PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. V. 



645 



Photograph by Wood 

 at 4'' 30"!, G. M. T. 



Eginitis at 6' 1 40. 

 Fig. 241. 



Hartmann 

 at -jh gm. 



We will now compare the astronomical observations here quoted, of the forms of the comet's tail, 

 with the forms that, according to our hypothesis and experiments, it would present if the tail-material 

 consisted of electrically radiant matter. 



In the first place it is obvious that here, as we maintained in our theory of the zodiacal light, we 

 must allow that the earth's magnetism will try to keep the electric radiant matter away from the earth 

 except in the polar regions. In the plane of the earth's equator, the negative electric corpuscle-rays 

 that come out of space straight towards the earth, even when at a distance of millions of kilometres, 

 arc deflected westwards as seen from the earth and this in inverse degree to the stiffness of the 

 rays. Compare with this the bulk of the rays in the experiment illustrated in fig. 219. The fact that the 

 material of the comet's tail has to some extent given rise to phenomena that could not be distinguished 

 from the phenomena of zodiacal light as a number of accounts state is therefore in perfect accord- 

 ance with our theories. In the same way the astonishingly short tail of the 2oth May as compared with 

 that of the 171)1 and of the 23rd may be explained (see sketch fig. 238). 



What should we have ex- 

 pected to see on the morning 

 side, when the huge comet's tail 

 was approaching the earth from 

 May 17 21, if the tail had 

 ((insisted of negative* electric 

 radiation? 



We obtain clear informa- 

 tion on this point by a compa- 

 rison with fig. 219. 



The bulk of the rays must be deflected westwards. This at once explains the fact that the tail of 

 the romet appeared morning after morning in almost the same position, although the comet had crossed 

 the sun's disc. 



We should further expect two branches from the tail, extending north and south and pointing 

 towards the poles of the earth. This is seen on a closer inspection of the experiments shown in fig. 

 ;2oo, where the rays strike the floor and roof of the vacuum-box, in fig. 215 especially Nos. 4 (see 

 letterpress p. 588) and 14 and in fig. 219. 



1-Yom the position of the earth's axis, one would have expected the in-drawing towards the north 

 xile of the earth; and INNES' observations seem to indicate this. There appear to have been two such 

 tranches in the comet's tail, one with a north, the other with a south direction. Dr. JURIT/. says in his 

 iccount (see above): "These two fainter tails were apparently between // and 20 long. The apptarattct 

 if the three ///-trins of liglil produced on me exactly the impression of the mouth of a great transparent cone 

 nto zi.i/iic/1 the earth was rushing". 



We have reproduced here (fig. 237) one of the figures from Innes' account, to which we have added 

 IK position of the magnetic equator. This, it will be observed, falls just in the dark space between the 

 wo branches of the tail, which is in itself a very remarkable fact. It is doubtful, however, whether 

 here is much to be concluded from this circumstance; but it calls to mind the phenomena illustrated 

 n the above-mentioned figures, where there is a similar division of the cathode-rays in the magnetic 

 quntor on the morning side of the terrella. 



The fact that the main direction of the comet's tail, i. e. of the rays, is oblique in relation to the 

 arth's magnetic equator, makes the whole thing a little less clear, as a comparison with the experiments 

 ihown in figs. 215 & 219 is in this case rather imperfect. 



Birkeland. The Norwegian Aurora Polaris Expedition, 19021903. 



