THE AURORA. 473 



formation. In 1870, the aeronaut Rollier, escaping from besieged 

 Paris in a balloon, landed in Norway, and he claims to have heard 

 a persistent sound the whole time lie was in a certain cloud, accom- 

 panied by a strong odor like ozone, very irritating to the bronchial 

 tubes. A fine aurora was observed at precisely that time. Berg- 

 mann compared this odor to that of sulphur, and Trevelyan to that of 

 electricity (?). 



It is not necessary, at this juncture, to enter into a discussion of 

 the extent, position, or periodicity of auroras, nor their relation to 

 sun spots. But some new facts may be added that are valuable. 

 There is a close resem.blance between certain effects of the aurora and 

 cirro-cumulus and cirro-stratus clouds, w^hich are the highest clouds; 

 so that it is difhcult to tell whether a given effect is due to real 

 auroral light or to these high clouds lit up by reflected light. Some- 

 times these bands of clouds stretch out in long parallel lines; again, 

 they are due north and south and lie parallel to the needle of the 

 compass, and are then called " polar bands." It has been discovered 

 that when these polar bands occur during the day an aurora follows 

 that night. 



As long ago as 1580 and 1590, Tycho Brahe observed that the 

 appearance of these polar bands and halos coincided with the pres- 

 ence of spots on the sun. Klein, in our own day, comparing twenty- 

 five years of observations made with the greatest care at Cologne 

 by Dr. Garthe, has confirmed the fact that these high cirrus clouds 

 and polar bands follow as to their frequency the same laws as the 

 spots on the sun; they succeed each other or even coexist. Many 

 observers of the aurora are of the opinion that the appearance of 

 the aurora depends on the presence of these clouds in the sky, which 

 are in turn due to magnetic disturbances of the earth's photosphere 

 originating in the sun spots. There are other evidences that the 

 aurora is intimately connected witli material particles in the atmos- 

 phere, like clouds and fog; for instance, when two or more rays in an 

 aurora cross, the light is augmented, or when an aurora makes a fold 

 on itself as in drapery forms, showing that there are two material 

 ''thicknesses" of the substance; moreover, the wind also acts on 

 the aurora, which is torn after a tempest, showing that the wind has 

 acted upon luminous clouds which are part of the aurora; finally, 

 the presence of clouds seems to favor the formation and development 

 of auroras. So there is an intimate relation between them. 



The rare phenomenon, St. Elmo's fire (balls of fire alighting on 

 ship masts and spars), is frequent during auroras, but this is about 

 the only form of electric disturbance of the atmosphere occurring 



