AURORA. BOREALIS. 183 



their disturbing the magnetic needle. But the disturbance of 

 the needle can he accounted for in a more simple manner. 



Some philosophers asserted that the auroras are thousands 

 of miles ahove the surface of the earth, hut the most generally 

 accepted theory is that they are within our atmosphere ; 

 for aeronauts have frequently heen elevated ahove them. 



The principal facts to be borne in mind concerning aurora* 

 are these: 



Firstly, that (in our latitudes) they are always seen to the 

 northward of us, never to the south even although persons to 

 the south of us may see auroras between us and them. 



Secondly, that when they are visible, the evening is usually 

 calm and clear. 



Thirdly, that they influence the compass needle and affect 

 the telegraphic lines. 



Fourthly, that they have different shades of colour. 



We have already shewn by the experiment of the " magnetic 

 curves " that there is a magnetic force or current continually 

 extending from the Poles towards the Equator. The poles of 

 a magnetic battery if introduced into a vase of oxygen will 

 exhibit light, and we have proved that sunlight is produced in 

 Nature from a similar action. We have, moreover, shown, that 

 when the poles of atoms are disturbed, opacity is the result in 

 daylight, and light is the consequence in darkness. We have, 

 therefore, two explanations open to us. The magnetic upper 

 current issuing from the north, being, of necessity, accom- 

 panied by mineral gases which are exuded principally from 

 the polar latitudes, comes iu contact, on a clear and calm 

 evening, with the vegetable gases in the lower atmosphere ; 

 either, then, a chemical action is induced between the two gases, 



