206 ELEMENTS OF STATIC ELECTRICITY. 



greater part of the mass of the atmosphere lies near the 

 earth's surface ; being included, probably, within the 

 first nine miles ; while the auroral stratum is supposed 

 to have an altitude of sixty-nine miles. Hence this 

 atmospheric electric action would be supplementary to 

 that of the earth, already described; and would have 

 an east and west as well as a north and south direction, 

 as described on page 182. 



The influence of the sun and moon, already referred 

 to, must intensify the effects produced by other causes : 

 so that we should expect to find maximum and miiv 

 imum auroral effects, corresponding to an increase or 

 decrease of intensity, in solar or lunar influence. Ob- 

 servation has shown, that such an auroral maximum 

 occurs, during the recurrence, once in eleven years, of 

 the -period of the maximum solar disturbance; that 

 auroras are then more frequent and brilliant than at 

 other times : and we may reasonably infer, that future 

 observation will show the existence of electric maxima 

 and minima, analogous to the tides, and auroral effects 

 corresponding to them. 



