lis THIRD REPORT — 1833. 



hypotheses with actual observation. The most recent attempt 

 of this kind is that by Professor Hansteen. He adopts Halley's 

 hypothesis of four magnetical poles, but considers that they all 

 revolve, and in different periods, the northern poles from west 

 to east, and the southern ones from east to west. The results 

 calculated on this hypothesis agree pretty nearly with the ob- 

 servations with which they are compared ; but as considerable 

 uncertainty attends magnetical observations, excepting those 

 of the variation made at fixed observatories, and especially the 

 early ones of the dip and variation, on which the periods of the 

 poles and their intensities must so much depend, it would cer- 

 tainly be premature to say that such an hypothesis satisfactorily 

 explains the phaenomena of terrestrial magnetism. If we admit 

 that the progressive changes which take place in the direction 

 of the needle are due to the rotation of these poles, we must 

 look to the oscillations of the same poles for the cause of the 

 diurnal oscillation of the needle. Any hypothesis which by 

 means of two or more magnetic poles will thus connect the 

 phaenomena of magnetism, is of great advantage, however un- 

 able we may be to give a reason for the particular positions of 

 the poles, or for their revolution. Hansteen refers these to 

 the agency of the sun and moon. 



Without assigning any cause either for the direction of the 

 needle, or for the progressive change of that direction, attempts 

 have been made to account for its diurnal oscillations. But 

 before taking a review of these, it is necessary that I should 

 state more particularly the precise nature of the phsenomenon. 

 This I cannot do better than by referring to the results de- 

 duced from Canton's observations*. From these it appears 

 that in London, during the twenty-four hours, a double oscilla- 

 tion of the needle takes place, the absolute maximum west 

 happening about half-past one in the afternoon, and the abso- 

 lute maximum east, that is, the minimum west, about nine in 

 the morning; besides which there was another maximum east 

 about nine in the evening, and a maximum west near midnight 

 or very early in the morning, the two latter maxima being small 

 compared with the absolute maxima. Colonel Beaufoy's very 

 extensive series of observations, made when the variation was 

 between 24° and 25° west, (Canton's having been made when 

 it was 19°,) give nearly the same results, the absolute maxima 

 happening somewhat earlier, and the second maxima west 

 about eleven in the evening. 



Canton explained the westerly motion of the needle in the 



» Philosophical Transactions, 1759, p. 398, and 1827, pp. 333, 334. 



