On Some Points Connected with Terrestrial Magnetism

Abstract

THE remarks in NATURE, vol. xxii. p. 169, of Messrs. De La Rue and Müller in connection with their most interesting and important researches on rarefied gases induce me to ask the privilege of stating somewhat more fully than I did on June 17 what I conceive to be the position filled by a working hypothesis such as that then mentioned in the science of terrestrial magnetism. Let me begin with the aurora. Here we have a phenomenon which invariably accompanies magnetic storms, on which occasions it occurs simultaneously over a large portion of the globe. Again the recent researches of the above-named gentlemen render it very probable that auroral displays do not occur at a very great height, while it is conceivable that they may occur at times at an altitude of a few thousand feet. Here then we have a phenomenon which is intimately connected with sudden changes of the earth's magnetism. To this we may add earth currents as another phenomenon of the same kind, so that we have earth currents and auroral displays invariably associated with magnetic storms, when these are of marked violence. Now what is the nature of this connection? When we examine the formal laws of these associated phenomena we find that these lead us (almost irresistibly, as I think) to conclude that earth currents and aurorae are secondary discharges caused by sudden changes in the earth's magnetism, no matter how these changes are produced. So strong is the evidence of form in this instance that the late eminent magnetician John Allan Broun expressed to me his belief that earth currents and aurorae were connected with magnetic storms in the way above mentioned.