NA TURE 



[January 5, 1899 



mathematical argument is, after all, only organised 

 common sense," is indisputable, but so far from making 

 the task undertaken in any way easier, it really em- 

 phasises the enormous difficulty. Hut Prof. Darwin 

 has avowedly taken pains to render an intricate subject 

 intelligible, and it will probably be generally agreed 

 that he has achieved an unqualified success. 



In publishing in book form the lectures which were 

 delivered last year at the Lowell Institute, Boston, 

 Massachusetts, the author has been distinctly well- 

 advised to recast their form, and to eliminate all traces 

 of the lecture-room. It is surely evident that there is 

 one style appropriate to the platform and another to the 

 essay, and that the two are of necessity mutually in- 

 compatible. It appears very unfortunate that the habit 

 of publishing lectures in the form in which they have 

 been delivered is becoming so prevalent. The result is 

 that a little trouble is saved on the part of the author, 

 always more or less to the detriment of his work. Is it 

 easy, for example, to imagine a more irritating book 

 than Tyndall's " Sound " ? No doubt that is an extreme 

 case, but the personal form of address is always 

 objectionable to the reader, and ought to be eliminated. 



As soon as one begins to e.\amine the book in detail, 

 one is struck by the excellence of Prof. Darwin's judg- 

 ment in the choice and arrangement of his subject- 

 matter. A liberal and comprehensive interpretation is 

 placed upon the scope of tidal and kindred phenomena, 

 so that a wide field of recent investigation is surveyed. 

 But cognate branches and developments are always 

 displayed in due order and significance of relation, and 

 digressions, as they may appear to be at first sight, will 

 be found in reality to be in perfect harmony with a 

 continuous purpose. 



The book begins where physical inquiry ought always 

 to begin, in methods of observation : this course has the 

 further advantage that the reader is not dismayed by 

 difficulties at the outset. The construction and use of 

 gauges for recording marine tides having been explained, 

 the study of the changes of level in lakes is introduced 

 in the second chapter. These Seiches, as they are 

 called, constitute a distinct and exceedingly interesting 

 phenomenon, which has not hitherto received the recog- 

 nition it merits. Most readers, we fancy, will find much 

 that is novel in this account. Dr. Forel's work on the 

 subject was begun about a quarter of a century ago, but 

 his researches, carried out on Lake Geneva with remark- 

 able skill, have only been imitated elsewhere within the 

 last year or two. There can be no doubt that highly 

 important results will follow from the systematic ap- 

 plication of Forel's methods which has been begun on 

 Lake George, in New South Wales, and on the great 

 lakes of North America. Not only will an appreciative 

 description of Dr. rorel's work with those instruments of 

 his own invention, the Plemyrameterand the Limnimeter, 

 be found in this chapter, but also an excellent account of 

 Mr. F. Napier Denison's application to this case of 

 Helmholtz' theory of the waves generated at the surface 

 of separation of two layers of fluid in relative motion. 

 In dealing with the peculiar behaviour of the waters in 

 tidal rivers. Captain Moore's work in observing the "bore" 

 on the Tsieng-Tang-Kieng is described, and illustrated 

 by reproductions from photographs. 

 NO. 1523. VOL. 59] 



.•\t this point Prof. Darwin introduces an historical 

 sketch, in composing which he has had the happy idea of 

 levying contributions from his colleagues at Cambridge 

 in the form of extracts from the early philosophical and 

 mythological writers of such nationalities as Chinese 

 and .Arabic. Only after so much by way of introduction 

 does he attack the mechanical theory. .Admirably lucid 

 chapters deal with the statics of the tide generating force 

 and the deflection of the vertical. This provides an 

 opportunity of describing his own researches with the 

 bifilar pendulum on lunar gravity, of which this popular 

 account is most welcome. This in turn leads to a short 

 discussion of those seisniological problems which are 

 now attracting wide attention. The distortion of the 

 earth's surface is also discussed as a disturbing factor in 

 the problem of the direct measurement of the tidal force. 

 The famous equilibrium theory of the tides is next 

 examined, its value being insisted on as a statement of 

 the statical conditions of the problem. But Prof. Darwin, 

 unlike most of his predecessors among the popular 

 writers on the subject, does not stop at this unsatisfactory 

 stage, but goes on to consider the hydrodynamics of the 

 tide-wave Prof Darwin's treatment of free and forced 

 waves in canals in different latitudes, and of tides in 

 lakes and land-locked seas, leaves no ground for criti- 

 cism, although it is impossible but that the unmathe- 

 matical reader will find great difficulty in following the 

 reasoning which the mathematician apprehends through 

 the medium of differential equations. .Any one who has 

 fairly well mastered the foregoing chapters should have 

 little difficulty in understanding the one in which Prof. 

 Darwin expounds the great modern method of harmonic 

 analysis, though here again it is scarcely possible for any 

 but the mathematician to realise that this powerful theory 

 leads to a unique solution of the problem, or, in other 

 words, that it has any sounder foundation than juggling 

 empiricism. A clear insight ought to be gained into the 

 method by which tidal observations are reduced and 

 made to provide the raw material for the tide-predicting 

 machine. The practicability of tide prediction having 

 been realised, it only remains to discuss the degree of 

 accuracy which has been attained. This naturally leads 

 to a consideration of the discrepancies, and the effects of 

 wind and barometric pressure and the variation of 

 latitude are discussed as disturbing factors. Thus the 

 end is reached of that section of the book which deals 

 with the subject in its more direct aspect. 



It is impossible here to give any detailed account of 

 the remaining contents of the book, although they are of 

 absorbing interest and eminently c.Tpable of popular 

 treatment. In masterly chapters Prof Darwin e.xamines 

 the effects of tidal friction, and discusses those particular 

 cases in which they seem to have been most clearly 

 manifested. The chapter on the possible equilibrium 

 figures of a rotating mass of liquid is a good example of 

 the author's judgment in the arrangement of his materi.il, 

 beginning as it does with the more easily understood 

 experiments of Plateau, and leading up to Poincare's 

 remarkable work. The Nebular Hypothesis as giver* 

 here in outline by Prof Darwin does not strain the limits 

 of conceivability as it commonly does when expounded 

 by less able writers, but is brought well within the bounds 

 of rational probability. .An excellent summary of those 



