176 COSMOS. 



and over a great extent of the South Sea, almost as far as the 

 meridian of Pitcairn and the group of the Marquesa Islands, 

 between 20 north and 45 south lat. One would almost be 

 inclined to regard this singular configuration of closed, almost 

 concentric lines of declination, as the effect of a local cha- 

 racter of that portion of the globe ; but if in the course of 

 centuries these apparently isolated 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 

 angular 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, whilst 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 morn- 

 ing until 1|- at mid-day, whilst 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 declina- 

 tion are to be observed. This fourth curve, which might be 

 called the curve of no motion, or rather the 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 ; f 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 



less than 2 (Erman, in Pogg. Annal., bd. xxxi. 129). Yet Cornelius 

 Schouten, on Easter Sunday, 1616, appears to have found the declination 

 null, somewhere to the south-east of Nukahiva, in 15 south lat. and 

 132 west long., and consequently in the middle of the present closed 

 isogonal system. (Hansteen, Magnet, der Erde, 1819, 28.) It must 

 not be forgotten, in the midst of all these considerations, that we can 

 only follow the direction of the magnetic lines in their progress, as they 

 are projected upon the surface of the Earth. 



* Arago, in the Annuaire, 1836, p. 284, and 1840, pp. 330-388. 



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



