PRESIDENTIAL ADDRESS, S.T.E., 1874. 221 



three elements, deflection, dip, and total intensity, 

 all vary every day with the ordinary diurnal 

 variation, and irregularly with the magnetic storm. 

 The magnetic storm is always 'associated with a 

 visible phenomenon, which we call, habitually, 

 electrical : aurora borealis, and, no doubt, also the 

 aurora of the southern polar regions. 



We have the strongest possible reasons for be- 

 lieving that aurora consists of electric currents, 

 like the electric phenomena presented by currents 

 of electricity through what are called vacuum 

 tubes, through the space occupied by vacuums of 

 different qualities in the well-known vacuum tubes. 

 Of course, the very expression " vacuums of dif- 

 ferent qualities " is a contradiction in terms. It 

 implies that there are small quantities of matter 

 of different kinds left in those nearest approaches 

 to a perfect vacuum which we can make. 



It is known to you all that aurora borealis is 

 properly comparable with the phenomena presented 

 by vacuum tubes. The appearance of the light 

 the variations which it presents, and the magnetic 

 accompaniments, are all confirmatory of this view 



