13 



ing 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 northern. That such should be the natural 

 consequences of such disturbances is too evident to require 

 further explanation. 



Besides these regular changes to w T hich the needle and the 

 mercury 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 

 appearance 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 me- 

 ridian; and that the beams perpendicular to the horizon are 

 only those on the meridian. It has been found that on the days 

 w T hen 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 

 change in the density or property of the currents, and not from 

 any changes in their directions. That there is magnetic matter 

 in the atmosphere is indubitable, and that this matter is con- 

 stantly acted upon by the currents cannot be doubted. How- 

 ever, it must not be expected that the variations are alike at all 

 places, because the disturbances in the equilibrium of the mag- 

 netic currents are influenced by various causes of a local nature. 

 Resembling the weather in this respect, these variations may 

 differ at different places at the same instant of time. 



The next point to be considered is the direction in which the 

 magnetic currents move. If the currents, as we have previously 

 stated, emanate from the south pole of the earth and enter into 

 the north pole, we should expect somewhat different appearances 

 in the south aurora compared with the north ; because from the 

 former they rise from an aqueous element, which must produce 



