METEOROLOGY. 



423 



indicated au electro- motive force equivalent in extreme cases to sis 

 Daniell's cells. But a few hundreds of Daniell's cells would be quite in- 

 sutiicieut to explain the auroral discharge. There is, however, one in- 

 stance of electric phenomena where we have tremendous tensions to 

 deal with ; and atmospheric electricity of tension sufficient, as in ordi- 

 nary lightning-, to strike across a mile of air at ordinary density would 

 probably be competent to strike across many miles of rarefied air. It 

 has long since been remarked that displays of aurorae seem in some way 

 or other in high latitudes to take the place of thunder-storms in low 

 latitudes. Well, then, I will explain what I imagine takes place. I do 

 not enter into any speculation as to the cause of atmospheric electricity. 

 We know as a fact, from its manifestation, that it exists, and that is 

 sufficient for my purpose. Suppose, now, that the air, especially the 

 higher portions of the air, over a large tract of country, say to the 

 north of us, were more or less highly electrified — positively or negatively 

 as the case may be — we will supijose positively; if the electric tension 

 were sufiicient, we might, since dense air is a non-conductor, have a 

 discharge taking place in the higher regions of the atmosphere where 

 the air is rarefied, and accordingly opposes less resistance to the change. 

 "In the figure the great circle P E ^ e is supposed to represent a sec- 

 tion of the earth by a plane passing through its center. This faint shad- 

 ing outside represents the atmosi)here, the height of course being enor- 

 mously exaggerated. 



Meridian section of the Earth. 



"Suppose in some way or other a portion of the upper atmosphere, as 

 C, got considerably charged positively or negatively, say positively, it 

 would act by induction on the earth below. The opposite electricity, 



