\']2 



NATURE 



[Dec. 23, i! 



morphological characters, are unmistakably identical with the 

 hypha; found in specimens of the non-choleraic intestine, lung, 

 skin, kidney, mesenteric glands, preserved,- as stated above, over 

 the summer. 



In conclusion I wish to say that I shall be most happy to 

 place at the disposal of Messrs. Roy, Brown, and Sherrington, 

 as also of Messrs. Vines and Gardiner, the materials or sections, 

 mounted and stained, of the various non-choleraic tissues in 

 which are present tlie mycelial threads of common mould 

 identical with the Cambridge cholera fungus. E. Klei.n' 



94, Philbeach Gardens, Earl's Court, December 18 



The Longitude of Rio 



May I ask for a few lines in which to correct an erroneous 

 impression naturally made by a sentence in my recent paper on 

 "Ten Years' Progress in Astronomy," which you have honoured 

 me by reprinting in Nati;re. The sentence relates to the 

 longitude of Rio ; and although it does not really assert that 

 the error in this longitude was first detected and corrected by 

 our American naval officers, yet I must frankly admit that the 

 connection and form of expression are such that this would be 

 the natural, though incorrect, inference. The fact is that 

 Admiral Mouchez and his coadjutors in the French Navy had 

 already, by their chronometric and other work, brought the un- 

 certainty to very narrow limits (say ± 2s.) before the tele- 

 graphic campaign of the Americans. The history of the case is 

 peculiar, but too long to be given here : it affords an excellent 

 example of the uncertainty of longitudes based on lunar obser- 

 vations. 



The misleading form of the sentence is due to a little care- 

 lessness on my part in cutting down the much more extended 

 statement I had made in the first draft' of the paper. The 

 available limits of time and space compelled me to compress my 

 material to the utmost. 



I cheerfully make this correction in justice to Admiral 

 Mouchez, who has called my attention to the matter. 



Princeton, N.J., December 9 C. A. YoUNG 



An Error in Maxwell's "Electricity and Magnetism " 



Many of your readers will be aware that Maxwell (ii. § 544) 

 deduces the equations of induction of currents from the laws of 

 electro-dynamics with the aid of the principle of energy, using 

 a proof taken from Helmholtz. I find that this proof is 

 erroneous ; and, as a point of considerable physical interest is 

 involved, I wish to call attention to the error in your columns. 



We suppose two circuits carrying currents to be moving rela- 

 tively to each other. Let A'l, A'j be the resistances, /j, /j the 

 currents, ^j, ,4, the electromotive forces of the batteries, and 



dV 

 - 1^1; — the rate at which work is done by the external forces 



which are moving the circuits. Then A-^l^ + .4.,^., '^ the rate 

 at which the batteries are doing work, and Aj/,- + Ji^Z-i^ is the 

 rate at which energy is being changed into heat in the wires. 

 So Maxwell says we have — 



dF 

 ^i/j + A^h-IJ. — =1 R^I{- + RJ} . . (I) 

 dt 



and it is this equation that is wrong. He has omitted to take 

 into account the change in the electro-kinetic energy which is 

 taking place. If, for instance, the two batteries were suddenly 

 thrown out of the circuits, the quantity of heat that would after- 

 wards appear, either in the wires or in the form of sparks, would 

 depend on the relative position of the circuits. And the energy 

 that would then appear as heat previously exists in the form of 

 electro-kinetic energy. 



Let M be the coefficient of mutual induction. Then, if we 

 neglect the rate of change of the currents, the rate of increase 



dM 

 of the electro-kinetic energy is /j/, — . So, instead of (l), we 



" dt 

 should write — 



dV dM 



Wj/i -H A J., - /j/, — = RJ.? -h RJ,^ 4- I J., — . (2) 

 dt - - - ^^ 



If we assume the accepted equ.ations of induction of currents, 

 viz. — 



dM \ 

 A, = A\A -I- /, - 



dt \ 



dM \ • • • • \i> 

 A., = RJ.. -t- /i — 



<// ' 

 neglecting, as before, the rate of change of the currents, we 



dM dV 

 see that — = — • 

 dt dt 

 And therefore the decrease of electro-kinetic energy is equal 

 to the work done by outside mechanical forces on the system. 

