A.— MATHEMATICAL AND PHYSICAL SCIENCES; 



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series corresponds to a uniform magnetisation of the earth, and thus a 

 physical interpretation was obtained. In this uniformly magnetised 

 sphere the magnetic potential at any point outside the sphere is given by 

 H,, r" cos 8/(f where 



d is the distance of the point from the centre ; 



r is the radius of the earth ; 



the angular north polar distance, 

 and H, the surface magnetic intensity at the equator. 

 The north magnetic pole is situated approximately at 78° N. 69° W., and 

 the whole magnetic field rotates with the earth. 



Secular Variation and Possible Cause. 

 What do we know of this so-called permanent field ? As already stated, 

 sxich variations as occur daily, monthly or suddenly, as in the case of 

 magnetic storms, are phenomena to be associated with other terms of the 

 Gaussian expansion, and are not to be attributed to changes in the main 

 or permanent field. Indeed, our information with regard to changes in 

 this field is very limited, and the difficulty that confronts us to explain 

 the existence and maintenance of the field is correspondingly great. The 

 observed facts are that the declination, inclination and intensity are not 

 constant, and over long periods of time their values change appreciably. 

 The changes over a comparatively short period are well illustrated by 

 the results obtained at Kew Observatory during a period of 50 years. 



Very long period observations are not so reliable because of the dis- 

 similarity of instruments and methods of observation, but it is certain 

 that in London the declination has changed slowly from about 11° east in 

 1580 to 24° 30' west in 1816, since when it has fallen to about 13° west 

 in 1920. Similarly, the inclination has changed from about 72° 0' in 1580 

 to 74° 42' in 1720, since when it has gradually changed to about 66° 40' 

 in 1920. Examination of the available data leads to the conclusion that 

 the magnetic field may be regarded as moving westwards along a parallel 

 of latitude at the rate of a few seconds of angle per day, the rate of move- 

 ment being such that, if continued for some hundreds of years, the field 

 would make a complete revolution round the earth, the motion being 

 in the opposite direction to that of the earth's rotation. The secular 

 variation may therefore be regarded as caused by change in direction 

 of the axis of magnetisation. If outer space is a conducting medium, 

 there will be relative motion between the magnetic field of the earth 

 and it, and the moving field will induce currents in the outer con- 

 ducting medium, and these currents in turn will react and induce other 



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