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SCIENCE 



N. S. Vol. XXX. No. 763 



late years, and nowhere more so than in the 

 attempts to trace back astronomical systems to 

 an earlier state on the theory of a general evo- 

 lution of the arrangement of matter. One can 

 not read Sir George Darwin's work in this 

 line without being impressed with the caution 

 which he continually exercises and wishes to 

 impress on the reader for any results, espe- 

 cially those which are numerical, that he may 

 print. It is interesting to learn his own 

 opinion of the outcome of the controversies 

 which have sprung from the theories set forth 

 years ago in his papers, and I can not do better 

 than to quote his estimate as given in the 

 preface to Volume II. In referring to the 

 papers on Tidal Friction and Cosmogony, 

 which fill this volume, he says : 



For tlie astronomer who is interested in cos- 

 mogony the important point is the degree of 

 applicability of the theory as a whole to celestial 

 evolution. To me it seems that the theory has 

 rather gained than lost in the esteem of men of 

 science during the last twenty-five years, and I 

 observe that several writers are disposed to accept 

 it as an established acquisition to our knowledge 

 of cosmogony. 



Undue weight has sometimes been laid on the 

 exact numerical values assigned for defining the 

 primitive configuration of the earth and moon. 

 In so speculative a matter close accuracy is un- 

 attainable, for a different theory of frlctionally 

 retarded tides would inevitably lead to a slight 

 difference in the conclusion; moreover, such a real 

 cause as the secular increase in the masses of the 

 earth and moon through the accumulation of 

 meteoric dust, and possibly other causes are left 

 out of consideration. 



The exact nature of the process by which the 

 moon was detached from the earth must remain 

 even more speculative. I suggested that the fis- 

 sion of the primitive planet may have been 

 brought about by the synchronism of the solar 

 tide with the period of the fundamental free oscil- 

 lation of the planet, and the suggestion has re- 

 ceived a degree of attention which I never antici- 

 pated. It may be that we shall never attain to a 

 higher degree of certainty in these obscure ques- 

 tions than we now possess, but I would maintain 

 that we may now hold with confidence that the 

 moon originated by a process of fission from the 

 primitive planet, that at first she revolved in an 

 orbit close to the present surface of the earth, and 



that tidal friction has been the principal agent 

 which transformed the system to its present con- 

 hguration. 



After some remarks on the diflSculties in the 

 way of the acceptance of the theory due to the 

 time element and the probable date of solidifi- 

 cation he refers to the arguments of Lord 

 Kelvin based on temperature gradients and 

 points out how the possible effects of radio- 

 activity and the " colossal internal energy 

 resident in the atom " may tend to nullify any 

 estimates based on temperature alone. Sum- 

 ming up, he adds : 



It is very improbable that tidal friction has 

 been the dominant cause of change in any of the 

 other planetary sub-systems or in the solar system 

 itself. Yet it seems to throw light on the dis- 

 tribution of the satellites amongst the several 

 planets. It explains the identity of the rotation 

 of the moon with her orbital motion, as was long 

 ago pointed out by Kant and Laplace, and it tends 

 to confirm the correctness of the observations 

 according to which Venus always presents the 

 same face to the sun. 



The arrangemept adopted for the order of 

 the papers is not entirely chronological. Pro- 

 fessor Darwin remarks that in the case of his 

 own work, his papers fall into a few well-de- 

 fined groups and that this fact furnished him 

 with the opportunity to place together those 

 papers which deal with particular subjects. 

 As a result of this arrangement the first part 

 of Volume I. consists of what is practically a 

 treatise on the tides and tidal prediction, the 

 second part containing two papers on the 

 lunar disturbance of gravity, while the whole 

 of Volume II. is devoted, as stated above, to 

 tidal friction and cosmogony. Appended to 

 each volume is a chronological list of all his 

 papers with references to the volumes in 

 which they are or will be contained. 



The collection will thus be something more 

 than a reprint. In general the reproduction 

 is literal, but here and there are notes pointing 

 out an occasional error or giving a reference 

 to later work by himself or others which has 

 tended to modify the earlier conclusions or to 

 fill gaps which had existed. One sometimes 

 wishes that these notes had been more fre- 

 quent so as to include references and perhaps 



