NA TURE 



THURSDAY. NOVEMBER 9, 191 1. 



TIDAL ACTION AND COSMOGONY. 



The Tides and Kindred Phenomena in the Solar 

 System. The substance of lectures delivered in 1897 

 at the Lowell Institute, Boston, Massachusetts, by 

 Sir G. H. Darwin, K.C.B. Third edition. Pp. 

 xxiv + 437. (London: John Murray, 1911.) Price 

 •js. 6d. net. 



THE first edition of this book appeared in 1898, 

 and was reviewed in Nature, vol. lix., p. 219. 

 The work has since taken rank as a classic in its way, 

 and has been translated into several languages. There 

 is no need, therefore, to dwell at any length on its 

 general character and purpose. It aims at giving, 

 with as few technicalities as possible, a summary of 

 the researches on tidal theory, and more especially on 

 the theory of tidal evolution, which have been the 

 principal occupation of the author's long scientific 

 career. It would be misleading to describe it merely 

 as a "popular" book, for it is a valuable guide even 

 to experts who might otherwise be dismayed by the 

 long and intricate calculations in which the original 

 investigations necessarily abound. In this respect it 

 may perhaps be compared with the celebrated " Exposi- 

 tion du Syst^me du Monde " which Laplace added as a 

 supplement to the detailed work of the " Mecanique 

 Ci'leste." It is true that the subject-matter is in the 

 present case more speculative, but the purpose is the 

 same, and the execution not unworthy of the great 

 exemplar. 



In this third edition some additions and alterations 

 have, however, been made which call for notice. 

 Tfiese consist in part of supplementary notes to the 



rlier chapters, in which brief summaries are given 

 oi recent work bearing on the various topics. Thus 

 at the end of the first chapter there is an interesting 

 account of deep-sea tide gauges, with a tantalising 

 indication of the valuable results which might be 

 obtained if an extended and systematic use of such 

 appliances were practicable. The appreciative account 

 of Forel's early investigations of the "seiches" of the 

 Lake of Geneva is appropriately supplemented by a 

 reference to Chrystal's recent work on the Scottish 

 lochs. The remarkable observations of Dr. Hecker 

 on the lunar disturbance of gravity, which are the 

 successful realisation of an interprise in which Sir G. 

 Darwin himself led the way, are described with 

 generous enthusiasm. In the absence of any authori- 

 tative account in English of these experiments, it is 

 perhaps to be regretted that the description is not still 

 more ample. Their value consists, of course, in the 

 inferences which they justify as to the degree of 

 rigidity of the earth. The other lines of evidence 

 bearing on the same question, viz. those based on 

 observations of the fortnightly tide, and on the free 

 (Eulerian) nutation of the earth's axis, are also ex- 

 plained and discussed, the general conclusion being 

 that the earth's surface yields, under the action of 

 tidal or precessional force, about two-fifths as much 

 as if it were absolutely devoid of rigidity. The more 

 n'ccnt work of Love and Larmor, in which a greater 

 NO. 2193, VOL. 881 



degree of precision is given to the inferences which 

 can be drawn from the respective lines of argument, 

 was probably judged to be of too refined a character 

 to admit of elementary treatment. 



The views of the author as to the age of the earth, 

 and the scale of geological time, have, as explained 

 in this edition, undergone considerable modification. 

 In 1898 the arguments of Lord Kelvin, which assigned 

 a comparatively moderate age to the solid earth, were 

 in the ascendant; but the rapid and startling succes- 

 sion of discoveries in radio-activity have undermined 

 much of the reasoning based on such considerations 

 as the output of energy from the sun and the secular 

 cooling of the earth. Moreover, the researches of 

 Strutt claim positively to demonstrate an enormous 

 antiquity for certain rocks, far transcending the limits 

 formerly accepted by physicists. On the other hand, 

 an altogether independent estimate by Joly, based on 

 a comparison of the amount of sodium in the ocean 

 with the amount annually carried into it by rivers, 

 would give to the existing ocean an age of about ion 

 million years at most, a figure which is on the scale 

 of Kelvin's results. The discrepancy is at present 

 unexplained, but we may note an interesting essay 

 on the subject by Joly, in a recent number of the 

 Phil. Mag. (September, 191 1). Sir G. Darwin in- 

 clines, on a review of the whole evidence, to the 

 belief that there is no assignable limit to the fund 

 of time on which biologists may draw for the purpose 

 of their evolutionary theories. 



The last three chapters of the book are almost 

 altogether new. The first of these is devoted to a 

 general explanation of the theory of " figures of equili- 

 brium " of rotating fluid, a classical problem with 

 which the names of Maclaurin, Jacobi, Roche, Poin- 

 car6, and Darwin will always be associated. The 

 theory of the stability of the various forms is of too 

 technical a character to admit of really elementary 

 treatment, but the summary will be welcome even to 

 mathematicians, as a survey of a somewhat intricate 

 subject. The succeeding chapter, one of the most 

 fascinating in the book, gives a striking application of 

 the theory to the phenomena of variable stars. The 

 evolution of binary stars, a very attractive subject, is 

 also considered, but the intricacy of such speculations 

 is exemplified by the fact that a new source of in- 

 stability, known as "gravitational instability," comes 

 into play in the case of a gaseous mass. The conclud- 

 ing chapter treats of the nebular hypothesis of Laplace, 

 and of the more important modifications of it which 

 have been proposed by subsequent writers. In par- 

 ticular the " planetesimal " hypothesis of Chamberlin 

 and Moulton, which was suggested by the observed 

 prevalent spiral configuration of many nebulae, is dis- 

 cussed at some length. Some speculations have an 

 invincible attraction for the mind, but the critical 

 reader will have an uneasy feeling that here at any 

 rate he is making doubtful ventures into a mysterious 

 region, far away from any secure dynamical base. 



It would be wrong to end this notice without a 

 word of admiration for the modesty and restraint 

 which the author has shown in setting forth the 

 results of speculations in the origin and development 

 of which he has himself borne a predominant part. 



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