;88 



NA TURE 



[February 22, 1906 



Moreover, the author on p. 183 says that London 

 coal gas has an illuminating value of 16 to 17 

 candles, and a calorific value of about 668 B.T.U.'s, 

 which is much more nearly true for the gas supplied 

 by the Gas Light and Coke Co. The error is of 

 importance, as an engineer working at the problem 

 of the gas-engine and consulting records of efficiency 

 made with London gas might be seriously misled. 



In dealing with water gas, no mention is made 

 of the more modern processes such as the " Dellwik," 

 now so largely used for the production of blue gas 

 for welding, as well as for diluting coal gas. 



It is admitted in the preface that the article on 

 practical photometry has not been brought up to date, 

 and this is a pity, as more than seven pages are 

 devoted to the Bunsen photometer and the manipula- 

 tion of candles, now practically extinct in all but 

 name as a standard of light, whilst a couple of pages 

 on pentane standards would have been of real value. 



In spite of a few blemishes, the whole work is so 

 good that no engineering chemist can afford to be 

 without it. 



Die Photographic im Hochgebirg. By Emil Terschak. 

 Second edition. Pp. xxiii + 62. (Berlin: Gustav 

 Schmidt, 1905.) Price 2.50 marks. 

 Everyone who is of a roving disposition, and takes 

 hi- camera to Switzerland or the Tyrol, or any other 

 region where mountain climbing is pursued, should, 

 if he wishes to gain by the experience of others, read 

 this book. It is written by a photographer to photo- 

 graphers, and is not only very interesting to read, but 

 contains a great amount of very useful photographic 

 information" of a particular kind. 



The successful photography of mountain scenery, 

 of ice, snow, and clouds at high altitudes requires 

 not onlv forethought, but much experience. As it is 

 necessary lo carry all the apparatus that is required, 

 the equipment must be well attended to, and since 

 also one does not necessarily wish to climb high 

 altitudes to take again a particular view that ha- not 

 turned out photographically successful, one must be 

 sure of securing a good negative at every exposure. 



The first edition of this book appeared in 1000, but 

 the author has since gained much more useful know- 

 ledge which he has embodied in the present edition. 

 The book is clearly printed in Roman characters on 

 good paper, and the illustrations are numerous and 

 well reproduced. 



The Roval Medical and Chirurgical Society of London. 

 Centenary 1805-1905. Written at the request of 

 the President and Council bv Dr. Norman Moore 

 and Stephen Paget. Pp. 337- (The Aberdeen 

 University Press, Ltd., 1905.) _ . 



Though not the oldest of the medical societies ot 

 I ondon the Roval Medical and Chirurgical Society 

 holds a position' second to none, and the presen! 

 volume of chronicles will not only be welcomed by 

 its Fellows as giving a history of their society, but 

 forms a useful record of the art and science of 

 medicine during the nineteenth century, with com- 

 ments bv the compilers. A noteworthy feature of the 

 volume is the list which is given for each year of the 

 principal papers read before the society, both pub- 

 lished and unpublished, extracts being given from the 

 more important ones. Thus, for the year 1833, we 

 find Hilton's unpublished account of Trichina spiralis 

 in human muscle, which ante-dated Paget's discovery 

 of this parasite. Short bibliographies of all the presi- 

 dents and a full index complete this interesting 

 volume, which contains several illustrations of the 

 various premises occupied by the society and a photo- 

 gravure frontispiece of William Saunders, the first 

 president. R- T. Hewlett. 



NO. 1895, VOL. 73] 



LETTERS TO THE EDITOR. 



[Tiu ■ Editor does not hold himself responsible for opinions 

 expressed bv liis correspondents. Neither can lie undertake 

 to return, or to correspond with the writers of. rejected 

 manuscripts intended fur this or any other part of NATURE. 

 No notice is token of anonymous communications.] 



Tidal Researches. 

 In Nature for January 11 (p. 24.8) appear some criticisms 

 upon my paper entitled " Cotidal Lines for the World." 



The critic says : — " The construction of these charts is, 

 unfortunately, but vagueh indicated." In reply to this 

 it may be said thai the charts embody all data known 

 to me at the time of their construction, and to such data 

 references as copious as space seemed to permit are given. 

 What is meant by cotidal lines is given in § 17. Notions 

 relating to the local modifications or peculiarities of 

 cotidal lines have been given in considerable detail by 

 means of lemmas and examples. In the construction of 

 these lines, large detailed charts showing soundings 

 wherever known were employed, and these depths were 

 carefully considered in each step of the process. The 

 ranges of tide written along the shore-lines simply re- 

 present data, and in no way depend upon any theory or 

 hypothesis. The same is essentially true of the cotidal 

 lines where observations or data are sufficient. Wherever 

 harmonic constants are available, the length of the series 

 analysed is of secondary importance in the construction of 

 cotidal lines, the results from two months being about as 

 in Factor} .1- those from twenty years. 

 If we are not permitted to extend cotidal lines outward 

 from the shore, we might about as well draw them upon 

 the land as upon the water, for in either ease they would 

 enh serve to point out the shore values. The reviewer 

 thinks well of Berghaus's chart, and so do I. However, it 

 is difficult to believe that a philosophical critic could long 

 rest content with cotidal lines extending but a short dis- 

 tant- off shore, and forming no connected or consistent 

 ,i,lnn. Of course, the attempt, on my part, at covering 

 .ill seas does not imply that all charts are equally good. 

 In some instances the data were very meagre, and atten- 

 tion was directed to this fact more than once in the paper. 



It seems strange that any serious misunderstanding could 

 exist in reference to the method employed in inferring the 

 times when the water particles are at elongation in par- 

 ticular directions. Does anybody doubt the conclusions 

 reached in § 56, part iv. A? If these conclusions are 

 wrong, let us hear the correct ones. It §J 60-65, 

 part iv. A, are not clear as they stand, it seems as if 

 § 24, part iv. B (to say nothing of a reply to former 

 criticism, Nature, April 2;, 1903), oughl i<> remove all 

 obscurity. 



Perhaps the following remarks may be of si, me service 

 in this connection : — 



Unless the free period of a body of water, or of some 

 portion of this body, approximately agrees with the period 

 of the tidal forces, the tide in the body proper must be 

 small, and generally smaller than the theoretical equil- 

 ibrium tide for the body in question. But in many parts 

 of the oceans the tide is several times greater than that 

 which could be raised by the forces, even if w e could 



.„|i| sufficient depths and sufficiently complete 



boundaries for enabling equilibrium tides to occur. Hence 

 regions the dimensions el which approach critical values 

 mus i rxisi in the oceans and account for the principal 

 tides If the aerial vibrations accompanying a musical 

 t one act upon a series oi resonators suited to various 

 pitches, the one or more constructed for the given tone 

 will respond to it. while all others will be practically- 

 silent; that is, tie dominant impressed motions belong to 

 resonators having critical dimensions, and not to the 

 resonators in general. 



That stationary oscillations of unexpectedly large ampli- 

 tude exist in the oceans there is abundant evidence. In 

 l. h 1 ■! glance at the charts under criticism will show 

 regions of large ranges over each of which the time of 

 tide varies but little. As a nodal line is approached the 

 range diminishes, and the time of tide changes rapidly in 

 a comparatively short distance. Moreover, the dimensions 

 of the oceans "are such that areas having nearly critical 



