TERRESTRIAL MAGNETISM. 



the atmosphere accessible to our observation. Neither 

 the principal epochs of the diurnal and annual variations of 

 the declination (the annual have been accurately represented 

 for the first time by Sabine from a vast body of observa- 

 tions), nor the periods of the mean intensity of the earth's 

 force, ( 80 ) agree with the periods of maxima or minima in 

 the temperature either of the atmosphere or of the 

 earth's outer crust. The turning-points in the most 

 important magnetic phenomena are the solstices and the 

 equinoxes. 



The epoch at which in loth hemispheres the intensity 

 of the earth's force is greatest, and the direction of the 

 dipping-needle is most nearly vertical, is that of the 

 greatest proximity of the Earth to the Sun,( 81 ) when also 

 the Earth has the greatest velocity in its movement of trans- 

 lation in its orbit. But at the period of perihelion 

 (December, January, and February), and at that of aphelion 

 (May, June, and July), the circumstances of temperature 

 in the two hemispheres, northern and southern, are diame- 

 trically opposed to each other : it follows, therefore, that the 

 turning-points, or change from decreasing to increasing 

 magnetic force, and vice-versa, cannot be ascribed to the 

 sun as the source of heat. 



Annual mean values derived from the observations at 

 Munich and Gottingen have led the active director of the 

 Royal Bavarian Observatory, Lament, to infer the remark- 

 able law of a variation-period of 10 J years in the amount of 

 the mean diurnal variation of the magnetic declination in 

 different years. ( 82 ) In a series lasting from 1841 to 1850, 

 the monthly means of the variations of the declination 

 attained their minimum in 1843^ and their maximum in 



