1832] WRITINGS OF JOSEPH HENRY. 63 



trial magnetic intensity, to be nine hundred and seventy- 

 nine seconds. The accidental errors in the above observa- 

 tions do not probably exceed in any case one second. 



At the time of registering the above observations, I had 

 not seen the following remark of Prof. Hansteen, which 

 was subsequently met with in the 12th volume of the Edin- 

 burgh PhilosophicalJournal: — "A short time before the aurora 

 borealis appears," says Prof. Hansteen, "the intensity of 

 the magnetism of the earth is apt to rise to an uncommon 

 height; but so soon as the aurora begins, in proportion as its 

 force increases, the intensity of the magnetism of the earth 

 decreases, recovering its former strength by degrees, often 

 not till the end of twenty -four hours." * This statement, 

 founded on observations made in Norway, is a precise de- 

 scription of the phenomenon observed in Albany ; and 

 should it be found a general, or even a frequent occurrence, 

 that a great increase of intensity precedes the appearance of 

 the aurora, it would perhaps reconcile many apparent dis- 

 crepancies in the different accounts of magnetic influence 

 of the meteor. 



Prof. Hansteen also remarks, in the same paper, that "The 

 polar lights seem to be the effect of an uncommonly high 

 magnetic intensity, which lets itself off, as it were, by the 

 aurora, and thus sinks under its common strength." Noth- 

 ing however can with certainty be deduced from these ob- 

 servations, in reference to this supposition ; since the mag- 

 netic intensity at any place, as exhibited by the vibrations 

 of the horizontal needle, may change while the absolute 

 force or intensity of the whole earth remains the same. If 

 we represent by F the whole force in the direction of the 

 dipping needle, by «5 the dip in degrees, and by jffthe hori- 

 zontal force, we shall have, by a well known law, 



cosm 8 



*I find the same observation has also been made by Humboldt; and also 

 a similar one by Van Swinden, who remarks, that the variation of the 

 needle increases when the aurora borealis is approaching. Journal Royal 

 Institution. Young's Natural Philosophy, vol. 2, p. 442. 



