1881. J un Magnetic Disturhancef Aurorce, and Earth Currents. GG9 



The lower regions of tljc earth's atmosphere offer great resistance 

 to the passage of electricity, but as we ascend the pressure diminishes, 

 and the electric resistance diminishes, until at last, at a height of 

 between 30 and 40 miles, a level is reached where the air offers least 

 resistance to the passage of electricity, where the pressure is about 

 * 397 of a mm. ; and above that level the electrical resistance again 

 increases, so that at a height of about 80 miles the battery of 11,000 

 cells would not cause a spark to pass. 



If we take a tube which has not been very highly exhausted, we 

 see that the light from the positi-ve pole extends nearly through the 

 tube, and the dark space around the negative pole is small. As the 

 exhaustion proceeds and the pressure of the air is diminished, the 

 electric spark passes through greater and greater lengths and changes 

 its character, until we get to the pressure corresponding to the least 

 resistance. Beyond that the resistance increases, the dark space 

 around the negative pole expands, and the molecules fly about more 

 freely ; those on the negative pole being charged with electricity, 

 and being repelled from it, proceed for a long distance in straight 

 lines, and possess the power of causing bodies on which they strike 

 to glow. In Mr. Crookes's tubes we get very beautiful effects from 

 this glowing of the glass tube itself, or from the glowing of 

 substances in the path of the stream. We may regard this as a 

 stream of molecules of gas charged with electricity, and we see the 

 difference between this stream and the electric current in a vacuum 

 tube at lower exhaustion by the action of the magnet upon it. In one 

 case the current going through the molecules from pole to pole in the 

 tube is bent out of its course by the magnet, and symmetrically by 

 the two poles, and returns to its path, the line of least resistance 

 through the molecules; whereas the stream of molecules at the higher 

 exhaustion carrying their electricity with them, are carried away by 

 the electric charge upon them, and get utterly lost and scattered on 

 striking the side of the tube, yielding up a great deal of energy in 

 the form of heat to the tube, or to the glowing platinum or other 

 substance in the tube. 



The aurora, as seen in the north-eastern parts of Siberia, where it 

 is very often very brilliant, is described as consisting of single bright 

 pillars rising in the north and in the north-east, gradually covering a 

 large space of the heavens. These rush about from place to place, 

 and, reaching up to the zenith, jn-oduce an a2)pearance as if a vast 

 tent was spread in the heavens, glittering with gold, rubies, and 

 sapphires. 



More exact attempts have been made to describe the aurora, and 

 perhaps I may be allowed to quote Dalton's description of an aurora 

 as seen by him. 



A remarkable red appearance of clouds was noticed in the southern 

 horizon, which afforded light enough to read by, and a remarkable 

 effect was expected. He says: — "There was a large luminous 

 horizontal arch to the southward, and one or more concentric arches 



