654 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, IQO2 1903. 



has been said above. I here reproduce some of the most characteristic curves from the period between 

 the i yth and 2oth May. 



The atmospheric-electric measurements included measurement of the conductivity by readings, i. e 

 not registerings. These readings were principally taken at Haldde Observatory. 



In these measurements I received capable assistance from Herr FEYLING, telegraph-director at 

 Bossekop, who kindly accompanied me in order to assist in the observations. Several long series of 

 5-minute readings were taken, alternately with positive and negative potential. The observations made 

 on the 2oth May were of special interest. Their results are described in Art. 93 on p. 449. 



The above is a comparison of the available observation-material concerning Halley's comet, May, 

 1910, with our experimental results, and may serve to strengthen the theory that the conn 

 material consists of electric corpuscles radiated from the comet. 



THE SATURNIAN RING. 



127. That Saturn's rings cannot be rings of coherent matter, either solid or liquid, has 

 been well established by theory, which showed that the equilibrium of such an object would neo 

 be unstable. 



The alternative hypothesis that the rings are clouds of minute satellites, or perhaps mere particles, 

 too small to be individually visible, but so numerous as to look, in our telescope, like a continuous 

 mass, was investigated by MAXWELL in his Adams prize essay, published in 1859. Although the stability 

 of such a ring of particles can hardly be said up to the present to have been strictly proved, MAXWELL'S 

 hypothesis has gained more and more adherents among astronomers, especially since the not> 

 addition to our knowledge of the rings of Saturn, made by KEELER, that the different parts of the rings 

 have a rotation in conformity with KEPLER'S third law. The extreme thinness of the rings has been demon- 

 strated at the times at which the plane of the rings passes through the earth. Even with the 36-inch 

 telescope of the Lick Observatory, the rings were completely invisible in these circumstances. This shows 

 that the entire ring must be so thin that its edge is quite invisible, even in the full light of the sun, 

 at the distance which separates us from the planet. On the other hand, the objects composing it must be 

 completely opaque, as is shown not only by their disappearance in the circumstances we have mentioned, 

 but by the darkness of the shadow which they cast upon the planet when the sun illuminates them 

 obliquely. The cloud of these very small satellites seems to be so dense that a ray of light cannot 

 penetrate the mass. 



At present MAXWELL'S hypothesis seems to be a strong one, although it seems almost incredible 

 that such a ring of cosmic dust should be able to exist for ever, so to speak, without other governing 

 forces than gravitation, when the ring is less than 21 kilometres in thickness ('), with an external radius 

 of 135,100 kilometres. 



Some astronomers, however, appear to be beginning to doubt this hypothesis. 



HERMAN STROVE, after having proved that their total mass is certainly less than VasTao f l ' iat c 

 Saturn( 2 ), says that these rings appear to be composed solely of an "immaterial light", mere dust-films 

 or wreaths of mist. 



(') RUSSELL, Astrophys. Journ , vol. XXVII, 1908, p. 233. 



( 2 ) Publications de 1'Observatoire Central Nicolas, se>ie II, t. XI, 1898, p. 232; and YOUNG, General Astronomy, p- 3! 

 Boston, 1900. 



