366 AURORA BOREALIS. 



They are most probably in the upper regions 

 of the atmosphere, what thunder storms are near 

 to the surface of the earth.* The aurora? are 

 seldom, if ever, seen between the tropics, be- 

 cause the upper atmosphere is not condensed 

 there, being of uniform temperature. Even near 



* Most of the appearances presented by the aurorae have been 

 imitated by artificial means. Dr. Priestley inserted with cement, 

 into the top of a tall receiver, a wire not very acutely pointed. 

 He then exhausted the receiver, and presented the knob of the 

 wire to the conductor, when every spark passed through the 

 vacuum in a broad stream of light, which often divides into a 

 variety of beautiful rivulets, that are continually changing their 

 course, uniting and dividing again in the most pleasing manner : 

 thus representing in miniature what has been called the streamers 

 of the aurora. When the wire is electrified minus, or negatively, 

 instead of streams of fire, there is a uniform luminous appear- 

 ance like a white cloud, or like the milky way in a star light 

 night, which remains for a considerable time, representing the 

 apparently stationary and diffused light of the aurora. (Priestley's 

 History of Electricity, p. 524.) 



We can assign no limits to the various phenomena which the 

 electric fluid may exhibit under different circumstances. The me- 

 dium in which it is formed, in the upper regions of the atmosphere, 

 is doubtless greatly rarefied ; perhaps equal to that of a receiver 

 which has been rarefied 100 times. It would seem, however, 

 that the aurora sometimes reaches the surface of the earth. It 

 is related by Bergman, that persons travelling over the high 

 mountains of Norway, have been enveloped in it ; and Captain 

 Ross states, that he has witnessed it very near to the surface of 

 the earth ; and that it has caused the deflection of his compass 

 needle as much as thirty and forty degrees. M. Hansteen also 

 found that when the aurorse were vivid, the horizontal needle was 

 restless, quivered, and varied from three to five degrees from its 

 ordinary place. 



