MAGNETIC INTENSITY. 83 



both hemispheres, is identical with the period at which the 

 earth is nearest to the sun,* and consequently when its ve- 

 locity of translation is the greatest. At this period, however, 

 when the earth is nearest to the sun, namely, in December, 

 January, and February ; as well as in May, June, and July, 

 when it is farthest from the sun, the relations of temperature 

 of the zones on either side of the equator are completely re- 

 versed, the turning points of the decreasing and increasing 

 intensity, declination and inclination can not, therefore, be 

 ascribed to the sun in connection with its thermic influence. 

 The annual means deduced from observations at Munich 

 and Gottingen have enabled the active director of the Royal 

 Bavarian Observatory, Professor Lamont, to deduce the re- 

 markable law of a period of 10| years in the alterations of 

 declination, j In the period between 1841 and 1850, the 

 mean of the monthly alterations of declination attained very 

 uniformly their minimum in 1843^, and their maximum in 

 1848^. Without being acquainted with these European re- 

 sults, General Sabine was led to the discovery of a periodic- 

 ally active cause of disturbance from a comparison of the 

 monthly means of the same years, namely from 1843 to 1848, 

 which were deduced from observations made at places which 

 lie almost as far distant from one another as possible (Toron- 

 to in Canada, and Hobarton in Van Diemen's Land). This 

 cause of disturbance was found by him to be of a purely ccs- 

 mical nature, being also manifested in the decennial periodic 

 alterations in the sun's atmosphere.^ Schwabe, who has ob- 

 served the spots upon the sun with more constant attention 

 than any other living astronomer, discovered (as I have al- 

 ready elsewhere observcd), in a long scries of years (from 



* Sabine, On the Means adopted for determining the Absolute Values, 

 Secular Change, and Annual Variation rfthe Terrestrial Magnetic Force, 

 in the Phil. Transact, for 1850, pt. i.,'p. 216. In his address to the 

 Association at Belfast (Meeting of the Brit. Assoc. in 1852), he like- 

 wise observes, " that it is a remarkable fact which has been estab- 

 lished that the magnetic force is greater, in both the northern and 

 southern hemispheres, in the months of December,- January, and 

 February, when the sun is nearest to the earth, than in those of May, 

 June, and July, when he is most distant from it ; whereas, if the ef- 

 fects were due to temperature, the two hemispheres should be oppo- 

 sitely, instead of similarly, affected in each of the two periods re- 

 ferred to." 



f Lamont, in Poggend., Annalen, bd. Ixxxiv., s. 579. 



$ Sabine, On periodical Laws discoverable in the mean Fffects oft/ie 

 larger Magnetic Disturbances, in the Phil. Transact, for 1852, pt. i., p. 

 121. Vide supra, p. 75. Cosmos, vol. iv., p. 85. 



