Each observer sees the summit of the auroral arc at his magnetic
meridian; it is, therefore, only those who are on the same magnetic
meridian who see the same summit, and who are able by simultaneous
observations to take its height.

If the summit of the arc pass beyond the zenith of the observer, the
latter is surrounded by the matter of the aurora borealis. This matter
is nothing else than aqueous vapors traversed by the discharges, and
which are in general luminous only at a certain height from the ground,
either because the air is there more rarefied, or because they are
themselves congealed, and more capable, consequently, of liberating
their electric light. Then it is, that, from being nearer to the spot
where the phenomenon is taking place, the observer hears the
crepitation, or whizzing, of which we have spoken, especially if he be
in an open country and in a quiet place. But if the arc do not attain
to his zenith, he is situated beyond the region in which the meeting of
the electric currents takes place; he sees only an arc a little more
elevated to the north or the south, according as he is situated in one
hemisphere or the other; and he hears no noise, on account of his too
great distance. The crepitation is the result of the action of a
powerful magnetic pole upon luminous electric jets in its immediate
neighborhood. With regard to the sulphurous odor which some observers
have perceived, it arises, as does that which accompanies the fall of
lightning, from the conversion into ozone of the oxygen of the air, by
the passage of electric discharges.

Gisler says, that on the high mountains of Sweden the traveller is
sometimes suddenly enveloped in a very transparent fog, of a
whitish-gray color inclining a little to green, which rises from the