VARIATIONS OF TERRESTRIAL MAGNETISM. 297 



if the phenomenon continues to follow the same course ; the mag- 

 netic pole will then be on the other side of the north pole in 

 reference to us. Since 1666 the inclination in Paris has been 

 continually decreasing : it will attain a minimum when the declination 

 is null. 



The variations with a short period seem to be connected with 

 the apparent motion of the sun, of the moon, etc., and are governed 

 by laws which at present are not well known. 



The mean values of the declination, for instance, have in one 

 and the same place a well-marked daily oscillation with two maxima 

 and two minima. The amplitude of the excursion of the magnet is 

 far greater during the day than during the night, and the time of the 

 extreme variations is very different according to the positions of the 

 stations. Thus, while the average maximum westerly deviation 

 over a whole year is at 9 a.m. at Hobart Town (Tasmania), Batavia, 

 the Cape, and St. Helena, this time corresponds to the maximum 

 easterly variations in the northern hemisphere. The maximum 

 westerly excursion is at i p.m. at Toronto (Canada), London, and 

 Paris; at 2 o'clock at St. Petersburg; at 3 at Nertchinsk and 

 Pekin. The hours of these maxima and minima vary, moreover, 

 with the seasons. The other magnetic elements, inclination and 

 components of the force, present analogous oscillations. 



By eliminating the mean daily variation from observations rela- 

 tive to the various magnetic elements, we may refer them to the 

 lunar day, and we thus find a regular variation in the residual effects. 



There is, further, an annual periodical variation. 



Finally, the accidental variations themselves, which seem to 

 be produced simultaneously over a great extent, if not over the 

 whole surface of the globe, and which are ordinarily known as 

 perturbations or magnetic storms, appear also to occur in certain 

 annual or secular periods as regards their main effects/ These 

 perturbations are directly related to the phenomenon of the aurora 

 borealis, and are accompanied by accidental currents in telegraph 

 wires. 



