246 AUEOEA BOREALIS. 



electric currents circulating around it, wliicli electric currents are sensibly dis- 

 turbed hy tbe action of the larger planets. Such an effect might be expected 

 to take place simultaneously in both hemispheres, and in conformity with the 

 results of experiments with artificial magnets, the exhibition of light should take 

 place chiefly in the region about the magnetic poles of the earth. 



We are thus led to regard great auroral displays as no longer an exclusively 

 atmospheric phenomenon, and as being to an important extent the result of the 

 influence of extra-terrestrial forces. But if these extraordinary electric currents 

 are mainly determined by extra-terrestrial forces, then since the earth exhibits 

 many of the properties of a great and permanent magnet, the two magnetic 

 poles of the earth ought to exert opposite influences, and we should expect 

 that the currents in the neighborhood of the two poles would move in contrary 

 directions. We are thus naturally led to infer a system of circulation some- 

 what similar to that siaggested by Mr. B. V. Marsh*, and which is illustrated 

 by figure 6, where N and S are supposed to represent the north and south 

 Uiagnetic poles of the earth, n and * the poles of an imaginary magnet, repre- 

 senting the magnetism of the earth. The east and west bands represent 

 auroral arches upon Avhich stand auroral streamers. The dotted lines 

 represent magnetic curves passing from auroral streamers in the northei-n 

 hemisphere to streamers in the southern hemisphere, showing the path pursued 

 by the currents of electricity in passing from one hemisphere to the other above the 

 atmosphere. It is not clear from Mr. Marsh's paper that he supposed a regular 

 flow of positive electricity through the earth from north to south, and above the 

 atmos[>here from south to north, but this seems to me to be necessary to render 

 hLs hypothesis complete. 



If, then, we regard great auroral displays as mainly determined by terrestrial 

 forces, the system of circulation previously described seems the most natural 

 one; but if they are determined mainly by extra-terrestrial forces, the system of 

 circulation just described appears the most probable. The two hypotheses 

 substantially agree, so far as the phenomena can be observed in the northern 

 liemisphere, but they lead to opposite results in the southern hemisphere, where 

 the first hypothesis supposes that the motion of positive electricity along the 

 surface of the earth is from south to north, and the latter supposes it to be from 

 north to south. If the direction of this motion could be determined by direct 

 observation, it would decide between the two hypotheses; and such observations 

 nn'ght doubtless be made in Australia. During the auroras of August 28 and 

 29, 1S59, the wires of the electric telegraph in Australia were disturbed to such 

 a degree that it was almost impossible to transmit any continuous message.f 

 It does not appear that any measures were adopted to determine the direction 

 of these electric currents. If, during some future auroral display, such obser- 

 vations could be made, they would probably furnish the experimeutum crucis 

 to decide between these two hypotheses. 



In attempting to explain the phenomena of the aurora, I have been led to 

 describe hypotheses which, by the progress of science, are liable any year to be 

 disproved. These hypotheses, therefore, must not be regarded as established 

 principles, but simply as convenient formulse for connecting fiicts which other- 

 wise appear disjointed. Such hypotheses often prove useful for the promotion 

 of science by suggesting new observations or researches, like the one just men- 

 tioned respecting the direction of electric currents in the southern hemisphere. 



In conclusion, I will make a few suggestions addressed especially to the me- 

 teorological observers of the Smithsonian Institution : 



1. It is desirable that there should be preserved a continuous and complete 

 record of all visible auroras at a variety of stations. Such observations are de- 



* Americ-au Journal of Science, n. s., v. 31, p. 311. 

 t American Jcurual of Science, u. s., v. 'i'i, p. 8. 



