246 RATE OF REVOLUTION OF THE MAGNETIC POLES. 



When we consider the phenomena of terrestrial mag- 

 netism carefully, it appears to indicate the action of a 

 power external to the earth itself, and,, as Hansteen 

 conceives,, having its origin from the action of the sun, 

 heating, illuminating, and producing a magnetic tension, 

 in the same manner as it produces electrical excitation 

 and actino-chemical action. 



The movements of these magnetic poles have been 

 the subject of extensive and most accurate observation 

 in every quarter of the globe. In London, during 

 1657-1662, there was no magnetic variation ; tlie agonic 

 line passing through it. The variation steadily increased, 

 until, in 1815, it amounted to 24 15' IT", since which 

 time it has been slowly diminishing. In addition to 

 this great variation, we have a regular annual change 

 dependent on the position of the sun, in reference to the 

 equinoctial and solstitial points, which was discovered 

 by Cassini, and investigated by Arago and others. 

 Also a diurnal variation, which movement appears to 

 commence early in the morning, moving eastward until 

 half-past seven, A.M., when it begins to move westward 

 until two, P.M., when it again returns to the east, and in 

 the course of the night reaches the point from which it 

 started twenty-four hours before. 



We have also remarkable variations in what is 

 termed the dip of the needle. It is well known, that a 

 piece of unmagnetized steel, if carefully suspended by its 

 centre, will swing in a perfectly horizontal position, but, 

 if we magnetize this bar, it will immediately be drawn 

 downwards at one end. The force of the earth's 

 polarity, attracting the dissimilar pole, has caused it to 

 dip. 



There is, in the neighbourhood of the earth's equator, 

 and cutting it at four points, an irregular curve, called 

 the magnetic equator, or aclinic line, where the needle 



Gilbeyt's Annalef". Brewstei's Magnetism : Encyclopedia "Mitro- 

 politana. 



