THE AURORA. 201 



in the western hemisphere, is shown at Lat. 22 N., Long. 

 75 W. from Greenwich ; and its northern limit, on 

 the same meridian, at Lat. 58 N. In the eastern 

 hemisphere, its southern and northern limits, on the 

 same meridian, are between 47 N. and 77 N. 



The increased width and number of the lines, towards 

 the northern limit, show a great increase in the fre i 

 qnency, brilliancy, and duration of the auroras in that 

 region. 



It is also found, that the position of this auroral belt 

 varies at different seasons of the year; reaching its 

 southern limit near the equinoxes, and its northern 

 limit near the solstices. 



The results given in the above chart must be regarded 

 as approximate, rather than strictly accurate ; as the data 

 on which they are based were more or less imperfect. 



CAUSES OF THE AURORA. Having now examined 

 the various phases of auroral phenomena, and their 

 location, we are prepared to investigate more fully the 

 causes by which they are produced. 



The earth has already been described as a thermo- 

 electric battery, and the atmosphere as a Ley den jar; 

 the one a generator and the other an accumulator; and, 

 in the combination of the two, we may look for the 

 principal cause of the aurora. 



We have seen that electric movement is from higher 

 to lower temperature, producing earth currents on east 

 and west lines, governed by the earth's rotation, and 

 by solar and lunar influence. But the greater differ- 

 ence of temperature between the equatorial and polar 

 regions must produce north and south currents of far 

 greater energy than these east and west currents. 



It has also been shown, that a change of potential, in 



