( 267 ) 



recorded at Greenwich (and also at Toj'onto) that they show a 

 tendency to recur after a synodic rotation of the sun and that some- 

 times even two and more returns occur. His conclusion is: 



"Our magnetic disturbances have their origin in the sun. The solar 

 "action which gives rise to them does not act equally in all directions, 

 "but along narrow, well defined streams, not necessarily truly radial. 

 "These streams arise from active areas of limited extent. These active 

 "areas are not only the source of our magnetic disturbances, but 

 "are also the seats of the formation of sun-spots." 



As soon as I could find the necessary leisure I prepared a list for 

 the period 1880 — 1899, containing 1149 disturbances and immediately 

 after made some statistical calculations based on them. 



A discussion of such statistical results is always better made by 

 the author of the list, than by anotlier person for whom it is impos- 

 sible to consult the original sheets. 



Though intending to publish the list, statistics and some repro- 

 ductions in full, I wish to give a preliminary account of my results, 

 because these questions are now of actual importance. 



Rules followed in preparing the list. 



An exact definition of what is understood by the expression 

 "magnetic storm" has never been given ; certain features however 

 are characteristic to it, viz : 



1. The sudden commencement. 



2. The postturbation. 



3. The increased agitation. 



Concerning the second, which 1 called the postturbation '), the 

 w^ell known fact may be remarked, that during 'a storm the mean 

 level of the components of the force changes, till a maximum digres- 

 sion is reached, and afterwards returns slowly to its old value. 



In 1895 I called attention to this phenomenon and investigated 

 its distribution over the earth. 



This research enabled me to give the following description of the 

 postturbation. 



During a magnetic storm a force appears contrary to the earth's 

 ordinary magnetic force, loith this dijj'erence, that its horizontal com- 

 ponent is directed along the meridians of the regular part of the 

 earth's magnetism, consequently not pointing to the magnetic pole, but 

 to the mean magnetic a.vis of the earth. 



') Gf. Meteorologische Zeitschrift 1895, p. 321. Terrestrisch Magnetisme I p. 95, 

 11 115, V 123, VIII 153. 



18* 



