336 P ro f' Oersted on Electro-magnetism. [Nov. 



as they must do, if the variation depended upon the increase or 

 decrease of the magnetic powers of the poles. 



It is more probable that the electro-magnetic state of the sur- 

 face of the earth determines these changes. Not having a suffi- 

 cient number of observations upon this subject to found princi- 

 ples upon, nor having sufficiently developed the principles of the 

 electro-magnetism of the earth to be able to arrange the obser- 

 vations which we possess, we ought to content ourselves at first, 

 with indicating the acknowledged analogy which the diurnal and 

 annual variations of the needle, with the periods of the day and 

 seasons. 



I have framed and examined many hypotheses as to the cause 

 of the variations of the needle without satisfying myself. The 

 different direction which the electro-magnetic belt receives by 

 the united action of the annual and diurnal motions of the earth, 

 the yearly and daily variations which occur in the figure of the 

 electro-magnetic belt, the discharges which may occur when the 

 electro-magnetic effect is at its maximum, the inequalities which 

 are produced by the different effects of the sun upon the land and 

 the sea, are considerations that have not yet given me sufficient 

 agreement with the phenomena which have been observed in 

 different parts of the earth. The frequent and unforeseen varia- 

 tions of the magnetic needle seem to depend upon electro-mag- 

 netic discharges, of which we have not at present any experi- 

 mental knowledge. Among such discharges I particularly 

 reckon polar light, known by the name of aurora borealis. I do 

 not, on this account, oppose the opinion of the celebrated Biot ; 

 for I think it very probable that these discharges occur in certain 

 clouds. Tempests have also a well-known influence upon the 

 magnetic needle, which no longer surprises us after having found 

 magnetism in every electrical discharge. It appears to me also 

 very probable that several discharges occur in the air, and, per- 

 haps, even in the earth, without our perceiving them. Among 

 other irregularities of the needle are those which embarrass 

 persons who have geometrical operations to execute in the hot 

 days of summer in the open air ; these seem to be owing to such 

 imperceptible discharges. I hope that in future the magnetic 

 needle will be used as a meteorological instrument. Weak 

 needles seem to be preferable for this purpose, because the 

 directing power of the earth produces only a weak action upon 

 them, while a neighbouring electrical discharge has a marked 

 effect. This agrees perfectly with the observations of Cassini, 

 according to which a weak needle was subject to many irregula- 

 rities, which he did not observe with a stronger one. But I would, 

 above all, propose strong needles in meteorological researches, 

 suspended, however, in such a manner that the magnetism of the 

 earth may possess but little or no influence upon it. In order, how- 

 ever, to determine the direction of the discharges, it is requisite to 

 have needles differently suspended. The mode of suspension repre-. 



