ATMOSPHERIC ELECTRICITY. 



397 



seen in its greatest perfection within the arctic and an- 

 tarctic circles.* 



The aurora is not a local phenomenon, but is seen simultaneously at places 

 widely remote from each other, as in Europe and America. 



The general height of the aurora is supposed to be between one and two 

 hundred miles above the surface of the earth ; but it sometimes appears 

 within the region of the clouds. 



Auroras occur more frequently in the winter than in the summer, and aro 

 only seen at night. They afleet in a peculiar manner the magnetic ncedlo 

 and the electric telegraph, and as the disturbances occasioned in these in- 

 struments are noticed by day as well as by night, there can be no doubt of the 

 cxM^urrence of the aurora at all hours. The intense light of the sun, however, 

 renders the auroral light invisible during the day. 



Fig. 331. 



The accompanying figure represents one of the most beautiful of the au- 

 roral phenomena. 



It has often been asserted, and on good authority, that sounds have been 

 heard attending the phenomena of the aurora, hke the rustling of silk, or the 

 Bound and crackling of a fire. On this point, however, there is great differ- 

 ence of opinion. 



Axiroras appear to be subject to some variation in their appearance, extend- 

 ing through a circle of years. Thus, from 1705 to 1752, the northern lights 

 became more and more frequent, but after that for a period they were seen but 

 rarely. Since 1820 they have been quite frequent and briUiant. 



• In the arctic and antarctic circles, when the Run is absent, the aurora appears with a 

 magnificence unknown in other regions, and affords light Bufi5cient for many of the ordi- 

 Dary out-door employmeuts. 



