270 



NA TURE 



[January 23, 1896 



Mr. Shelford Bidwell (Phil. Trans., 1888, pp. 216-7). He has 

 been followed by Prof. Ewing, on pp. 242-3 of his " Magnetic 

 Induction." 



On the other hand, Maxwell, I.e., speaks of a tension along 

 the lines of force, and even more explicit statements to the same 

 «ffect are due to Kirchhoff ( ?F«^^. Ann., Bd. 24, p. 52, and Bd. 

 25, p. 6oi)and Prof. J. J. Thomson ("Applications of Dynamics 

 to Physics and Chemistry," p. 52). The latter says: ". . . 

 there are the strains arising from Maxwell's distribution of 

 stress. Kirchhoff has investigated the effect of this on a small 

 soft iron sphere placed in a uniform magnetic field and has 

 shown that it would produce an elongation . . . along the lines 

 of force and a contraction at right angles to them. We may 

 therefore assume that in general this distribution of stress causes 

 an expansion ... in the direction of the lines of force ..." 

 {italics mine). 



Prof Thomson's view was accepted and restated by myself in 

 ■discussing the phenomena in cobalt {Phil. Trans., 1890, p. 339). 

 The fact that my views escaped Dr. More's notice is not sur- 

 prising, but it is strange that he overlooked Prof. Thomson's 

 statement, and that I overlooked Mr. Shelford Bidwell's. My 

 belief now, as before, is that the view of Kirchhoff and J. J. 

 Thomson is the correct one. 



Let us take the case of a long iron bar magnet suspended 

 vertically. Suppose it cut in two transversely at a section A ; 

 then, supposing the magnetism sufficiently intense, the lower 

 portion will, we know, adhere to the upper until a considerable 

 weight, W, is hung from the lower end, B. If w be the weight 

 •of AB, the magnetic force over A must balance W + w, acting 

 downwards, and so must itself act upwards, or away from AB. 

 In other words, the magnetic force across A acts precisely like 

 the elastic stress across a section of a rope fastened at one end 

 and pulled at the other. The rope, as every one will realise, is 

 wnder tension not pressure, and so clearly is the iron bar. 



A more general way of looking at the matter is as follows : 



A' 



B B' 



''I l"L''' 



Take imaginary sections A and B of a long uniformly mag- 

 netised straight or bent bar, and regard coterminous surfaces 

 AA' and BB' as separated by distances small compared to A B. 

 Taking the usual elementary theory, we see the force acting on 

 the distribution -t- m over A is F - F' -F F" to the left, where 

 F arises from - vi over A', - F' from - in over B, and F" from 

 -f m over B'. Now clearly F' and Y" are approximately equal, 

 and each is small compared to F, so that A is obviously pulled 

 to the left. Similarly B is pulled to the right, or AB is under 

 tension. 



Of course if the bar, when supported horizontally on a fric- 

 tionless table, were actually cut through at A, foreign material — 

 a finger, for instance— inserted between the naturally co- 

 terminous surfaces A and A' would experience compression. I 

 cannot avoid the conclusion that Mr. Shelford Bidwell and Dr. 

 More have fallen into confusion through regarding the problem 

 from this point of view, forgetting that the surfaces supposed to 

 attract one another have nothing between them to squeeze. 



That the question is of vital importance to a true understand- 

 ing of the phenomena in iron is unquestionable if Dr. More's 

 experiments— which seem carefully designed— gave anything 

 like the true results. A reversal of the sign in his correction 

 B^SlJSwl in his tables I. to IV. —necessitated by the view I 

 advocate— would completely alter the character of the physical 

 ■deductions to be made. The occasion for the correction, I may 

 add, is as clearly indicated by Mr. Shelford Bidwell as by Dr. 

 More, but the experimental data obtained by the former led him 

 to attach less importance to it. 



It is not my intention to discuss the actual magnitude of the 

 magnetic stress or of the consequent alteration in length. An 

 interesting experimental paper on the former subject by Mr. E. 

 Taylor Jones, with references to recent conflicting authorities, 

 appeared in the Phil. Mag:, March 1895, PP- 254-267. I 

 may say, however, that the theoretical importance of investiga- 

 tions such as those of Prof. J. J. Thomson into the mutual rela- 

 tionships of magnetism and elasticity renders it all the more 

 ■desirable that any doubt as to the true nature of the experimental 

 results should be removed. To be altogether satisfactory, these 

 investigations require trustworthy magnetic and elastic data for 

 one and the same specimen. C. Chree. 



