PART I. ON MAGNETIC STORMS. CHAP. II 



159 



Fig. 73- 



Several bundles of rays are here 



seen issuing from two points, one 



of which is in the magnetic equa- 

 torial plane, and the other a little 



above it, the rays being directed 



towards the terrella. 



"Fig. 74 shows a comparison 



between the observed and the 



theoretical districts of precipita- 

 tion ('). It will be seen that the 



similarity is striking. 



"In this connection I will 



mention that the same calculations 



may be employed as regards the 



earth, for the purpose of finding 



the districts of the precipitation of 



electric (negative) corpuscles com- 

 ing from the sun. All the data 



necessary for such a calculation 



will be found in my Geneva paper (1. c., chap. IV). 



" Let O (fig. 75) be the centre of the earth, P the north pole, OM the earth's magnetic axis, OAB the 



magnetic equatorial plane, and OS the direction to the centre from which the corpuscles emanate (the 



sun). OS is calculated from the 

 time of the phenomenon, by well- 

 known formulae from spherical 

 astronomy. The angle ifj is 

 thereby found, i. e. the altitude 

 of the sun above the magnetic 

 equatorial plane, or in other 

 words, the sun's altitude above 

 the horizon at the point M. 



"The angle of deflection <P 

 (calculated positive ( 2 ) westwards) 

 answering to i//, is now obtained, 

 as regards the simplest trajec- 

 Fl - 74 ' lories, with sufficient accuracy by 



the tables given in 14 & 15 



of my paper. They give the following curves, in which </> is the abscissa and ip the ordinate (fig. 76). 

 "The continuous line is the curve for the northern hemisphere, the broken line, symmetrical with 



the first, that for the southern. 



"For each value of tp, we generally find that there are several values of answering to various 



trajectories from the same point of emanation; and this gives correspondingly various districts of 



precipitation ( 3 ). 



(') See "Sur les Trajectoires des Corpuscles Electrises", etc., by Carl St0rmer, 16, Archives de Geneve, July October, 



1907; and a lecture on the same subject given at the International Mathematical Congress at Rome, April, 1908 

 ( 2 ) For positive rays, must be calculated positive eastwards. - ( 3 ) I. c. 14, 15 & 18. 



