PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 609 



from east to west. GYLLENSKIOLD indicates about 39' as an average of six observations of the 

 angular velocity of the luminous wave. Supposing the thickness of the rays to be 10', we obtain about a 

 quarter of a second as the time which the corpuscular rays take to pass from near the southern to the northern 

 auroral zone, and vice versa. We suppose here rays of a certain rigidity. In reality, rays of a somewhat 

 different rigidity will, of course, occur, and the conditions will then be correspondingly more complex. 

 We will not here enter more closely into the theoretical problems as to the explanation of the so-called 

 folds and whirls in an auroral curtain. We will only say that we suppose that where such phenomena 

 occur, the angle between the rays and the magnetic lines of force is nearly o, or the angle lies in the 

 magnetic meridian. 



We have taken for granted that the auroral drapery is formed by negative corpuscular rays of a 

 kind similar to ft rays, and have thus assumed that a or other similar positive rays take no part in the 

 formation. 



There might in itself be much that would lead one to think of rays in connection with auroral 

 draperies, but there are decisive points that to my mind contradict such an assumption. 



In the first place the auroral draperies appear, as a rule, in the time between the positive polar 

 storms in the afternoon and the negative storms at night, i. e. just at the time when the negative 

 corpuscular rays fall most vertically and farthest in towards the earth. During the positive storms in 

 the afternoon, the rays are bent westwards along the auroral zone, and in the night, during the negative 

 storms, they are bent eastwards, always supposing that our results from the terrella experiments can be 

 transferred to the earth. 



A precipitation towards the earth of a rays or other positive rays from the sun, would come in 

 on the morning side of the earth, not on the evening side as the negative rays do; and it would be a 

 remarkable coincidence if the positive rays were to go right round the earth and descend farthest into 

 the asmosphere on the evening side, at the very place where all experience would lead us to expect the 

 lowest precipitation of negative rays. 



The way in which the phenomena are here compared, furnishes an explanation of an observation 

 that is sure to be made whenever bright draperies are seen near the zenith in the neighbourhood 

 of the auroral zone. The magnetic needles in the magnetometers then always, as far as I can 

 learn, oscillate backwards and forwards, with alternately great positive and negative deflections. 



From these points of view, it will be easily understood that the connection between the magnetic 

 perturbations and aurora cannot be either simple or direct. Very early observers have proved that they 

 are not the very same conditions that give rise simultaneously to the most powerful magnetic storms 

 and the brightest aurora; but it is certain that when one of these phenomena manifests itself with great 

 intensity, the other infallibly occurs, although there is not on that account any easily definable proportion 

 to be found between their intensities. 



During the last couple of years, attempts have been made in different ways, upon the basis of the 

 corpuscular rays, to obtain a plausible explanation of the formation of the auroral curtains. 



VILLARD( I ) has tried, upon the basis of some beautiful experiments, to conceive the auroral 

 drapery as formed by cathodic rays emanating from cirrus clouds, and afterwards drawn towards a 

 terrestrial magnetic pole, e. g. the north pole, whence the ray returns after having penetrated far into 

 the atmosphere and formed an auroral ray. He conceives then that the ray returns and goes towards 

 the south pole, where the same ray penetrates far into the atmosphere and forms] a southern auroral 

 ray. The ray then returns again and goes towards the magnetic north pole, and forms there a new 

 auroral ray by the side of the first one, and so on, times out of number. 



(') VILLARD : Les rayons cathodiques et 1'aurore boreale. Paris, 1907. 



