M. Necker is not the only observer
who has not heard the noise; neither have MM. Lottier and Bravais, who
have observed so great a number of aurorae, ever heard it; and a great
many others are in this case. This may be due to the fact that it is
necessary to be very near to the aurora in order to hear the
crepitation in question, and also to the fact that it is possible that
it does not always take place, at least in a manner sufficiently
powerful to be heard.
We have just been pointing out, as concomitant effects of the aurora
borealis, a noise of crepitation analogous to that of distant
discharges, and a sulphurous odor similar to that which accompanies the
fall of lightning. M. Matteucci also observed at Pisa, during the
appearance of a brilliant aurora borealis, decided signs of positive
electricity in the air; but of all phenomena, those which invariably
take place at the same time as the appearance of the aurora borealis
are the magnetic effects. Magnetized needles suffer disturbances in
their normal direction which cause them to deviate generally to the
west first, afterwards to the east.
Pages:
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302