A great deal of obscurity, as yet hangs over the way in which the sound from the earth, reaches the auditory nerves, whether by vibration, communicated from the earth's surface to the stratum of immediately superincumbent air, and by it to the ear; or directly from the vibrating surface of the earth, through the bones and other tissues of the human frame, to the ear. Upon this point, the remark made casually by one of the persons at Polla, that he heard the noise, come up through his body or legs from the earth, is not to be wholly lost sight of. The probability is, that the sound, from its starting-point, travels to the ear both ways; viz., through the earth directly and the human body, to the ear; and through the earth, (vertically, or nearly so, to the nearest points of air,) and thence through the atmosphere, to the ear: and as the rate of transit is very much more rapid, probably, in the solids, than in the air, this condition would add another, to the causes already assigned, for the continuity of the sound. (Compare 'First Report on Facts of Earthquakes, Reports Brit. Ass., 1850, cap. 23.')
One circumstance connected with the sounds of earthquakes, has hitherto received but imperfect explanation, namely, the arrival at a point distant from the origin, of the sounds before the shock. The fact, has been apparently sufficiently attested, as occurring occasionally, and some years since it was adduced, as one tending to cast some doubt, upon the general theory of earthquake dynamics, which, based upon the movements of elastic waves, has since been universally accepted. The phenomenon admits of simple probable explanation, and may be treated, along with that of the tremors before and after the shock, in the following chapter.
