April 9, 1896] 



NATURE 



533 



plates of structural details have been added, two of which 

 are devoted to the structure of the mouth in the principal 

 genera of bees. The first, however, which includes details 

 of general structure, very carefully indicated, will prove 

 of the greatest value to entomologists taking up the study 

 of the order Hymenoptcra. There is a larger edition of 

 the work, with coloured plates of the various species ; but 

 of these we cannot speak, as they are not before us while 

 writmg. We hope that Mr. Saunders' labours may induce 

 many residents in the country to take up the study of the 

 order Hymc?ioptera, and ultimately to extend their re- 

 searches beyond Aculeata to the far larger and much 

 niore neglected, though hardly less interesting, section 

 of Terebrantia, which includes the sawflies, gallflies, 

 ichneumons, &c. The parasitic groups are so numerous 

 as to render the Hymenoptera probably the largest of all 

 the orders of insects, though they have hitherto received 

 far too little attention from British entomologists. 

 Os/wald's Klassiker der exakten Wissenschaften, Nos. 



67 to 75. (Leipzig: Wilhelm Engelmann, 1895-96.) 

 We have before us nine volumes recently added to Prof 

 Ostwald's very handy and useful series of reprints and 

 German translations of classical papers. No. 67 is A. 

 Gopel's "Theorias transcendentium Abelianarum primi 

 ordinis adumbratio levis," published in 1847. This is 

 edited by Dr. H. Weber, and translated into German by 

 Dr. A. Witting. No. 68 should be of interest to chemists, 

 for It contains papers by Lothar Meyer (1864-69) and 

 Mendelejeff (1869-71) on the " Naturliche System der 

 chemischen Elemente." This volume is edited by Dr. 

 Karl Seubert, who adds to it some notes on Newland's 

 work in connection with the discovery of the periodic 

 law. A translation of Maxwell's papers on P^araday's 

 lines of force, read before the Cambridge Philosophical 

 Society in 1855-56, appears in No. 69 of the series, edited 

 and annotated by Prof. Boltzmann. The following volume 

 (No. 70) is taken up with Seebeck's papers (1822-23) on 

 ^' Magnetische Polarisation der Metalle und Erze durch 

 Temperatur-Differenz," its editor being Dr. A. J. v. 

 Oettingen. No. 71 contains Abel's investigations of the 

 series 



m{m 



O^.y, m{m-i){,n-2) ^^^ 



I 1.2 'I 



published in CreWe?, Journal in 1826. In the volume en 

 titled "Chemische Analyse durch Spectralbeobachtungen" 

 (N0.72), Kirchhoffand Bunsen's contributions to spectrum 

 ■analysis in i860 are reprinted, with two coloured plates and 

 seven figures in the text. The editor of this volume is Prof. 

 Ostwald. Under the title "Zwei Abhandlungen iiber 

 spharische Trigonometrie " (No. 73), translations, by E. 

 Harnmer, are given of two papers by Euler — one on the 

 outlines of spherical trigonometry (1753), and the other 

 on general spherical trigonometry (1779). In No. 74, 

 German readers have provided for them a translation of 

 Berthollet's discussion of the laws of affinity (1801), 

 ■edited by Prof. Ostwald. Finally, Prof. Groth edits a 

 German edition of the work of the Finland mineralogist, 

 Axel. Gadolin, on the " Herleitung aller krystallo- 

 graphischer Systeme mit ihren Unterabtheilungen aus 

 ■einem einzigen Prinzipe," which forms No. 75 of this 

 valuable series. Our only regret is that English readers 

 have not a similar collection of edited reprints and trans- 

 lations of scientific classics. 

 T/te Metric System of Weights and Measures. By G. 



T. P. Streeter, B.A. Pp. 43. (London : Gee and Co., 



1896.) 

 Thls short treatise is not only concerned with the metric 

 system, but also contains "certain arithmetical principles, 

 problems and formuhe, and an appendix on the common 

 chemical reactions." The arithmetical contents may be 

 useful as a supplement to ordinary books on arithmetic, 

 but the statement of chemical reactions is " cram," pure 

 and simple. 



NO. 1380, VOL. 53] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 p-essed by his correspondents. Neither can he undertake 

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

 manuscripts intended for this or any other part of Nat ORE. 

