SICT. XXX. DISTURBANCES. 303 



whole globe is by the same chart about the 25 south 

 latitude and 12 west longitude. In the northern hem- 

 isphere the foci of maximum intensity are in lat. 54 32' 

 N., long. 261 27' E., and lat. 71 20' N., long. 119 57' E., 

 according to M. Gauss's calculations. The magnetic 

 intensity appears to be doubled in the ascent from the 

 equator to Baffin's bay. 



Such are the principal phenomena of terrestrial mag- 

 netism, but it is subject to secular, periodical, and tran- 

 sient disturbances still imperfectly known. In the north- 

 ern hemisphere, the poles, the lines of equal and no 

 variation, the equator, and in short the whole system is 

 gradually moving toward the east, so that the relations 

 observed in Europe two centuries ago have now reached 

 the limits between Europe and Asia, while other parts 

 of the system have moved gradually over to us from the 

 west. In the southern hemisphere the secular motion 

 of the poles and of the whole system is in a contrary 

 direction. The cause of these secular disturbances is 

 altogether unknown. 



The horizontal needle or compass at any one place is 

 also subject to periodic and transient perturbations. 

 Great disturbances occur on the same day, or nearly on 

 the same day, in different years, from causes unknown. 



There are also disturbances which, according to the 

 observations of M. Kreil, in Milan, depend on the decli- 

 nation of the moon and her distance from the earth ; 

 others of shorter duration seem to be intimately con- 

 nected with the motion of the sun in regard to the mag- 

 netic meridian of the place of observation. In conse- 

 quence of the latter, the needle in the same place is 

 subject to diurnal variations: in our latitudes the end 

 that points to the north moves slowly westward during 

 the forenoon, and returns to its mean position about ten 

 hi the evening; it then deviates to the eastward and 

 again returns to its mean position about ten in the 

 morning. 



M. Kupffer of Casan ascertained that there is a noctur- 

 nal as well as a diurnal variation, depending in his opinion 

 upon a variation in the magnetic equator. Magnetic 

 storms, or sudden and great but transient disturbances, 

 take place occasionally in the compass, which are per- 



