170 THEORIES AS TO DISTURBANCES. 



CHAPTER IX. 

 DISCUSSION OF DISTURBANCES. 



85. In discussing the magnetic disturbances recorded in the Antarctic and the co-operating stations it 

 is desirable to pay some regard to existing knowledge and theories. It has long been known that large 

 magnetic disturbances are usually felt over at least a large portion of the Earth's surface. In a considerable 

 number of instances large storms have been ushered in by comparatively sudden changes in the magnetic 

 elements, which the researches of ADAMS, ELLIS, VAN BEMMELEN, and others have shown to be in all 

 probability simultaneous in their incidence wherever recorded. It has been observed that magnetic 

 storms are more numerous near sun spot maximum than near sun-spot minimum, and many persons have 

 suspected a very intimate connection between the two phenomena. In temperate latitudes a display of 

 aurora is practically always accompanied by a magnetic storm, and another invariable accompaniment of 

 the larger and more rapid magnetic changes is the existence of earth currents. 



Turning to theories, a considerable number have been advanced. It has been supposed that the Sun is a 

 powerful magnet, and that the regular diurnal variation of terrestrial magnetism and the irregular changes 

 arise from the direct magnetic action of the Sun acting, as we may say, at a distance. Another hypothesis 

 is, that electric waves or Rontgen rays, arising in the Sun, travel to the Earth, and on their arrival set up 

 aurora and magnetic disturbances. Others have postulated the existence of negatively-charged particles 

 (AnRHENius), or kathode rays (BIRKELAND), or some analogous form of ray discharge (MAUNDER). The 

 discharge, whatever its nature, on reaching the Earth's atmosphere, occasions electric phenomena which 

 create variations in the Earth's magnetic field. ARRHENIUS has calculated that his hypothetical negative 

 particles will travel from the Sun to the Earth in about 45 hours. BIRKELAND has produced artificially, in 

 the neighbourhood of a small magnetic sphere, or terella, in a high vacuum, electric discharges bearing a 

 close resemblance to various types of aurora, and has attempted to establish a direct connection between 

 individual auroras and the magnetic disturbances recorded during them. Adopting BlRKELAND's 

 hypothesis of kathode rays, STORMER has carried out elaborate mathematical calculations as to the paths 

 of these rays within the Earth's magnetic field, and has concluded that they can approach the surface only 

 in the regions surrounding the magnetic Poles. Mr. MAUNDER has not attempted to assign any exact 

 physical properties to the discharge which he postulates, beyond supposing that it emanates from a sun- 

 spot, and travels out not in all directions but in a compact mass, like a water jet from a hose. The 

 commencement of a magnetic storm answers to the Earth commencing to cross the jet, and the storm 

 continues until the transit of the jet has been accomplished. Supposing the sun-spot to continue active 

 for a time in excess of one complete revolution of the Sun, the Earth will naturally cross the jet again after 

 an interval of about 27J days, and a second magnetic storm will be experienced. 



If magnetic storms are due to electric carriers or ions of any type, the incidence of the magnetic storm 

 will depend on the Earth's magnetic field, and until the properties of the ions are known the laws governing 

 the intensity of disturbances at different parts of the Earth's surface cannot be inferred. If the carriers are 

 kathode rays, then, assuming STORMER'S calculations correct, magnetic disturbances would seem to be 

 necessarily always greater near the magnetic Poles than towards the magnetic Equator, but further than 

 this we cannot apparently go at present. 



86. Direct magnetic action from the Sun was considered long ago by KELVIN, who made a rough 

 calculation of the amount of energy which on this hypothesis would be expended during a largo magnetic 

 storm. The result which he reached seemed to KELVIN inadmissably large. Physicists' ideas, however, as 

 to solar energy have considerably altered of late, and I think it will be generally allowed that in our 

 present state of knowledge it is safest to admit the possibility of expenditures of energy which 20 years 

 ago would have appeared incredible. It must also be allowed that, whilst physical speculators in the 

 past do not seem to have been deterred by the absence of outward and visible signs of magnetism in the 

 Sun, still the discovery by Professor HALE of the Zeeman effect in light received from sun-spot areas 

 affords a direct evidence which has hitherto been lacking. Dr. SCHUSTER, it is true, has already 

 discounted Professor HALE'S discovery to some extent, by publishing in ' Nature ' an estimate of the 



