358 Dela Rive on the Aurora Borealis. 
portion of the atmosphere saturated with humidity, giving rise to 
mists; and by this means it may easily pass to the earth and 
combine with the negative electricity with which the earth itself 
. ischarged. It consequently results that there are constant cur- 
rents of positive electricity rising from different points of the 
earth’s surface into the upper regions of the atmosphere, which 
pass towards the poles, and then return beneath the earth’s sur- 
between the two systems. We may add that the experiments 
made with the electric telegraph have demonstrated that the ter- 
restrial globe is an almost perfect conductor of electricity, com- 
pensating by its mass, for what it wants in the conductibility of 
the materials which constitute it. Thus the existence of the 
currents, whose course I have traced, rests on well established 
principles, with a foundation of simple experiment. 
But more than this: their existence is demonstrated by facts 
long studied and established,—those pertaining to the diurnal va- 
riation of the magnetic needle. 
this deviation is precisely that which should be occasioned by 
currents passing along the surface of the globe from the north 
pole to the equator, augmenting in intensity with the heat of the 
day and diminishing as it decreases. The diurnal variation is at 
its maximum (13/ to 16’) in those months in which the sun is 
longest above the horizon, May, June, July, August. It is at its 
minimum (4/ to 5’) during the winter months. The variation is 
escend at the polar regions, and thus traversing the globe, 
