PART II. POLAR MAGNETIC PHENOMENA AND TERREL1.A EXPERIMENTS. CHAP. IV. 557 



auroral zone or to the south of the southern auroral zone. The smaller precipitation, B, is due to quite 

 another class of rays, which, unlike those of A, operate close to the terrella north of the northern 

 auroral zone or south of the southern auroral zone. 



The aim of the experimental investigations here described, is to obtain a clear idea of the general 

 course of these two classes of rays. We shall first show, by numerous experiments, how the rays 

 forming A curve round the terrella, rising and falling above and below the equator, when not arrested 

 by the screens. In the next place, the rays forming B will be investigated by an altogether different 

 series of experiments. We shall see, among other things, that rays of the first class will give us a 

 natural explanation of the negative polar storms, while the rays belonging to the second group will 

 help to explain the positive polar storms. In the following pages we shall speak of these two classes 

 of rays as rays of group A and rays of group B. 



In fig. 202, eight more photographs are given, representing various experiments. No. i of 

 these answers to No. 5 of fig. 291, the only difference being that here the terrella is magnetised with 

 10 amperes instead of 30. Otherwise everything is the same. Only the effect of the precipitation A is 

 visible, that of B, for reasons that will be made clear later, being no longer found on the horizontal 

 screen. Some of the rays forming A now fall upon the terrella, and we obtain a figure upon the front 

 of it that resembles the luminous figures shown in figs. 66 and 68 in Section I. 



Photograph No. 2 also shows conditions similar to those of No. 5 of the preceding Plate, except that 

 the hour-angle of the vertical screen is 180. In the precipitation B there appears the shadow of one 

 of the cylindrical pegs. In some of the experiments it sometimes happens that two shadows of the same 

 peg are seen, one of them being cast by rays of the precipitation A, the other by B. 



Photograph No. 3 was taken for the purpose of examining the sharp line of precipitation that forms 

 the eastern limit of A on the horizontal screen. The experiment was made under the same pressure 

 and with the same discharge-current as before, and the magnetising current to the terrella was of 30 

 amperes. The hour-angle of the vertical screen was 90, and the photograph was taken 45 to the 

 west of it. The experiments were made by turning the terrella in such a manner that the vertical screen 

 came near the line of precipitation on the horizontal screen. If the vertical screen were turned ever so 

 little more to the east than the line of precipitation on the horizontal screen, no precipitation was found 

 upon the vertical screen. On the other hand, if it were turned less to the east than that line, precipita- 

 tion appeared upon the vertical screen in the form shown in this photograph. We at once get the 

 impression that the rays bend down towards the line of precipitation, where, if they could get through 

 the horizontal screen, they would cross one another, so that the rays that at first were above the screen 

 would go below it, and vice versa. It is then a natural proceeding, if we wish to study the rays in A, 

 to experiment, as we have done, with a horizontal screen alone, in which there is a slit parallel with the 

 powerful line of precipitation; and also with a vertical screen alone, containing a radial slit at the mag- 

 netic equator. This vertical screen must be so bent that the line of precipitation, right from the terrella, 

 can be made to fall upon the slit in the screen, along the entire length of the line, so that all the rays 

 can get through the slit simultaneously. The arrangement of the experiments is also clearly seen from 

 their accompanying photographs, which will soon be described. A preliminary experiment was made, 

 and this is shown in No. 4 of fig. 202. Here a slit was made in the horizontal screen, which, however, 

 the first time, was not given the right direction along the line of precipitation. Next, a hole was made 

 in the vertical screen, near the auroral zone. We at once discover that close to the slit in the horizon- 

 tal screen, the rays leave the under side and form a second precipitation upon the north side of the 

 terrella, while the rays from above go through the screen, and form corresponding precipitation upon 

 the south side of the terrella. We have thus brought out the second of the remarkable instances of 

 precipitation represented in fig. 68, Section I. 



Hirkdanil. The Norwegian Aurora Polaris Expedition, 19021903. 