 No notice is taken of anonymous communications.'] 



The Stress in Magnetised Iron, 



Mr. Wilberforce's letter (Nature p. 462) raises some 

 points I ought to notice. In treating of the stress and strain, 

 my phraseology has, I think, been extremely " unmaterialistic," 

 in the sense that I have said little or nothing about a magnetic 

 "ether," and have employed rather the language of action at a 

 distance. Maxwell doubtless would have put things very 

 differently, but my own experience has been that when one 

 wishes to avoid confusing ordinary people on such questions as 

 the sign of a stress or strain, the less one says about " ether " 

 the better. My discussion of Maxwell's electrostatic medium 

 ^Proceedings Edinburgh Math. Soc, vol. xi. p. 107) will show, 

 I hope, that his standpoint is not unfamiliar to me. The 

 question really at issue is the existence and sign of certain strains 

 in iron and other gross materials, and I judged Prof. Ewing's 

 mode of presenting the case, which I practically followed, 

 to be as clear as any. If strict Maxwellians object to the 

 association of his name — which I did not originate — with stresses 

 answering to the strains in question, by all means let us use 

 another term, say " Q stresses," so long as their existence is 

 queried. 



In the accompanying figure— slightly modified from that on p. 

 270— suppose for the moment a" a', ab, &c., to be straight lines. 

 The conclusion I reached that the stress on the element AB of a 

 uniformly magnetised bar, with air gaps aa', bb', is a tension 

 meets, I am glad to see, with Mr. Wilberforce's support. This 



:ic=i 



implies his belief that the reasoning of Mr. Shelford Bidwell 

 (yPhiL Trans., 1888, pp. 216,217), Prof. Ewing ("Magnetic 

 Induction," § 145), and Dr. More {Phil. Mag., October 1895, PP- 

 349. 350) — who in the places cited have treated the existence of 

 air gaps as immaterial — should have led them to the conclusion 

 that " Q stresses " exist, and that they cause a lengthening, not 

 a shortening, of magnetised iron. 



Mr. Wilberforce's reconciliation of Prof Ewing's present 

 views with my own is based, I rather fear, on a fallacy. Let us 

 consider the accompanying figure, still supposed to represent a 

 straight uniformly magnetised bar. 



When gaps aa', bb' exist, there must, as Mr. Wilberforce 

 says, be forces at a", b" to balance the attractions exerted by 

 AB. Let, however, a' move up to A, and b' to b, and 

 equilibrium will still exist when the forces at a", B"are supposed 

 to be reduced to fcero. Hence, Mr. Wilberforce argues, in a 

 continuous bar the " Q stresses " at A and B cease to exist. Let 

 us push the argument a little further. Equilibrium will still 

 exist when equal pressures of any magnitude are applied at a" 

 and b", so that apparently the conclusion to be deduced is that 

 the "Q stresses "are Y>xG%%\\rQ% wholly arbitrary in magnittide, 

 which Euclid, I fear, would have declared to be absurd. 



The explanation of the paradox is, I think, that when we 

 treat a' a", b' B"as finite, we must suppose the conditions such as 

 to maintain unaltered the state of uniform magnetisation 

 originally postulated, and this does not leave the magnetic 

 stresses at a" and b" arbitrary. We may of course use a magnetic 

 bar for transmitting stresses other than the " Q stresses." For 

 instance, if we employ two magnetising coils, carried by the bar 

 itself, their mutual attraction or repulsion will introduce stress 

 into the bar. (This is, I think, analogous to the case of glass 

 and tinfoil introduced by Mr. Wilberforce.) AH such stresses 

 must be allowed for, but I think it is expedient when possible to 

 avoid confusing them with the " Q stresses." 



Mr. Wilberforce seems to me to attach too much importance 

 to the criterion of equilibrium. The equilibrium of the element 

 AB in the figure would be equally secured whether the stresses 

 transmitted across the air gaps were pressures or tensions. 



My illustration of the rotating anchor ring was introduced 

 because Prof. Ewing seemed unable to realise the existence of 

 a uniform stress — whether tension or pressure — in a simpler 

 system than that composed of an outer hollow ring pushing or 



