Geology and Magnetism. 499 



during the day, and rising again to its former height in the night. 

 In the northern regions, such as Denmark, Iceland and Greenland, 

 the diurnal variations, are greater and less regular, in the height of 

 the mercury as well as the direction and dip of the needle ; but in 

 advancing from the north to the equator the diurnal variations 

 diminish. In the southern hemisphere the daily variation of the 

 needle is in an opposite direction, the north end of the needle mov- 

 ing to the east at the same hours that it does to the west in the 

 northern hemisphere. 



To give an occular illustration, we may conceive the magnetic 

 current as a string from pole to pole : if the string be drawn from 

 its meridional position at the equator, its relative direction or the 

 angle formed between it and the meridian will be the same ; but if 

 we compare the direction on each side by looking towards the north 

 alone, we shall observe that the bend of the string will be — say 

 north-west in the southern hemisphere and north-east in the north- 

 ern. That such should be the natural consequences of such dis- 

 turbances is too evident to require further explanation. 



Besides these regular changes to which the needle and the mer- 

 cury in the barometer are subject, they are often affected with 

 sudden and extraordinary movements, to which Baron Humboldt 

 has given the name of magnetic hurricanes, during which the 

 needle often oscillates several degrees on each side of its mean 

 position. The vibrating action of the needle during the appear- 

 ance of the aurora is well known. It is also known that the 

 luminous beams of the aurora are more or less parallel, or rather 

 corresponding to the convergence of the dipping-needle ; that the 

 rainbow-like arches are seen on either side of the meridian ; and 

 that the beams perpendicular to the horizon are only those on the 

 meridian. It has been found that on the days when the southern 

 aurorae take place, the same phenomenon is observed also in the 

 north : the one appears to be the cause of the other, and is therefore 

 simultaneously produced, similar to the electric sparks seen at the 

 poles of an artificial globe. 



Although the aurora lights are generally accompanied with 

 oscillations of the magnetic currents, it is not necessary that they 

 should always be so affected, because the light may arise from a 



