PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 605 



of the equator to be precipitated in the "auroral zone". It was this shadow of the suspension rod 

 that first suggested the idea of constructing the high eight-armed screen and making these ex- 

 periments which have been of such great importance to the theory of the positive and negative polar 

 storms, as seen in Article 113. Such screens, placed on the edge above the polar regions of the terrella, 

 have already been used previously, but the results of the experiments were not so clear (see fig. 136, 

 page 302, Section I), because the screens were far too low. 



The illustrations show us the angle at which the most perpendicular rays fall towards the 

 terrella. The shadows behind the branches of the screen show, further, that the rays are most perpen- 

 dicular in the middle of the "auroral zone". On the southern edge of the zone, the rays fall most obliquely, 

 and on the northern edge more obliquely than in the middle, but less so than on the southern edge. 

 It appears moreover, although not positively, from the photographs, that the rays, at about the same 

 tension, descend somewhat more perpendicularly towards the terrella with strong than with slight 

 magnetisation. With the same magnetisation, the rays are also somewhat more perpendicular with low 

 than with high tension; but the difference does not appear to be so great. There are here, however, 

 several things to be taken into consideration. It must not be forgotten, for instance, that the shadow- 

 producing part of the screen does not remain the same in all cases, a fact of which proof is found in 

 the form of the precipitation on the western side of the screen-branches (see fig. 212, Nos.i, 4 and 7). 



We have endeavoured in the foregoing pages, by numerous experiments, to show how the rays 

 move round our terrella. It would have been of great interest if these experiments had been repeated 

 with our last terrella No. 7, which was highly magnetic, in the new large discharge-box measuring 

 70 litres, as we might then have chosen the magnetic conditions so that the luminous polar band would 

 have had an angular diameter of 45. We could then at once have transferred the results to the 

 earth, and in particular determined the perpendicularity with which, according to the theory, the auroral 

 rays might be expected to come towards the earth. We propose to make these more extensive experi- 

 ments, and the results obtained will be published in the second volume of the present work. 



In a general way, it can even now be established as a fact, that rays which are finally precipi- 

 tated in the "auroral zone", have first passed round the terrella, oscillating above and below the plane 

 of the magnetic equator. 



In the foregoing pages, we came, as a consequence of our experimental results, to the conclusion 

 that the continuous luminous ring in the "auroral zone" was produced by a countless succession of secon- 

 dary precipitations overlapping one another in such a manner that the luminous ring appeared to be 

 continuous. We remember, for instance, having once counted, on the night side of the terrella, about 

 20 distinct secondary precipitations, of which those of a higher order lay to the east of those of a 

 lower. The number of these precipitations was greatly multiplied in proportion to the increase of the 

 magnetisation of the terrella. It is this opinion of the constitution of the luminous ring which we shall 

 firmly maintain in endeavouring to develope a theory as to the formation of auroral draperies. 



The rays which are precipitated, for instance, on the night side of the terrella, a little eastward of 

 the place where other contiguous rays, originally from the same bundle of rays, are precipitated, will 

 thus have travelled considerably farther than those rays which are precipitated on the west side, close 

 by. They may, in fact, have been deflected below the level of the equator towards the south pole, and 

 then have risen again and been precipitated in the northern "auroral zone". It will consequently be 

 observed that the rays in the precipitation-zone are formed from separate, relatively small groups of rays 

 which have intersected the plane of the equator several times, before they are at last precipitated. We take 

 then first a group of rays in the northern "auroral zone", which have passed n times through the 

 equator. The nearest companion group which had nearly been precipitated in this zone, has subse- 



Birkeland. The Norwegian Aurora Polaris Expedition, 19021903. 77 



