Brute animals, being more sensitive than men of the slightest movement of the earth, are said to evince extraordinary alarm, and it has been often observed that even the dull hog shows symptoms of uneasiness previous to the shock. During the great Neapolitan earthquake of 1857 an unusual halo-like light was seen in the sky before, and not long before, the shock. Mr. Mallet was at first inclined to look upon this notion as a superstitious tale; but, finding it widely diffused in a country where communication is bad and news travels slowly, no longer doubted that it was founded on fact.[10] Conjectures would be useless as to its nature, but future observation directed to the point may determine whether some sort of auroral light may emanate from the vast depths of rock formation under the enormous tensions and compressions that must precede the final crash and rupture; or whether volcanic action, going on in the unseen depths below, may give rise to powerful disturbances of electric equilibrium, and hence to the development of light; just as from volcanic mountains in eruption lightnings continually flash from the huge volumes of steam and floating ashes above the crater. Humboldt is also of opinion that, though in general the revolutions which take place below the surface of the earth are not announced beforehand by any meteorological process, or a peculiar appearance of the sky, it is not improbable that during violent shocks some change may occur in the condition of the atmosphere. Thus, during the earthquake in the Piedmontese valleys of Pelis and Clusson, great alterations were observed in the electrical tension of the atmosphere without any appearance of a thunder-storm. Footnote 10: Mallet, ‘The Great Neapolitan Earthquake of 1857,’ vol. i. p. 323.