PART. II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 603 



On a preliminary comparison with the investigations of the ordinary position of the auroral arc at 

 several polar stations, it appears as if these arcs have the direction corresponding to the experiments, 

 and the translatory motion of the arcs in a corresponding manner also makes its appearance. 



INVESTIGATIONS REGARDING THE ANGLE FORMED BY THE PRECIPITATED RAYS 



WITH THE MAGNETIC LINES OF FORCE. 



118. We will now proceed to the description of the experiments represented in fig. 221, and 

 discuss the facts resulting from them. 



The experiments were made in order to make it somewhat clear how steeply towards the terrella 

 the rays are precipitated in the "auroral zone" under the different experimental conditions, especially when 

 the magnetic stiffness of the rays is modified in proportion to the magnetisation. The plate is unfortunately 

 not so good as could be desired. 



These investigations are of great importance to our present theory on the auroral draperies, as 

 we suppose that the auroral rays in the draperies are formed by those pencils of rays which come as steeply 

 as possible towards the earth, where they are entirely absorbed by the atmosphere after having rendered 

 the air luminous over a more or less wide expanse. 



This is to some extent a modification of the opinion I have previously expressed, as I formerly 

 supposed that the rays of an auroral drapery were formed by secondary beams produced in the atmo- 

 sphere by the influence of the primary cosmic and corpuscular current which forms the auroral arc 

 itself. In a certain degree, something valid will remain in this older theory ; but it seems more 

 natural to suppose that rays with such tremendous power of penetration as that dealt with here, 

 must be the same stiff rays that we suppose to be emitted from the sun. The state of the atmosphere 

 of the earth is hardly such as to permit the formation of such stiff rays. I have therefore been 

 brought to take a different view of the matter, which was further confirmed by my terrella experiments, 

 namely, that auroral rays are formed by the rushing in of distinct pencils of cosmic rays towards 

 the earth almost exactly along the magnetic lines of force, without any turning, worth mentioning 

 about those lines These cosmic rays, which thus penetrate the atmosphere, are entirely absorbed, 

 and therefore never return into space. 



During the experiments about to be described, the terrella maintained an unaltered position in 

 the discharge-tube, the line from the centre to the magnetic south pole being in a horizontal plane 

 with an eastern hour-angle of 270. The photographs have been taken from a place in that plane 

 which also has an hour-angle of 270, so that the eight branches of the screen are seen edgewise. 



The discharge-current, during all the experiments, was about twenty milliamperes. The photographs 

 i, 2 and 3 were taken with a magnetisation current of 10 amperes, the first at a tension of 

 2800 2600 volts and a pressure of 0.009 mm., the second at 2200 2100 volts and a pressure of 

 3.017 mm., ar >d the third at 1800 1700 volts and a pressure of 0.05 mm. The photographs 4, 5 and 6 

 ire from experiments during which the magnetisation current was 20 amperes and the tension respectively 

 2500 2100 volts, 2200 2000 and 2000 1800 volts, and the pressure respectively 0.007 mm , 0.017 mm. 

 >nd 0.025 nim. The photographs 7. 8 and 9 are of experiments during which the magnetisation current 

 ,vas 30 amperes and the tension respectively 3000 2600 volts, 2400 1800, and 1700 1500 volts, 

 .vhile the pressure was respectively 0.007 mm -> 0.022 mm. and 0.026 mm. 



In some of the photographs, for instance Nos. 4, 6, 8 and 9, on the left of the third branch of 

 he screen, the shadow of the brass rod from which the terrella was hung in the magnetic equator 

 .vill be observed. We have seen this shadow rather more clearly on a large number of the previous photo- 

 graphs, and it immediately gives us an idea of the steepness with which the rays here pass through the plane 