January 11. 



NO. 1369, VOL. 53] 



The Metric System. 



Mr. Brook-Fox does not state accurately, in his letter to 

 Nature of January 9, what passed in relation to legislation on 

 the subject of Weights and Measures in British India in the 

 years 1 870-1 871. 



It is true that an Act was passed in April 1870, enabling the 

 Governor-General in Council to require the use of the Metrical 

 Weights and Measures by Government departments, railway or 

 other companies, registered under the law relating to such 

 companies, and persons exercising specified occupations or 

 trades ; but only after certain preliminary steps had been gone 

 through. 



This Act was disallowed by the then Secretary of State for 

 India, the Duke of Argyll, who had at first authorised legislation 

 on the subject, in the general direction given to this Act, 

 but considered that the actuaj form in which the measure was 

 passed went beyond his original instructions, as it might have 

 been extended to the dealings of persons engaged in business or 

 trade, and was otherwise considered to be too drastic. Objec- 

 tion was also taken to the inclusion of the metre as the standard 

 of length, the onginal authority to legislate having been limited 

 to the adoption of the kilogramme as the unit of weight. 



Immediately on the receipt of the Secretary of State's dis- 

 allowance of the Act of 1870, a new permissive Bill was prepared, 

 extending only to the standard of weight, which was again stated 

 to be the kilogramme, and of measures of capacity which are sub- 

 sidiary to those of weight ; and limited, as to the power of apply- 

 ing it, to the case of Government departments, municipal 

 bodies, and railway companies. 



This measure became law as Act xxxi. of 187 1, and is still in 

 force. But it has never been put into operation. This, however, 

 was not the result of the Duke of Argyll's action, but of the 

 change of policy that followed on Lord Mayo's most unfortunate 

 murder. Had Lord Mayo lived, he would certainly have given 

 effect to the Act, the importance of which he thoroughly recog- 

 nised. His successor was of a different opinion, and the com- 

 bination of circumstances that for the moment appeared to have 

 rendered possible the introduction of a rational system of weights 

 and measures into our Indian Empire, passed away, and has not 

 yet returned. 



As having had charge of the Bill of 1870, in the Council of 

 the Governor-General, I may be permitted, in conclusion, to 

 quote the words that I wrote twenty-five years ago with reference 

 to the fears expressed by the Secretary of State as to the danger 

 of precipitate action in such a matter as this. 



" The.history of this very Act regarding weights and measures, 

 which has been under consideration in one form or other for not 

 less than fourteen years, which in its last stage has taken seven 

 or eight years in coming to maturity, and to carry out the pro- 

 visions of which no measures have yet been taken, or can be 

 taken in all probability for another year at least, seems to me to 

 teach that the one thing perfectly certain in the future is that 

 the progress made in giving effect to any change in the present 

 case will be slow in a degree most painful to all who are con- 

 cerned in it ; and that what with active opposition, and what 

 with indifference, this Act is far more likely to remain a dead 

 letter on the statute book for an indefinite period, than to be 

 brought into operation with improper haste." 



January 13. Richard Strachey. 



Marsupial with an Allantoic Placenta. 



I HAVE just received from my friend and former pupil Mr. 

 J. P. Hill, Demonstrator of Biology in the Sydney University, 

 an advanced copy of the Abstract of Proceedings of the Linnean 

 Society of New South Wales for November 27, in which he 

 announces the discovery of a true allantoic and highly vascular 

 placenta, of a discoidal and most probably deciduous type, in the 

 Bandicoot {Perameles obesula). 



In the second part of his magnificent " Forschungsreisen im 

 Australien und dem malayischen Archipel," Prof R. Semon has 

 substantiated the discovery of apposition between the allantois 

 and sub-zonal membrane in Phascolarctiis, and shown that a 

 fusion is effected. He points out that the foetal appendages of 

 that animal are of a type involving certain Marsupialia in a 

 position intermediate between the Placentalia on one hand, and 

 the Monotremata and Sauropsida on the other, and regards the 

 earlier recognised Marsupial condition, in which the allantois is 

 remote from the serous membrane, as secondary and associated 

 with the loss of a respiratory function by that organ. 



I 



