August 1, 1896.] 



KNOWLEDGE. 



181 



BOOKS RECEIVED. 

 The story of Electrieitt/. By J. Munro. (Xewnes.) Illustrated. 



l9. 



Observations de V Eclipse Tot ale du Soleil dii 16 Avril, 1S93. Par 

 M. H. Deslaiidres. (Paris: Gautliier-Villars et Fils.) 



Modern Optical Instruments. By Henry Orford. (Whittaker 

 &, Co.) lUustrated. 



Thiifls Xeic and Old. Book VII. By H. O. Arnold-Forster. 

 (Casseli.) Illustrated. 



Results of Meteorological, Magnetical, arid Solar Ohservafiom at 

 Stoni/hurst College. Bv Rev. \V. Sidgreares, S.J., F.K.A.S. 



The Old Light and the Xew. Bv Wm. Ackroyd, F.I.C. (Chapman 

 & Hall.) Illustrated. Is. 6d. 



Phfisicsfor Students of Medicine. By Alfred Daniell,M.A.,LL.B., 

 D.Sc. (Macmillan.) 4s. 6d. 



The Law of the Sgmmetri/ of Composite Flowers, By W. W. 

 Strickland. (Ualton : E. S. Smitlison.) Illustrated. 



Text-Book of Zoologi/. By Dr. J. E. V. Boas. Translated by 

 J. W. Kirkaldy and E. C. Pollard, B.Sc. (.Sampson Low, Marston.) 

 Illustrated. 



A Geographical Kistorii of MammaU. By R. Lydekker, B.A., 

 E.R.S., etc. (Cambridge University Press.) Illustrated. lOs. 6d. 



The Evolution of Bird -Song. By Charles A. Witchell. (A. & C. 

 Black.) 5s. 



A Concise Handbook of British Birds. By H. K. Swann. 

 (Wheldon & Co.) 3s. 6d. 



Bheumatism. Bv T. J. Maelagan, M.D. Second Edition. 

 (A &C. Black.) lOs. 6d. 



Vo.Tometric Revelation. Written and compiled by J. Abner for 

 the Author, A. A. Xorth. (Authors' and Printers' Joint Interest 

 Publishing Co.) Illustrated. 



Plants of Manitoba. A Series of Chromolithographs. (Marcus 

 Ward.) lUs. 6d. 



The Biological Problem of To-day. By Prof. Dr. Oscar Hertwig. 

 Translated by P. C. Mitchell, M.A. '(Heiiiemann.) 



The Universal Lair of the Affinities of Atoms. Bv J. H. Loader. 

 (Chapman & Hall.> 2s. 6d. 



Waiiside and Jl'oodland Blossoms. Second Series. Bv Edward 

 Step, F.L.S. (Wame.) Illustrated. 7s. 6d. 



irhat it Costs to be Vaccinated. By Joseph Collinson. (William 

 Reeves.) 



Practical Sadiography. By H. Snowden Ward, F.R.P.S. 

 (Dawbum & Ward.) Illustrated. Is. 6d. 



Uttttrs. 



[The Editors do not hold themselves responsible for the opinions or 



statements of correspondents.] 



■ 



TIDAL WAVES AXD THEIR CAUSE. 

 To the Editors of Knowledge. 



Sirs, — I have long been of the opinion, so well expressed 

 by Mr. Woblwill in the .July Number of Knowledge, that 

 the usual explanation given of the coincident occurrence 

 of high water at the antipodes is unsatisfactory. 



Could not a theory of the tides be formulated more in 

 accordance with the facts and known dynamical laws ? In 

 the received theory our earth's solid nucleus is supposed 

 to be completely submerged beneath an exaggerated ideal 

 ocean, in which a tidal wave is supposed to follow con- 

 tinuously in the walie of the moon, without any interruption 

 whatever, this again being followed by another tidal wave 

 in the same hypothetical universal ocean at the antipodes, 

 the two waves always occupying the same relative position 

 to each other and to the moon, and consequently com- 

 pletely and continuously circumnavigating the globe as 

 our satellite apparently does from east to west. 



