Motion appears to increase the visibility of
the phenomena. Around the point in the heaven which corresponds to the
direction of the dipping needle produced, the rays appear to meet and
form the boreal corona. It is seldom that the appearance is so
complete, and is prolonged to the formation of the corona; but when the
latter appears, it always announces the end of the phenomenon. The rays
then become more rare, shorter, and less vividly colored. Soon nothing
further is seen on the celestial vault than wide, motionless, nebulous
spots, pale, or of an ashy color; they have already disappeared, when
the traces of the dark segment whence the appearance originated still
remain on the horizon."
The connection that seems to exist, says De la Rive, between the polar
light and the appearance of a certain species of clouds is confirmed by
all observers; all have affirmed that the polar light emitted its most
brilliant rays when the high regions of the air contained heaps of
cirri,--strata of sufficient tenuity and lightness to cause a corona to
arise around the light.
Pages:
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293