368 De la Rive on the Aurora Borealis. 
the aurora about the magnetic pole, is no longer under the influ- 
ence of the currents which circulate around it and not above or 
below, and it ought therefore to experience only a variable and 
irregular action. 
I have said that the aurora was probably of daily occurrence, 
and varied only in intensity. These differences in intensity are 
the reason for its being not always perceptible, and also for its 
less frequency remote from the magnetic poles. As to the differ- 
ences of number for each month, they are attributable to two 
causes—but especially to the unequal length of the nights, for there 
should be fewer in the shorter nights. Thus in May, June and 
July the fewest are seen, because the days are the longest, while 
that the auroras are most frequent at the times of the equinoxes, 
and especially the autumnal equinox. This is readily understood . 
if we consider that the vernal equinox is the time when the sun 
transfers to the northern hemisphere its powerful influence either 
direct or indirect in the development of electricity ; and that the 
autumnal should be followed with a large condensation of the 
vapors accumulated in the atmosphere during the months of sum- 
mer—a condensation which, as already explained, facilitates the 
neutralisation of the two electricities, developed in large quanti- 
ties during the summer, and augments consequently the intensity 
of the discharge at the pole. 
magnetic poles, which are the centers of the aurora, and which 
according to the surface about them would more or less facilitate 
the electric circulation: for it is evident that the naked soil would 
afford more ready circulation than a surface covered with a great 
thickness of ice. But, I repeat it, the fact of the periodicity 8 
far from proved. : 