 This result was long ago obtained by Sir William Thomson, as 

 is indeed noticed by Maxwell in this very article. 



Notwithstanding the use of the incorrect equation (i), Max- 

 well obtains a correct result. In fact, he falls into a second 

 error which exactly compensates for the first. He supposes /j 

 to be very small compared with /,, and says that we may then 

 with sufficient accuracy put A-^^ = K\h i° (')• I^"' ^^ (3) ^^ 



dM 

 see that the term thus neglected is /,/> — > which is not 



dt 

 negligible. 



As I have not had access to Helmholtz' original memoir, I 

 cannot say whether Maxwell has correctly transcribed his proof. 

 James C. McConnel 

 St. Moritz, Engadine, Switzerland 



Seismometry 



In reply to my letter answering Prof. Milne's assertions 

 (Nature, Nov. 25, p. 75), Mr. T. Gray (his associate in seis- 

 mometric work) says nothing in support of these, but attacks me 

 on two distinct and quite irrelevant issues. The tone of Mr. Gray's 

 letter (Dec. 9, p. 126) is unusual : as to that no answer is neces- 

 sary ; but the two questions of fact raised by him require reply. 



(i) Mr. Gray writes : — " He [Prof. Ewing] says, or leads one 

 to infer, that he introduced horizontal pendulums in seismology. " 

 On the contrary, wh.at I have said (in my memoir on "Earth- 

 quake Measurement," Tokio, 1883, p. 21) is this : — 



" It appears that the earliest attempt to apply the horizontal 

 pendulum to the measurement of earthquake-motions was made 

 by Prof. W. S. Chaplin, of the University of Tokio, about 1S78. 

 His apparatus consisted of a wooden rod, free to turn about a 

 vertical axis, and carrying at its end a rigidly attached block. It 

 was intended that the motion of the earth should be recorded by 

 a tracing-point fixed to the block, writing on a smooth surface 

 fixed to the earth below it. There was no multiplication of the 

 motion, and either for this reason, or because friction was not 

 sufficiently avoided at the joints and pointer, no results were ever 

 obtained, and the apparatus was abandoned." 



The passage Mr. Gray alludes to as having been "read in 

 my presence " was a casual reference by Prof. Milne to these 

 unsuccessful experiments. Prof Chaplin, their author, has him- 

 self written to me : — 



" I certainly think you were the first to use successfully a 

 seismograph depending on the principle of the horizontal pendu- 

 lum. I believe the records obtained by you with this seismo- 

 graph were the first obtained in Japan (and probably in the 

 world) which showed the motion of the earth during an earth- 

 quake from beginning to end of the shock. I cannot better 

 mark the effect which the first record produced than by relating 

 my own experience. I was, up to that time, working on an 

 instrument for determining the velocity and direction of an 

 earthquake ; and my design was founded on the idea that an 

 earthqual^e began with a sudden and violent shock. Your records 

 showed (I believe for the first time) that an earthquake often 

 began with an almost imperceptible motion, which increased in 

 amplitude and might have many maxi na ; hence my machine 

 would have been useless had 1 completed it." 



What I do claim in this matter is that I succeeded in 

 constructing the earliest successful seismograph capable of 

 making absolute measurements of the lijrizontal motion 

 throughout an earthquake, in conjunction with the time, 

 and giving records from which the amount, direction, velocity, 

 and acceleration of the successive movements could be, and 

 •were, for the first time determined. The earliest records, 

 referred to by Prof. Cha])lin, were obtained in November iSSo, 

 and are described in the Transactions of the Asiatic Society of 



