

 comes indecisive before the origin of the 

 moon is reached, which may signify that 

 this is not the true line of approach to the 

 origin of the moon, or that there is some 

 error or defect in the assumptions. It 

 would seem to be obvious, however, that if 

 the tidal function was the result of a slow 

 aggregation which began at an indetermi- 



