The diameter of the luminous ring is greater in proportion as the
magnetic pole is more distant from the surface of the earth, since this
pole must be situated upon the intersection of the plane of the ring
with the axis of the terrestrial globe; if we could determine
rigorously the position of the aurora borealis, we should then have the
means of knowing exactly that of the pole itself.
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.
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