214 



NATURE 



[August 17, 191 1 



sary. Now we know that the a radiation given out 

 during one radio-active transformation is, in fact, homo- 

 geneous. 



A most peculiar properly of aurora is the formation 

 of parallel bands. It seems as if the bands were mutually 

 1 led in some way. When one band leads the others 

 will form similar equidistant curves, and it is frequently 

 observed that if one of the bands moves, the others will 

 undertake a similar movement. 



I think these most peculiar properties will be immedi- 

 ately understood if we assume each band to be produced 

 by its own homogeneous radiation, and that the homo- 

 geneous radiations corresponding to the various bands 

 start from the same source. If so, the various groups 

 will be subject to the same conditions, with the exception 

 that they will be slightly differently bent by the fields of 

 force which they have to pass through on their way to 

 the polar regions. II, therefore, one homogeneous bundle 

 makes itself felt as a thin luminous band along the mag- 

 netii parallel, the rays from the other groups, having 

 started under the same initial conditions, must give rise 

 to bands similar in form, but with a difference in position, 

 corresponding to the difference of stiffness of the various 

 groups. 1 



Now the existence of homogeneous groups given out 

 from the same source is a simple consequence of the a-ray 

 hypothesis, because a number of different radio-active- 

 compounds will be formed through the atomic disintegra- 

 tion and eject groups of homogeneous a radiations. 



In that way the auroral drapery bands should form a 

 kind of magnetic spectrum of the a rays given out by 

 some radio-active matter present at the sun. Thus a 

 possibility opens out of studying and identifying the radio- 

 active substances cm the sun by examining the magnetic 

 a-ray spectrum produced by the earth's magnetic field in 

 the form of drapery bands. 



In the following table is given the " magnetic auroral 

 spectrum " of the radium family calculated on the 

 assumption that the earth's magnetic field is that of an 

 elementary magnet : — 



«„=ir,- ; 4=126*01. 



Substance tf. S<? B 



km. 



Radium O ... 



,, F ... 24 ... 



„ emanation.. 39 



A 64 ... 



„ C 133 



0„ i- the angular distance from the magnetic axis to 

 the radium band, which is farthest north, h is the height 

 to which the draperies are supposed to descend, d is the 

 distance from one of the bands to that of radium, S is the 

 distance in km. ami j3 flic- angular distance between succes- 

 sive bands. flu- draperies are supposed 10 be seen near he 

 zenith at the height /i=i2o km. Any attempt at identifi- 

 cation with actual observations is hardly possible with 

 the material at my disposal, but such identification may 

 be possible, e.g. through the development of the photo- 

 graphic methods. I think, however, that the values found 

 11 ol the right order. 



Summing up, 1 think we may say that, so far as our 

 present knowledge goes, the properties of auroral drapery 

 bands, and probably other forms of aurora, are well ex- 

 plained by assuming that they are caused by a rays from 

 some radio-active substances on the sun. In order that 

 the rays shall preserve homogeneity, it is necessary that 

 the radio-active matter is distributed in extremely thin 

 layers, an. I in such a way that the radiation gets out 

 without traversing other kinds of matter. 



Th.- arguments in favour of the a-ray hypothesis are, in 

 shoi 1 . Hi.' follow ing : — 



(Yi The straight-lined structure of the draperies is ex- 

 plained from the small scattering of a rays. 



I he rapidit} with which the luminosity stops at the 

 bottom edge "I the band is explained from the "range" 

 of the a rays. 



(3) The maximum intensity of the luminosity is 

 lined from the variation of ionisation along the path 

 of die a partii le. 



he explanation from the radiation theory of the 



1 irmei ; papers on corpuscular orbits. 

 INTO. 2 I Si. VOL. 87] 



ihm draper} bands i- in favour of a positive charge, while 

 the diurnal distribution uf aurora gives at present very 

 little information in this respect. 



(5) The parallel draper} hands are explained from the 

 fact that a mixturi of radio-active substances formed by 

 atomic disintej - out a mixture of homogeneous 

 groups of a rays. 



(6) Calculation of the height of aurora from the rela- 

 tion between velocity and range found by Bragg, Klee- 

 man, and Geiger has shown that a rays with velocities of 

 the same order as found for ordinary radio-active sub- 

 stances would get down to heights varying betwei 



and 300 km., which is, indeed, the interval of height most 

 frequently found for aurora. 



(7) According to Stdrmer's calculations, the draperies, 

 when formed by a rays, should appear at an angular 

 distance of about 17 from the magnetic axis, which gives 

 very nearly thi right position of the auroral zone. 



(8) Radiation of the /3-ray type cannot explain the struc- 

 ture "I the draperies on account of their great scatter- 

 ing, and being magnetically much softer than the o rays 

 tiny do not give the right position of the auroral zone 



(9) The spectrum of aurora has not yet been interpreted 

 in terms of spectra physically known. This negative result 

 indicates that the auroral spectrum owes its peculiai 



not so much to the gases present as to the peculiar way 

 in which the light is produced. 



In my opinion the spectrum should be produced by 

 a rays penetrating through the upper strata of the atmo- 

 sphere. It might be possible for those who possess a 

 sufficient quantity of radium to examine the spectrum pro- 

 duced when a rays pass through rarefied gases. 



L. Vegard. 



University of Christiania, May 30. 



Occurrence of a Fresh-water Medusa in Indian Streams. 



Dr. Annandale's interesting announcement, in Nature 

 "I August 3, of the discovery of a fresh-water Medusa in 

 streams of the Western Ghauts, emboldens me to mention 

 that, at the- beginning of the hot season of 1879 or 1880, 

 I saw ami handled one of these beautiful little creatures 

 in the lake at Purulia, in Chota Nagpore. 



This lake, as I i-emember it in those years — since when 

 I have nevei had an opportunity of revisiting it — was a 

 sheet of water of no very great size, and a maximum depth 

 11; about 1 e said to have been formed by damming the 

 mouth of a wide and shallow ravine so as to catch and 

 hold the ordinary surface drainage. Its flora and fauna 

 wrv ju 1 thosi 1 an ordinary Indian " tank "; in the cold 



it was used as a sort of port of call by wild-duck; 



and in the rainj season two little isleis that rose above its 

 waters becan 111 and unfragrant nurseries of 



night-herons and snake-birds. 



I caughl the Medusa -there was only one— when taking 



my ciis ijiN morning swim, and though I afterwards 



kept a look-out, I never saw another. 



I i lew friends 10 whom I ever mentioned the matter 

 always very politely changed the conversation : but Dr. 

 Annandale's discoven now leads me to think that it might 

 01 worth wlii!. in look for medusa- in the- pools of th< 

 Damuda, Subanrika, and Kasi Rivers, which run through 

 Chota Nagpon on their way to thi Baj "I Bengal. 



Heathlands, Belvedere, Kent. A. Aixock. 



Interglacial Conditions in the South of England. 

 Recen 1 observations here through excavations conn 

 with the opening up of the district have enabled me to 



appreciate the imporl of the letter which appeared in 



\uiri (Deceniliei 15, iqio, p. 206) from Mr. Hazzledine 

 Warren on the " Arctic Plants from the Valle} Gravels of 

 the River Lea Hi peaks of the evidence as leaving 

 " no doubt ih, n the Pleistocene age was closed by ■< partial 

 return to glacial conditions, succeeding an epoch when 

 temperate conditions prevailed." Reserving for the 

 moment a discussion in any detail of the evidence now to 

 hand in the upper Valley of the Stort (an affluent of 'he 

 Lea), I merely wish, with your courtesy, to say now that 

 the physical evidence bears out Mr. Warren's contention; 



