624 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



It would be natural here, under the theory of the zodiacal light, to lay great weight upon undoubtec 

 electric evaporation of the sun's surface, which must be assumed to accompany the emission of cathodt 

 rays in accordance with our experience of electric discharges from a cathode in high vacuum. 



In the following articles on comets' tails and Saturn's ring, due consideration has been paid tc 

 these conditions. Experiments have shown that considerable quantities of matter are in this way flum 

 out into the plane of the equator. It can be imagined that these grains of dust, moving under the 

 influence of gravitation and electromagnetic forces, become massed together by collision into greater am 

 greater globules. 



This brings us to the assumption of a dust-ring round the sun, undergoing constant renewal fron 

 the central body ; and we thus come nearer to the hypothesis most current at the present time, namelv 

 the so-called meteoric theory. 



That the spectrum of the zodiacal light suggests reflected sunlight can then also be explained b\ 

 the reflection of the light from these tiny particles originally produced by the radiant matter. 



.ertakci; 



122. Appendix. Since the above was printed, I, together with Mr. KROGNESS, have undertaker, 

 a journey to Egypt and the Soudan, for the purpose of beginning to make personal observations of the 

 zodiacal light. 



Of the expenses of this expedition one tenth was borne by the University, one tenth by my friend 

 Mr. SCHIBSTED, and eight tenths by myself. 



For the time being, our object was to find out whether the pulsations in the light discovered by 

 JONES were accompanied by simultaneous magnetic pulsations. 



During two months, March to May, 1911, observations and attempts to photograph the ligh 

 were carried on at Assouan by Mr. KROGNESS, and at Omdurman, near Khartoum, by myself. 



As the then much discussed question of the simultaneity of certain abruptly-beginning magnetic 

 disturbances seemed likely to be also of importance in connection with these observations, I publishc 

 in "Nature" for March 16, 1911, (No. 2159, p. 79), a letter requesting other observatories, especially 

 near the equator, to take "quick-run" registrations at the same hours at which we did so. 



At Assouan the instruments were set up in the depths of an ancient Egyptian tomb, in which the 

 temperature was fairly constant. Thanks mainly to Mr. KEELING, the superintendent of the Khedivial 

 Observatory at Helouan, we enjoyed all the facilities for our work that we could desire. 



Our observations of the zodiacal light were made every evening and night in favorable weather, 

 from camps out in the desert west of Assouan and south of Omdurman, where the light from the towns 

 in no way hampered the observations. It was a strange occpuation these observations every dark night 

 in the Soudanese Desert, accompanied only by a chance Abyssinian servant. 



The time however was not favorable, according to the general opinion of several inhabitants. The 

 zodiacal light could often be seen much brighter there than we saw it. 



As a rule the desert wind raised fine sandy dust, which caused the air to become thick, especially 

 near the horizon. Venus, moreover, at that time was very bright, and was situated near, and sometimes 

 in the very middle of, the cone of zodiacal light, where its presence was highly embarrassing. It was 

 impossible, for instance, to be sure of the pulsations in the zodiacal light, although we thought now and 

 then that we saw slight, rather sudden changes. 



Only on the 3oth April, the last day I was in Khartoum, just as I was about to leave it, the light 

 was unusually strong and right up in the zenith, and I was almost sure that I could see decided changes. 



I had no opportunity of noting the exact times of these changes when I unpacked my photographic 

 apparatus; but during that last hour at the railway-station before leaving, I succeeded in getting the best 

 photographs of the light that we took throughout the expedition. 