But what are the actual facts ? The terrestrial waters 

 only partially cover the solid earth, and are irregularly 

 divided by the two principal masses of the dry land — the 

 continents of the Old and New Worlds — which extend 

 north and south of the Equator almost to the l'ole3, so as 

 to effectually act as two great barriers to the continuous 

 movement of any tidal wave from east to west in the wake 

 of the moon. The general effect of this alternate exposure 

 of continent and ocean to lunar influence is that vibratory 



motion of the latter characteristic of the tides, each 

 antipodal oceanic basin oscillating backwards and forwards 

 twice during the time that the moon makes a single un- 

 interrupted apparent revolution roimd the earth. The 

 real tide caused by the moon is the primary wave that 

 immediately follows that body. This wave, being unable 

 to follow the moon in her entire circuit, ebbs and again 

 flows back upon the terrestrial barriers before-named, 

 which stretch north and south of the Equator, as a 

 secondary tide, whilst the moon pursues her course beneath 

 the horizon. It is easy to see that a primary wave in the 

 Atlantic will flow in the same direction as a secondary 

 wave in the Pacific, and vice rer^.l, so that we may some- 

 times get a simultaneous production of high water at the 

 antipodes as the efiect of the general arrangement of the 

 great continents and oceans. 



In a brief letter like this it is, of course, only possible to 

 deal with the problem in a very crude and imperfect 

 manner. But I am certain that if the geographical 

 relations of land and water were to be taken into con- 

 sideration instead of being totaUy ignored, a much more 

 correct theory of the mechanism of the tides than the 

 one at present accepted could be constructed upon sound 

 dynamical principles. 



Halifax. H. .\. Cookson. 



P.S.— Sir R. Ball, in his work on " Tune and Tide," 

 tells us that it is still " a moot point whether high water 

 or low water should be represented beneath the moon, 

 supposing the ocean to be vibrating with ideal tides" — 

 which is equivalent to admitting that the theory is very 

 far from being a complete solution of the problem. Mr. 

 Proctor goes even further, and says that " the theory of the 

 tides remains yet to be satisfactorily established ; '" and 

 ' that "although the received theory explains the statical 

 equilibrium of a tidal wave, the dynamical conditions of 

 the problem cannot be thus explained." 



►.♦H 



To the Editors of Knowledge. 



Sirs, — I cannot altogether follow Mr. Wohlwill's argu- 

 ment in the July Number, but I agree with him that 

 Mr. Cornish is inaccurate. 



The usual popular explanation that has been handed on 

 from book to book for several generations is to the effect 

 that the moon draws up the water into a heap on the side 

 of the earth nearest to itself, while it pulls the earth away 

 from under the water on the remote side of the earth. 



On this hypothesis the sheer direct liftin-i power of the 

 moon causes the tides to arise, and their position woidd be 

 such that the two opposite high tides would be always in a 

 line with the moon if there were no land to interfere with 

 the free motion of the water. Mr. Cornish seems to have 

 accepted this theory almost in its entirety. 



Now, from the mathematical theory of the tides (.«•(• 

 Airey, Brinkley, Abbot, etc.) we learn that they are pro- 

 duced solely by the taniifntial action of the moon, /.<•,, 

 tangentialiy to the earth. This consists in an acceleration 

 (or retardation) of the rotation of the watery shell of the 

 earth acting through alternate quadrants, as the earth, 

 with its oceans, rotates under the moon. Farther, it is 

 demonstrated that theoretically (/.-■., if there were no 

 obstructing continents) /<>"• imt' r would be found almost 

 directly in a line with the moon at any given moment. 



The whole subject of tide-production is an abstruse one, 

 requiring the higher mathematics for its investigation, 

 and is really incapable, apparently, of being described in 

 homely language. But this does not warrant the use of 

 simple language, if it is to be done at the cost of sheer 

 misstatements of scientific truths. 



