of the Aurora Borealis in North America. 295 



period to the middle of the next period, and last from twenty 

 to twenty-two years, making the interval from the end of one 

 to the beginning of the next about forty-five years. 



6. Belations to Magnetism. — While the forms of the aurora 

 usually appear to be under the control of magnetic forces, 

 yet this is not always the case, since the arches do not always 

 culminate in the magnetic meridian, nor do they always place 

 themselves at right angles to that meridian, nor does the 

 effect on the needle correspond to the different states of 

 intensity of the aurora. 



7. Geographical Belations. — The aurora borealis has remark- 

 able geographical relations, belonging chiefly to the higher 

 latitudes, and only in the great visitations descending below 

 the latitude of 40°, but descending lower on the western than 

 on the eastern continent, and prevailing more in the northern 

 than in the southern hemisphere. 



The foregoing seven propositions appear to embrace the 

 most important facts respecting auroral exhibitions, and are, 

 therefore, deemed to be, in the present state of our know- 

 ledge, the laws of the aurora borealis. But we cannot rest 

 satisfied to let this great visitation of the phenomenon, — 

 w^hich, in its whole progress and duration, we have enjoyed 

 fair opportunities of observing, — pass by, without an effort 

 to assign its true origin, and to explain the causes of its 

 mysterious phenomena. 



After thunder and lightning were first proved by Franklin 

 to be caused by electricity, it was taken for granted, almost 

 without discussion, that the aurora borealis was produced by 

 the same agent ; and this hypothesis has still very numerous 

 adherents. We endeavour, then, first, to shew that the 

 aurora borealis is not produced by electricity. We argue, — 

 1. That it is unsafe to infer an identity of origin from a 

 resemblance between the aurora and certain appearances of 

 electricity in passing through an exhausted tube, and that 

 the resemblance itself is greatly overrated. 2. That such 

 an origin is inconsistent with the great extent of the pheno- 

 menon. 3. That electrometers do not indicate the presence 

 of electricity in any unusual degree, during an aurora. 4. 

 That these exhibitions are scarcely known in the equatorial 



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