The phenomenon, the aurora borealis, or polaris, has never been satisfactorily explained. It is acknowledged as purely magnetic in character, and to be due to the passage of currents upward from the earth. It has received the regard due to a mere negative though brilliant exhibition, whereas the character, extent, and significance of its manifestations should have caused it to be greeted, and studied, as the index of the operation of very positive cosmical functions.
Humboldt regarded this process as "the restoration of a disturbed equilibrium;" and so indeed it is, but it is an equilibrium, not simply as between the earth, and atmosphere. Various observers have estimated the altitude to which the aurora sometimes reaches, at from 80 to 265 miles. The fact that the bulk of the atmosphere reaches but three miles above the earth's surface, forbids it to be regarded as purely a terrestro-atmospheric phenomenon.
While viewing the more striking and brilliant exhibitions of the aurora, the more undemonstrative and by far the most important and vital operations have been disregarded. The former may not be observed, except occasionally, and fitfully, can only be present when favoring meteorological conditions admit of its disclosure. The latter, more unobtrusive and even invisible to the naked eye, are incessantly, and at all seasons, in action, by day as well as by night.[9] May not this auroral display then be regarded in a measure as confirmatory of what the law of conservation had already suggested to us; the existence of a retro-current?
Well understood Processes in Confirmation.
The suggestion of a simple, adequate, and perfect theory is given us by an ordinary electro-magnetic battery. Let the conducting wire from such a battery extend half around the circumference of this globe. It is apparently as quiet and dormant as is our earth; yet in those cold plates, solutions, and wire, there lie the hidden elements of heat, light, and power. At the distant extremity of the wire, when not connected with the earth, we may have none of the manifestations of heat, light, or attraction—even though the plates are put into the solution. But let us now make the connection between the extremity of the wire and the earth, then the circuit is complete, and heat, light, and attraction are disclosed in highest degree.
Now from the Great Sun Battery,[10] in which we locate the one Great Universal Force: Newton's "Higher and Still Unknown Force," every one recognizes a current constantly present, setting towards the earth. That current is recognized as bringing us our light and heat. But without a retro-current, should we have a circuit complete? Should we have any of these phenomena?
Heat without Combustion.
Neither in the battery nor near the battery do the phenomena manifest themselves. Though the developer of light, heat, and power, the battery itself is neither luminous, hot, nor magnetic. "To explain the effects of the sun, therefore, there is not the least reason to infer that it is itself luminous, or even warm. Potential action generated in a dark, cold body, may produce great heat and light, at a distance from the seat of activity; and what is thus wrought artificially in a small way may surely be done naturally in a tremendous fashion by the grand forces of the sun."
Inter-currents.
It is now well known that a number of currents may pass in each direction, at the same time, over one and the same telegraph wire; and in like manner, great solar currents may pass to and fro without interference.