.MAGNETISM. 183 



fonth lat. One woull. almost be inclined lo regard this s ; u- 

 gular configuration of closed, almost concentric, lines of decli- 

 nation as the effect of a local character of that portion of the 

 globe ; but if, in the course of centuries, these apparently iso- 

 lated systems should also advance, we must suppose, as in the 

 case of all great natural forces, that the phenomenon arises 

 from some general cause. 



The horary variations of the declination, which, although 

 dependent upon true time, are apparently governed by the 

 Sun, as long as it remains above the horizon, diminish in an- 

 gular value with the magnetic latitude of place. Near the 

 equator, for instance, in the island of Rawak, they scarcely 

 amount to three or four minutes, while they are from thirteen 

 to fourteen minutes in the middle of Europe. As in the whole 

 northern hemisphere the north point of the needle moves from 

 east to west on an average from 8 in the morning until 1 at 

 mid-day, while in the southern hemisphere the same north 

 point moves from west to east,* attention has recently been 

 drawn, with much justice, to the fact that there must be a 

 region of the Earth between the terrestrial and the magnetic 

 equator where no horary deviations in the declination are to bo 

 observed. This fourth curve, which might be called the curve 

 of no motion, or, rather, t/ie line of no variation of horary 

 declination, has not yet been discovered. 



The term magnetic poles has been applied to those points 

 of the Earth's surface where the horizontal power disappears, 

 and more importance has been attached to these points than 

 properly appertains to them ;t and in like manner, the curve, 

 where the inclination of the needle is null, has been termed 

 the magnetic equator. The position of this line and its secular 

 change of configuration have been made an object of careful 

 investigation in modern times. According to the admirable 

 work of Duperrey,t who crossed the magnetic equator six times 

 between 1822 and 1825, the nodes of the two equators, that 

 is to say, the two points at which the line without inclination 

 intersects the terrestrial equator, and consequently passes from 

 one hemisphere into the other, are so unequally placed, that 

 in 1825 the node near the island of St. Thomas, on the west- 



* Arago, in the Annvairc, 1836, p. 284, and 1840, p. 330-338. 



t Gauss, Allg. Theorie des Erdmagnet., $ 31. 



t Duperrey, De la Configuration de I'Equateiir Magnttique, in tha 

 Annales de Chimie, t. xlv., p. 371 and 379. (See, also, Morlet, iu tho 

 Mlmoires prtscntts par divers Savons a V Aca i. Roy. dcs Sciences, t Hi, 

 P 2.) 



