AURORA. 205 



from 100 cells. And the short line from Boston to 

 Dedham, ten miles, showed the disturbing influence as 

 much as the longer lines." 



In these observations, as in those cited in Chapter XII, 

 it has been found that whenever the earth connection is 

 severed, at either end of the line, the current immedi- 

 ately ceases ; proving it to be an earth current, and 

 not a current in the atmosphere. 



The increased intensity of current, on lines having a 

 southeast and northwest direction, noticed during the 

 'aurora of Feb. 4, 1872, is confirmatory evidence of the 

 existence of resultant currents, as explained on page 

 202. 



The hours at which maximum and minimum effects 

 were observed, during the aurora of Nov. 17, 1882, 

 correspond exactly to the hours of maxima and minima 

 potential, and current intensity, already cited. A max- 

 imum having occurred during the three morning hours, 

 beginning just before daylight; a minimum late in the 

 afternoon, and a maximum again after sunset. 



Another cause of the aurora is found in the move- 

 ment of warm air from the torrid to the frigid zones, 

 and of cold air, at a lower altitude, from the frigid zones 

 to the tonid. The meeting and intermingling of these 

 opposite currents, at different temperatures, must give 

 rise to strong electric action in the atmosphere, similar 

 to that already described as taking place in the earth, 

 and coincident with it. And this action must occur in 

 the stratum next the earth, far below that assigned to 

 the aurora; its intensity increasing with the density of 

 the atmosphere, and hence being greatest at the earth's 

 surface. 



This becomes evident, when we consider, that the 



