244 



NA TURE 



{July 12, 1888 



A chapter on the motion of two spheres indicates the 

 attention given to this problem of late years, and may also 

 serve as a warning of its hopelessness. The anticipations 

 of its yielding an explanation of magnetic phenomena, to 

 which the first experiments by Bjerknes gave rise, have 

 been dissipated by the exhaustive mathematical treatment 

 it has received. 



The excellence of this work leads us to look forward 

 with great interest to the publication of the second volume, 

 which will deal with fresher and more suggestive portions 

 of the subject ; and the two volumes together will prove 

 of very great use to every student. The words on the 

 title-page, " with numerous examples," strike us as below 

 the dignity of a subject like hydrodynamics. The book 

 will certainly be appreciated for its own merits even more 

 than for its examination usefulness, to which aim too 

 many books conform. 



OUR BOOK SHELF. 



Sierra Leone ; or, the White Man's Grave. By G. A. 



Lethbridge Banbury. (London : Swan Sonnenschein, 



Lowrey, and Co., 1888.) , 



The author of this book explains that he does not offer it 

 as '' one of travel over unknown ground " or " as one of 

 dangerous adventures and hardships." His aim simply 

 is to bring before his readers a description of an English- 

 man's life in " the most interesting but deadly colony of 

 Sierra Leone." He has done his work well, and the book 

 will be cordially welcomed by all who have any special 

 reason for wishing to obtain clear and accurate informa- 

 tion about this part of the West African coast. The 

 volume consists chiefly of letters written while Mr. Ban- 

 bury was at Sierra Leone, and has therefore a freshness 

 and vividness which it would have been hard for him to 

 match in a more elaborate and formal work. The most 

 valuable chapters are those in which he sets forth the 

 impressions produced upon him by the natives, in whose 

 ideas and customs, as here depicted, there is an odd 

 mixture of Christianity and the lowest forms of paganism. 

 Mr. Banbury has a strong belief in the power of education 

 to improve the character of the native population, and he 

 urges that more strenuous efforts should be made for the 

 establishment of proper schools. It is tolerably certain 

 that if permanent good cannot be done to the colony by 

 this means there is no other way in which real progress 

 can be secured, for, as Mr. Banbury points out, the un- 

 healthiness of the climate prevents any large increase of 

 the number of European settlers. 



Nature's Fairy-Land : Rambles by Woodland, Meadow, 

 Stream, and Shore. By H. W. S. Worsley-Benison. 

 (London : Elliot Stock, 1888.) 

 This book consists of a series of papers selected from a 

 considerable number which have appeared in various 

 periodicals. The author has a clear, pleasant style, and 

 his vivid descriptions and explanations are well adapted to 

 awaken in the minds of young readers a genuine interest 

 in various aspects of scientific truth. The volume opens 

 with an attractive paper on " The Journeyings of the Rain 

 Drops," and this is followed by papers entitled " From 

 Root to Flower," " Out Among the Gorse," and " Com- 

 panions of the Corn." These three papers serve as an 

 introduction to other chapters on plant-life. There are 

 also interesting essays on such subjects as shells and 

 shell-builders, spiders, and the nests of fishes. 



Lessons in Elementary Mechanics. By W. H. Grieve, 



P.S.A. (London: Longmans, Green and Co., 1888.) 

 The second stage of mechanics is alone dealt with here, 

 and throughout, the author has rendered the various forces 



which produce motion, together with the laws which 

 regulate those forces, in a clear and simp'e style ; the 

 illustrations are numerous, and are specially adapted to 

 an elementary course. The work is suited to the require- 

 ments of the second stage of the revised code, and the 

 arrangement of the chapters is the same as that in the 

 Syllabus of Instruction adopted by the London School 

 Board. The examples at the end of each chapter are 

 instructive and well chosen, and the book concludes with 

 a series of examination papers and results to the 

 numerical questions. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 take to return, or to correspond with the writers of, 

 rejected manuscripts intended for this or any other part 

 of Nature. No notice is taken of anonymous communi- 

 cations. .] 



Photography of Lightning. 



So much interest is being taken at the present time in obtain- 

 ing photographs of lightning flashes, that perhaps some one 

 would be willing to take the necessary trouble, and use a 

 moving camera. A camera revolving or vibrating at any 

 ordinary pace would probably give each single flash unaltered, 

 but it might analyse multiple and complex flashes into their 

 constituents. 



The eye is so easily deceived as to what is really happening in 

 these sudden effects that very erroneous views may easily be 

 formed, and indeed are in some quarters now prevalent. 



Whether it is better to make the camera revolve as a whole, 

 or only the sensitive plate, or whether a revolving mirror should 

 be used with a stationary camera, are questions for experience 

 to decide. 



One good method, if not too troublesome in practice, would 

 be to arrange a double camera, with component axes parallel, so 

 as to photograph the same flash in both halves, but with the 

 sensitive plate of one fixed, of the other rapidly revolving. 

 Appearances really due to succession in time could be then easily 

 distinguished, and might be capable of interpretation. 



July 10. Oliver J. Lodge. 



Micromillimetre. 



I AM glad that the Council and Fellows of the Royal Micro- 

 scopical Society have seen their way to the adoption of the word 

 micron, but the letter in which Mr. Crisp announces this decision 

 to you is not, I think, calculated to give a correct impression of 

 the circumstances under which it was taken. 



Firstly, I need hardly say that I did not take exception to 

 the word micromillimetre, but to its use as equivalent to the 

 thousandth of a millimetre. 



In the next place, I wrote to the Secretaries of the Royal 

 Microscopical Society on behalf of the Council of the Physical 

 Society, of which no mention is made by Mr. Crisp. 



Thirdly, the proposal of the Council of the Physical Society was 

 that the word microti should be adopted. 



Lastly, I am myself quite in favour of this course, and in fact 

 moved its adoption by the Physical Society. 



The word micrometre must in accordance with the rules of the 

 B.A. Committee be a possible alternative just as a cubic decimetre 

 is alternative to a litre, and I think the disadvantage of the 

 multiplication of special names not based on a uniform system is 

 nearly as great as that of the possible occasional confusion 

 between micrometre and micrometer. This is however a little 

 matter as compared with the use of micromillimetre in two 

 different senses, and the official sanction of micron by the 

 French authorities (of which I was not aware when I originally 

 wrote to you) is quite sufficient to turn the scale in its favour. 



As some of your readers may not have seen the previous 

 correspondence, will you allow me in conclusion to state that it 

 is now generally agreed, 



(1) That the thousandth of a millimetre shall be called a 

 micron and denoted by /x. 



(2) That the millionth of a millimetre shall be called the 

 micromillimetre and denoted by fx. yu. 



Arthur W. Rucker. 



