3i4 



NA TURE 



[Feb. 5 , 1885 



without any mention of the magnetic units of the C.G.S 

 system, leading the reader to conclude that the volt is 

 equal to io 8 of the static C.G.S. units. These are grave 

 errors in a book designed for specialists. On p. 94 the 

 author, or editor, announces the insertion of some " data 

 given . . . by physicists known for their veracity!' Are 

 there any others ? 



OUR BOOK SHELF 

 Key to Magnus's Class-Book of Hydrostatics and Pneu- 

 matics. (London Science Class-Books.) By John 

 Murphy. 67 pp. (London : Longmans, Green, and 

 Co., 1S85.) 

 Mr. Murphv has rendered useful service to science 

 teachers by the publication of the solutions of the exer- 

 cises and problems given in Mr. Magnus's widely-known 

 volume. These problems cover the whole ground of 

 elementary hydrostatics and pneumatics ; and the solu- 

 tions are intelligently worked out in full. The work has 

 had the benefit of Mr. Magnus's own revision ; and this 

 should be a guarantee of the goodness of the methods 

 followed and of the correctness of the results. The 

 only fault we have to notice is a tendency to looseness 

 in the use of certain terms about which there ought not 

 in physical science to be the slightest vagueness : we 

 refer to the misuse of the words strain and pn ?sure 

 where the proper word should be force. A strain is an 

 alteration of shape or volume, and ought not to be con- 

 fused with the force which produces the strain. A pres- 

 sure is a force divided by an area and cannot be specified 

 except by naming both the force and the area on which 

 that force acts. Vet on p. 5 of Mr. Murphy's Key occurs 

 the statement that " the pressure or whole strain to 

 which the sphere is subjected equals the weight ... of 

 the liquid.'' It is greatly to be desired that this am- 

 biguity between pressure and force should as speedily 1 5 

 possible be removed from this and all other elementary 

 books, as it is misleading to beginners as well as in- 

 correct. 



Electrical Units. By Dr. R. Wormell, M.A. (London : 



T. Murby, no date.) 

 This little work of 48 pp. is apparently issued as an 

 - appendix to the author's class-book of " Electricity and 

 Magnetism." It contains a concise and easy account of 

 the units in ordinary use, and of the notion of dimensions 

 of units so puzzling to beginners. A number of useful 

 data of constants are given, and there are some numerical 

 problems for calculation added. Dr. Wormell's genius as 

 a teacher conies' out in several points : the transition from 

 magnetic to electro-magnetic units being particularly 

 neatly brought about on p. 10. A few slips should be 

 corrected at once. In the table on p. 1 the electro- 

 chemical equivalent of hydrogen is given as '00001055, 

 and on p. 14 as -0000105. According to the late results 

 of Kohlrausch and Lord Rayleigh it is -00001035. On 

 p. 6 the horse-power is wrongly stated as 746 kilogramme- 

 metres per second. On p. 15 there is a curious muddle 

 about units of capacity, arising partly from a confusion 

 between electrostatic and electromagnetic units. It is 

 certainly not true that a sphere of one centimetre radius 

 has a capacity about equal to that of " the whole Atlantic 

 cable"; neither is the farad the millionth part of the 

 microfarad. It also must strike the practical electrician 

 as rather a curious statement that (p. 33) the Swan lamp 

 is usually fed by the Gordon dynamo. We were under 

 the impression that only one Gordon dynamo had yet 

 been built, and that it had not been used since last winter. 

 The connections of the Brush armature on p. 37 arc 

 wrong ; and the author should not describe Edison's 

 armature as being like that of Gramme, when the fact 

 is that it pays royalty to Siemens as a Siemens armature. 



Weekly P ruble in Papers, with Notes, intended for the Use 

 of Students Preparing for Mathematical Scholarships 

 and for the Junior Members cf the Universities who 

 are reading for Mathematical Honours. By the Rev. 

 J. J. Milne, M.A. (London : Macmillan, 18S5.) 

 Mr. Walter Besant in a recent work entitled " In 

 Luck at Last," makes his heroine (a Maria d'Agnesi or a 

 Somerville) remark, " No life can be dull when one is 

 thinking about mathematics all day. Do you study 

 mathematics ? " For such a one this handy volume of a 

 hundred papers, each of which has at least seven ques- 

 tions, some of which bifurcate or trifurcate, will be a 

 charming companion. Though the range is limited to 

 the requirements of a LJniversity scholarship — this by the 

 way is fairly extended at the present day — yet there is 

 sufficient " variety " in the selection of problems to make 

 it what we state it to be above, " charming." The book, 

 as such a work ought to be, has been printed with very 

 great care, and, after a close perusal, we have detected 

 only two or three slight clerical errors. The compiler, 

 who is to be congratulated on his successful achievement 

 of a somewhat difficult task, proposes to bring out at a 

 future date a second volume containing his solutions to 

 the exercises. Wolstenholme's collection is, except under 

 the guidance of a judicious tutor, too hard and too full of 

 tricks for the class whose wants this manual is designed 

 to meet ; the boy who has mastered this collection, or a 

 fraction of it, will have realised what sort of questions he 

 will be called upon to " tackle " when he has an examina- 

 tion paper before him. 



LETTERS TO THE EDITOR 

 [ The Editor does not hold himselj responsible for opinions expressed 

 by his correspondents. Neither can he- undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notict is taken op anonymous communications . 

 [The Editor urgently requests correspondents to keep their letters 

 as s/iort as possible. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel facts.] 



Free Hydrogen in Comets 



Towards the enrl of an admirable mathematical paper on 

 the theory of the forms of comets, received this morning by post 

 from M. Bredichin, that very able Director of the Imperial 

 Observatory of Moscow (specifying himself, to •, in English at 

 the head of a pamphlet in the French language as " Associate 

 of the Royal Astronomical Society"), draws the conclusion that 

 the great comet of 1881 was a structure of compound hydro 

 carbon gas, while Halley's historic comet was "of a type whicli 

 corresponds to pure hydrogen." 



The distinguished author's position with regard to the cornel 

 of 18S1 I presume will be contested by no one ; for all the 

 spectroscopes of the time proved so abundantly that the light of 

 that comet was of the kind familiarly known among technists 

 as "the candle-spectrum," or the spectrum of the compound 

 gas CH, in the form of acetylene, perhaps, but entirely peculiar 

 to carbo-hydrogen. Who, however, can help the theorist of 

 Moskva's white-stoned city and golden domes to establish the 

 probabilty of a spectrum of pure and elemental hydrogen gas 

 fur Halley's cornel, before that wanderer in far off space shall 

 return in the beginning of next century, and then instantly 

 testify its chemical composition to the spectrum analysis of 

 that day ? 



One of the difficulties which M. Bredichin has to deal with 

 meanwhile, seems to be, that no trace of a pure hydrogen 

 spectrum has ever yet been seen in any comet, however much of 

 CH there may have been ; and he is driven to suggest that the 

 II, or hydrogen lines — S3 entirely different from those of CH — 

 were invisible on account of their faintness. But though that 

 idea, with some modification or explanation, may ultimately 

 turn out to be correct, it requires something more just now to 

 create many converts to it, particularly in face of the universal 

 experience of all dabblers in spectroscopy with vacuum tubes ; 

 for they know so well that, whatever the reputed gas in them 

 may be, intrusive lines of hydrogen, though present as an un- 



