AURORA BOREAL1S. 196 



heard any sound attending the phenomenon. If this negative 

 testimony should not be deemed equivalent to the positive 

 counter evidence of Heame on the mouth of the Copper River 

 and of Henderson in Iceland, it must be remembered that 

 although Hood heard a noise as of quickly-moved musket- 

 balls and a slight cracking sound during an aurora, he also 

 noticed the same noise on the following day, when there was 

 no northern light to be seen ; and it must not be forgotten 

 that Wrangel and Gieseke were fully convinced that the sound 

 they had heard was to be ascribed to the contraction of the 

 ice and the crust of the snow on the sudden cooling of the 

 atmosphere. The belief in a crackling sound has arisen not 

 amongst the people generally, but rather amongst learned 

 travellers, because in earlier times the northern light was de- 

 clared to be an effect of atmospheric electricity, on account of 

 the luminous manifestation of the electricity in rarefied space, 

 and the observers found it easy to hear what they wished to 

 hear. Recent experiments with very sensitive electrometers 

 have hitherto, contrary to the expectation generally enter- 

 tained, yielded only negative results. The condition of the 

 electricity in the atmosphere* is not found to be changed 

 during the most intense aurora ; but, on the other hand, the 

 three expressions of the power of terrestrial magnetism, de- 

 clination, inclination, and intensity, are all affected by polar 

 light, so that in the same night, and at different periods of the 

 magnetic development, the same end of the needle is both 



* [Mr. James Glaisher, of the Eoyal Observatory, Greenwich, in his 

 interesting Remarks on the Weather during the Quarter ending Decem- 

 ber 31st, 1847, says, "It is a fact well worthy of notice, that from the 

 beginning of this quarter till the 20th of December, the electricity of 

 the atmosphere was almost always in a neutral state, so that no signs of 

 electricity were shown for several days together by any of the electrical 

 instruments." During this period there were eight exhibitions of the 

 aurora boreal is, of which one was the peculiarly bright display of the 

 meteor on the 24th of October. These frequent exhibitions of brilliant 

 aurorae seem to depend upon many remarkable meteorological relations, 

 for we find, according to Mr. Glaisher's statement in the paper to which 

 we have already alluded, that the previous fifty years afford no parallel 

 season to the closing one of 1847. The mean temperature of evapora- 

 tion, and of the dew point, the mean elastic force of vapour, the mean 

 reading of the barometer, and the mean daily range of the readings of 

 the thermometers in air, were all greater at Greenwich during that 

 season of 1847 than the average range of many preceding years.] Tr. 



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