106 COSMOS. 



stations this opposition and the periodicity of the horary va- 

 riation in the dip have been firmly established by several 

 thousand regularly prosecuted observations, which have all 

 been submitted to a careful discussion since 1840. The 

 present would seem the most fitting place to notice the facts 

 that have been obtained as materials on which to base a 

 general theory of terrestrial magnetism. It must, however, 

 first be observed, that if we consider the periodical varia- 

 tions which are recognized in the three elements of terrestrial 

 magnetism, we must, with Sabine, distinguish, in the turn- 

 ing hours at which the maxima or minima occur, two great- 

 er, and therefore more important, extremes, and other slight 

 variations, which seem to be intercalated among the others, 

 as it were, and which are for the most part of an irregular 

 character. The recurring movements of the horizontal and 



variation is sufficiently sensible to be observed with a lens. I would 

 still insist upon the fact that changes of inclination are not sufficient 

 .to explain the change of intensity, deduced from the observation of a 

 horizontal needle. An augmentation of temperature, all other cir- 

 cumstances remaining the same, retards the oscillations of the nee- 

 dles. In the evening the temperature of my horizontal needle is al- 

 ways higher than in the morning ; hence the needle must on that account 

 make fewer oscillations in a given time in the evening than in the 

 morning ; in fact, it oscillates more frequently than we can account 

 for by the change of inclination, and hence there must be a real aug~ 

 mentation of intensity from morning till evening in the terrestrial mag- 

 netic force." Later and more numerous observations at Greenwich, 

 Berlin, St. Petersburg, Toronto, and Hobarton, have confirmed Ara- 

 go's assertion (in 1827) that the horizontal intensity was greater in 

 the evening than toward morning. At Greenwich the principal max- 

 imum of the horizontal force was about 6 P.M., the principal minimum 

 about 10 A.M., or at noon ; at Schulzendorf, near Berlin, the maxi- 

 mum falls at 8 P.M., the minimum at 9 A.M. ; at St. Petersburg the 

 maximum falls at 8 P.M., the minimum at llh. 20m. A.M. ; at To- 

 ronto the maximum falls at 4 P.M., the minimum at 11 A.M. The 

 time is always reckoned according to the true time of the respective 

 places (Airy, Magn. Obscrv. at Greenwich for 1845, p. 13; for 1846, 

 p. 102; for 1847, p. 241; Riess and Moser, in Poggend.,^lnm/erc, bd. 

 xix., 1830, s. 175 ; Kupffer, Compte rendu Annuel de P Observatoire, Cen- 

 trale Magn. de St. Peter sb., 1852, p. 28; and Sabine, Magn. Observ. 

 at Toronto, vol. i., 1840-1842, p. xlii.). The turning hours at the 

 Cape of Good Hope and at St. Helena, where the horizontal force is 

 the weakest in the evening, seem to be singularly at variance, and 

 almost the very opposite of one another (Sabine, Magn. Observ. at the 

 Cape of Good Hope, p. xl., at St. Helena, p. 40). Such, however, is 

 not the case further eastward, in other parts of the great southern 

 hemisphere. "The principal feature in the diurnal change of the 

 horizontal force at Hobarton is the decrease of force in the forenoon, 

 and its subsequent increase in the afternoon" (Sabine, Magn. Obs. at 

 Hobarton, vol. i., p. liv., vol. ii., p. xliii.). 



