April 27, 1899] 



NA TURE 



605 



Atmosphere," "Glaciers," "The Work of Underground 

 Water," "The Soil," and "The Rocks and their Order." 

 Closer examination, however, shows that these headings 

 are texts for exposition rather than descriptive titles, and 

 a great variety of unexpected information is brought out 

 in the course of the discussions, which are not cramped 

 by an undue eftbrt after conciseness of expression. Thus 

 the Stellar Realm includes a description of the solar 

 system, in which, incidentally, the fact that a planet 

 always keeps the same face turned towards the body 

 around which it revolves, is implied to prove that it does 

 not rotate on its axis. The chapter on the Atmosphere 

 embraces almost as much as the atmosphere itself: not 

 only a little on climate and a great deal on storms, but 

 all that is said about the oceans, tides, lakes and rivers 

 is included under this head ; yet glaciers have a chapter 

 to themselves. The greater part of the " Work of Under- 

 ground Water" is devoted to the phenomena of vol- 

 canoes, while earthquakes are dealt with apart under 

 " The Rocks and their Order," a chapter which also 

 includes a section on the moon. 



. The whole is written in a thoroughly interesting style, 

 like all Prof Shaler's work, and the frequent references 

 to his own researches give it an element of personal 

 attractiveness. He mentions that, in spite of his long 

 study of swamps, he never saw the phenomenon known 

 as Will-o'-the-\Visp, and is inclined to disbelieve in its 

 existence ; the reports of a moving light being, in his 

 opinion, due to subjective impressions induced by gazing 

 into darkness. 



But although an interesting book to read, and one 

 which is likely to attract the reader to more systematic 

 scientific studies, this work seems to leave an opportunity 

 for honest doubt as to the wisdom of piecing together 

 portions of discussions unequal in degree of detail with 

 the object of showing the uniformity and continuity of 

 natural processes. There is a want of some more 

 definite coordinating idea, such as would be supplied by 

 considering the progressive evolution of the world and 

 Its processes from the condition of a raw planet up to 

 its completion as the home of man. H. R. M. 



Admiral Sir Williatn Robert Mends, G.C.B. By 



Bowen Stilon Mends. Pp. xvi -I- 380. (London : 



John Murray, Albemarle Street, 1899.) 

 This biography is of special interest, as the life of 

 Admiral Sir William Mends was exceptionally active and 

 eventful. The facts are taken chiefly from his diary, and 

 from letters to his wife ; and his detailed account of the 

 months he spent in the Crimea during the war, gives 

 a good idea of the difficulties which had to be con- 

 tended with at the time. The author was evidently 

 keenly alive to the scientific side of his profession, for 

 he states that, so far back as 1830, the young midship- 

 men on board the Thetis had to make daily reckonings 

 of the latitude by star altitudes, and of the longitude by 

 lunar observations. The admirable descriptions which 

 he gives of the ships, afford us an excellent opportunity 

 of noting the advance which has been made in the navy, 

 and the new scientific applications which have been 

 continuously introduced. 

 The Great Salt Lake Trail. By Colonel H. Inman 



and Colonel W. F. Cody. Pp. xiii 4- 529. (New 



York ; The Macmillan Company, 1898.) 

 This book is of great interest, but of little or no scientific 

 importance. It is a valuable contribution to the history 

 of early travel in North America, and comprises stories 

 related by members of the first exploring and trapping 

 expeditions. The remarkable adventures of " Buffalo 

 Bill,' one of the authors, are also described. 



We feel, on finishing the book, that we have obtained 

 a great deal of information, and a good insight into the 

 gradual and steady growth of civilisation in the part of 

 the United States with which it deals. 



NO. 1539, VOL. 59] 



LETTERS TO THE ED/ J OR. 

 '^r/id Eiitor does not hold himself responsible for opinions ex- 

 f^resset by his correspondents. Neither can he undertake 

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

 niiiuiscripts intended for this or any other part of NATURE. 

 Xo notice is taken of anonymous com m unications.'\ 



The Interferometer. 



As I have been away from home, I have only to-day seen 

 Lord Rayleigh's letter on the interferometer, in your issue of 

 April 6, and I write at once to state that I agree heartily with 

 all he says. I am quite confident that the " structure " dis- 

 covered by Prof Michelson in the spectral lines is a real one 

 existing in the light emitted by the source, and I hold this 

 opinion after a careful study of Prof. Michelson's work, com- 

 bined with the fact that all the " structure " revealed by the 

 interferometer in the light emitted by a source placed in a 

 strong magnetic field is in general accord with my own 

 observations with a 21 '5 feet grating. 



The law deduced by Prof Michelson, from his observations 

 with the interferometer, for the amount of the magnetic (Zee- 

 man) effect, viz. that "it is approximately the same fur all 

 colours and all substances," was, however, at such utter variance 

 with all observations made with a good grating, that I felt 

 bound to question the general performance of an instrument 

 which yielded such a law. My opposition was directed against 

 this law, and if the interferometer had insisted on it, then the 

 interferometer must be discarded as a measuring instrument, 

 or be standardised according to some other scheme. 



I am very glad, however, to hear from Prof Michelson that 

 the law announced by him was probably generalised from in- 

 sufiicient data, and that the interferometer is not at fault. 

 This being conceded, and the law being abandoned, I am 

 thoroughly satisfied, and my confidence in the instrument is re- 

 established. 



I may just add that the question as to whether the working 

 of the instrument is prejudiced by " diffraction " or not, was 

 not raised by me, but by others ; nor did I intend for a moment 

 to support the idea that the phenomena were due to diffraction. 

 The main phenomena revealed are certainly not due to dif- 

 fraction, and it is not easy to see where diffraction can come in. 

 The light, of course, passes through a slit, and through other 

 pieces of optical apparatus ; and I thought it just possible that 

 some of the minor effects — some of the little "humps," per- 

 haps, in Prof. Michelson's drawings — might be due to trans- 

 mission through the slit or other pieces of apparatus. It is 

 very easy to determine, however, if diffraction plays any part, 

 for the effects due to it should be similar for all wave-lengths, 

 and should therefore attend all spectral lines varying merely in 

 scale for the different wave-lengths. It is hardly necessary, 

 however, to mention this obvious fact, and I am sure it has 

 been duly taken into account long ago by Prof Michelson. 



Thomas Preston. 



Bardowie, Orwell Park, Dublin, April 21. 



Absolute Determination of the Ohm. 



The recent great advances in methods of calculating co- 

 efficients of mutual and selfinduction, due to Prof Jones and 

 his colleagues in the work, has suggested that a method, which 

 occurred to me some years ago, for the absolute determination 

 of resistance might now be employed with, I think, some ad- 

 vantages over even the Lorentz method. It is as follows : — 

 Take an earth inductor coil, fixed so ihat it can revolve con- 

 tinuously, thus giving a pure sine voltage, due to cutting the 

 lines of the earth's magnetic field. Take the coils made by 

 the Committee fur the determination of the ampere, as described 

 by Prof Ayrton, and use the two outside coils as the primary 

 and the inside coil as the secondary of a transformer. Pass 

 the current from the earth inductor through one set of coils of 

 a Kelvin balance, and thence through the primary of the trans- 

 former. Then through the (rion-inductive or easily calculated 

 inductive) resistance required to be measured, thence back to 

 the earth inductor. Connect the secondary of the transformer to 

 the other set of coils of the Kelvin balance, and speed up the 

 earth inductor till the Kelvin balance arm becomes horizontal, 

 and the pull on the two sets of coils is equal. Then take out 

 the resistance we are measuring, and speed up the inductor 

 till the Kelvin balance comes to zero again 



