NATURE 



[April 29, nog 



tables, but may be used as they stand. The system of 

 formulae determines the simple equivalent of the two wires, 

 copper or iron, when their capacity is sufficiently small 

 to be left out of account. One wire is the return of the 

 other, and they are equal in all respects. 



Let (a, /i, a) be the radius, permeability, and resistivity 

 of a wire in C.G.S. units, / the frequency of alternation, 

 and c the distance between the axes of the wires. 



Writing 



A = 4to(/^//V )*, p = log,( J7r/fl. 10-"). 



Then 



(a) For copper wires, provided X>ii, so that the 

 frequency is high, if (L, R) be the inductance and resist- 

 ance per unit length of the pair, 



L = 4(i-^)log.^ + ^(i-?)-^ ,(p-4P + 3log.;-) 



\ pi J a A\ A/ A("p-\ Clt 



Srr/ aV a/ A(--p-V a) \-cY 



■|P'-2-3P + 2p-loge^j 



where X-', a*/c*, and a'/K'c'p have been neglected. 

 (6) For iron wires, ignoring also fi,\-' and fi-'. 



R 



4t/ 



aV a 4AV f'-=\ ^ a/ 'V 



•:) 



(f) For copper wires with low frequency, 

 2/8 



L = 4 log, - + - 



;(:-a.p.-^) 



8a- f 



,-2i3( 2aps--2ap7= 



- e= + 2/37) log,- 



R V 2a- / „ 

 S— = ^ - -TTf, ( fl - 2<ip.- + 27p: 

 071/ OS sc-U \ 



-^«:i 



^■^D 



;f(^e. 



8- 7= + 2a 



D = I - 40p; + 4op-:-, 2Zij2 — \, 



--i-ic'+ 



720" 



7C-' = I 



, 2;3c- 



24 



-:' + 



473 „. 

 2160' 



19, 



--:•' + • 

 3 120 



and 3'°, a^'c* have been ignored. 



(d) For iron wires under the same conditions, neglecting 



also II-' and a-z'/'c^, 



1358 4a-z' 



L = 4 log,- -4-5+1- 





p + loge- 



2-\ 12 ISO / fiC~ \ 24/ 



i=*) 



4a-c 



y(p + .og,^)(:-l=^) 



The results above appear to be capable of including all 

 important practical cases in which the condition of small 

 capacity is not violated. This condition restricts the length 

 of the wires. 



For a four-figure accuracy, the capacity must in general 

 satisfy the two conditions 



Ci-(3/-/-)-'io-^ 

 C+(6L/^/=)-iio-' 

 where C is the capacity per unit length and I is the length 

 of either wire. For a capacity of a microfarad per kilo- 

 metre C = io-°°. J. W. Nicholson. 



Trinity College, Cambridge, April 21. 



Gigantocypris and the " Challenger." 



The writer of the note on " Some Marine and Fresh- 

 water Organisms " (Nature, April 8) quotes from Herr 

 Luders (Zeiischr. wiss. Zool., xcii., [i], p. 103, 1909) the 

 statement that the giant Ostracod Gigantocypris was first 



NO. 2061, VOL. 80] 



obtained by the Challenger Expedition. It may perhaps 

 be worth while to point out that this statement has no 

 foundation in fact. It was first made in 1895 by Dr. 

 G. W. Miiller (Bull. Mus. Comp. Zool. Harvard, xxvii., 

 p. 165), who quotes a passage from the " Challenger- 

 Brief e " of R. v. Willemoes Suhm (Zeitsclir. wiss. Zool., 

 xxiv., p. 13, 1874), where it is stated that the Challenger 

 dredged between Prince Edward Island and the Crozets 

 a fragmentary specimen of a gigantic Ostracod. Dr. 

 Miiller suggests that this may have been a Gigantocypris, 

 and he continues : — " Leider fehlen niihere Angaben iiber 

 das Thier, und in den Challengerostracoden ist es nicht 

 erwahnt." Herr Liiders, in his recent paper, accepts the 

 identification, . and echoes the lament. .'\s a matter of 

 fact, the specimen described by Willemoes Suhm is still 

 safely preserved in the British Museum, but it is not an 

 Ostracod at all ! Long before Miiller conjectured that it 

 might be a Gigantocypris, Prof. G. O. Sars had described 

 and figured it as one of the two co-types of the remark- 

 able phyllocarid crustacean N-ehaliopsis lypica (Rep. 

 Phyllocarida Challenger, p. 22, 1887). Prof. Sars says ; — 

 " It is apparently this form that was mentioned by the late 

 Dr. V. Willemoes Suhm in a letter to Prof. v. Siebold 

 as a gigantic Ostracode. This strange mistake may be 

 readily explained by the incompleteness of the first speci- 

 men obtained, of which only the carapace and a small 

 fragment of the body was brought up in the dredge." 

 The statement might have been made still more emphatic. 

 The description and the dimensions given by Willemoes 

 Suhm, as well as the locality, put it beyond doubt that 

 he was speaking of the identical specimen which is figured 

 on Plate III., Fig. 5, of Prof. Sars's report. 



W. T. Calman. 

 British Museum (Natural History), Cromwell 

 Road, S.W., April 15. 



Persistent Trail of a Meteor on March 14. 



I RECENTLY scnt the Cape Astronomer Royal an account 

 of an unusual meteor which I saw, and he has suggested 

 that I forward an account to you. 



On the evening of March 14 I was walking along the 

 sea-shore looking south-west; the sun had set, and the 

 sky was still bright with sunlight. A few clouds were 

 slowly drifting from the south-east, when suddenly, about 

 7.45 p.m., I saw what looked like a large rocket dart from 

 behind a cloud, rush across the sky from west to east, and 

 disappear over the Table Mountain range in the direction 

 of False Bay. The track of the meteor was shown by a 

 brilliant, apparently glowing, streak of silvery light, which 

 remained stationary in the sky like a long ribbon of fire 

 for fully ten minutes. The " tail " then gradually assumed 

 a wavy form, and slowly faded out of sight. The 

 peculiarity consisted in the persistence of the " tail " or 

 track of the meteor, as I suppose it was. On looking 

 into Sir Robert Ball's book, "The Story of the Heavens," 

 I find an account strangely akin to mine, and I should 

 like to know the reason for the persistence of the luminous 

 track, which must have been very bright to have shown 

 so plainly against the sun-lit sky. Our southern skies are 

 wonderfully brilliant, owing, doubtless, to the clearness of 

 our air ; and I have often seen meteors flash across the 

 sky, but never before have I seen such a magnificent dis- 

 play as that described above. Edward J. Steer. 



Box 42, Cape Town, March 22. 



Lignum Nephriticum. 



I MUST thank Mr. Benham for directing attention 

 (April S) to the early observations of Boyle quoted by 

 Faraday. I have erred in good company ; Stokes himself 

 was apparently unaware of Boyle's experiment, and the 

 " Optics " of Basset, Glazebrook, Preston, Tait, and 

 Winkelmann all seem to regard Brewster and Herschel 

 as the first discoverers of fluorescence. 



Dr. Stapf's letter in Nature of April 22 confirms the 

 conclusions of a recent correspondence in the Gardeners' 

 Chronicle ; letters of March 20 and April 3 give reasons 

 for assigning Lignum Nephriticum to a Mexican tree 

 known as Coatii or TIapalcypatli. John H. Shaxby. 



University College of South Wales and Monmouth- 

 shire, Cardiff, April 23. 



