Sometimes these clouds are grouped and arranged
almost like the rays of an aurora borealis; they then appear to disturb
the magnetized needle. Father Secchi has remarked, that magnetic
disturbances are manifested at Rome whilst the sky is veiled with
clouds that are slightly phosphorescent, which, at night, present the
appearance of feeble aurorae boreales.
After a brilliant aurora borealis, we have been able to recognize, on
the following morning, trains of clouds, which, during the night, had
appeared as so many luminous rays.
The absolute height of aurorae boreales has been very variously
estimated by different observers. It has long been thought that we
might determine it by regarding, from two places widely distant from
each other, the same part of the aurora,--the corona, for example. But
we have started from a very inaccurate assumption, namely, that the two
observers had their eyes directed to the same point at the same
time,--whilst it is now well proved that the corona is an effect of
perspective, due to the apparent convergence of the parallel rays
situated in the magnetic meridian; so that each observer sees his own
aurora borealis, as each sees his own rainbow.
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