314 METEOROLOGY AND ALLIED SUBJECTS. 



tion whether any noise is developed in connection with the aurora. It 

 is remarkable that in the Southern Hemisphere no one has ever noticed 

 or recorded the least suspicion of noise during an exhibition of the 

 aurora australis. 



(25) Although a definite conclusion as to the precise nature of the 

 aurora is not yet attainable, yet it is worth while to review the numerous 

 hypotheses that have been suggested. We select a few from the very 

 complete review given by Fritz. The hypothesis of an electric discharge 

 dates from the memoir by Canton, London, 1753, and is presented in a 

 form corresponding to our present knowledge of the electricity by De 

 la Eive, 18G5, according to whom positive electricity flows from the 

 equator toward either Pole accompanying thunder storms. But a sim 

 ilar difficulty seems to beset both these explanations, in that no one c«u 

 explain why the two kinds of electricity are not followed by a state of 

 equilibrium immediately after the discharge has taken place. 



Mayer, the founder of the mechanical theory of heat, has sought to 

 explain the origin of atmospheric electricity as due i^rincipally to the 

 friction between the ocean and the trade-winds of the tropical regions, 

 a hypothesis that is partially supported by the observations of Secchi 

 and others, according to whom the north and south winds have definite 

 influences upon the position of the magnetic needle. Muncke and 

 ]\roser have between them elaborated a therm o-electro-maguetic hypoth- 

 esis, according to which, under the influence of the sun's heat, the 

 rotating earth must be an electro-magnet. If the earth becomes mag- 

 netic then, in consequence of its perpetual rotation, the iron contained 

 within the earth's crust will become permanently magnetic, and the 

 magnetic variations, partly periodical and partly appearing as pertur- 

 bations, are fairly explained. The i^eriodical changes would be caused 

 by the periodical changes in the relative positions of the earth, the 

 moon, and the sun, and the variations in the radiation of heat from , 

 the sun's surface, while the extraordinary and irregular disturbances 

 in terrestrial magnetism would be due to the disturbances and changes 

 occurring on the sun's surface, similar to the changes perpetually occur- 

 ring in the earth's atmosphere. The origin of the electricity to which 

 the auroral light is attributable is explained by F. Mohr as due to the 

 friction of currents of air flowing over each other in different directions, 

 and the greater frequency of appearances in high latitudes depends on 

 the dryness of the air. This view is also sustained by Prestel. For 

 the great aurora of February 4, 1872, Mohr computes the total 

 amount of air in motion, and finds an average over the whole of Europe 

 of over one thousand million pounds of air in movement for each Eng- 

 lish mile square, so that the aurora is only an extremely small effect^ of 

 an immense force. But if we attempt to extend the Mohr-Prestel 

 theory to the observations at Spitzbergen and Franz- Josef Land, then 

 the results appear quite different. 



Balfour Stew..;t considers the aurora as a secondary electric current 

 due to small but sudden changes in the earth's magnetism, produced 



