Even when observers in the same place agreed in hearing the sound, it presented itself to them under different aspects. Thus, at Hereford, a crash or bomb-like explosion was noticed by some, but not by all, observers; at Ledbury, the sound according to one began like a rushing wind and culminated in a loud explosive report, another heard a noise like distant thunder, which ended when the shock began, while a third heard no sound at all. At places more distant from the epicentre, the same diversity, both in character and intensity, is manifested. Thus, at Birmingham, the accounts refer on the one hand to the distant approach of a train and the rising of the wind, on the other to the reports of large cannons and to a noise as if tons of _débris_ had been hurled against the wall of the house; at Bangor, to muffled thunder, wind through trees, and a loud rumbling sound. The first explanation of these apparent anomalies which presents itself is inattention on the part of the observers; but it is one that will not bear examination, though it may apply in some cases. The sound is too loud, at any rate near the epicentre, to escape notice, and it is generally heard before the shock begins to be felt. Moreover, as described in the last chapter, three out of every four earthquakes in Japan are unaccompanied by recorded sound, and the Japanese as a race cannot be accused of such constant inattention. The defect, it can hardly be doubted, is inherent to the observer, and not dependent on the conditions in which he is placed. That the higher limit of audibility varies with different persons has long been known; and there can be no reason for doubting that there is a similar variability in the lower limit. Thus, to some observers, the sound remains inaudible throughout, however intently they may be listening. Again, it is found that, the deeper the sound, the greater must be the strength of the vibrations required to render them audible. As the vibrations which reach an observer increase in period, it may therefore happen that, sooner or later, the strength of some does not attain or exceed that limiting value, and, at that moment, the sound will cease to be heard. Moreover, for vibrations of a given period, this limiting value varies for different persons. Thus, to one observer, the sound may become inaudible, while another may continue to hear it. Lastly, the vibrations which affect an observer at any moment are of various strength and period. One may hear all perhaps, while a second may be able to hear some and not others. Thus, to one observer, the sound may be like a rising wind, to another like a heavy traction-engine passing; one may hear the crashes which accompanied the strongest part of the shock, while a second may be deaf to the same vibrations; to one the sound may become continually louder and cease abruptly, to another it may increase to a maximum and then die away.