Excluding the Ischian earthquakes, which belong to a special class, it is evident that there is generally some slight preparation for a great earthquake. For a few hours or days beforehand, weak shocks and tremors are felt or rumbling noises heard within the future meizoseismal area. But, unfortunately, it has not yet been found possible to distinguish these disturbances from others of apparently the same character which occur alone, so that for the present they fail to serve as warnings. In Japan, where the organisation of earthquake-studies is more complete than elsewhere, it is possible that a vague forecast might be made, if the distribution of the fore-shocks of the earthquake of 1891 should prove to be a general feature of all great earthquakes. It was at first supposed that this earthquake occurred without preparation of any kind; but a closer analysis of the records shows that during the previous two years there was a very decided increase in the seismic activity of the district, and also that the distribution of the epicentres marked out the future fault-scarp, and at the same time exhibited a tendency to comparative uniformity over the whole fault-region. For the present, then, the only warning available is that given by the preliminary sound, which may precede the strongest vibrations by as much as five or ten or even more seconds. Though two or three seconds may elapse before its character is recognised, the fore-sound thus allows time for many persons to escape from their falling houses. Some races, however, are less capable of hearing the sound than others, and this may be one reason why Japanese earthquakes are so destructive of human life. DISTURBED AREA. It is usual with some investigators to measure the intensity of an earthquake roughly by the extent of its disturbed area. The depth of the seismic focus must of course have some influence on the size of this area, and this condition is only neglected because we have no precise knowledge of the depth in any case. Thus, Mr. Oldham regards the Indian earthquake of 1897 as rivalling the Lisbon earthquake of 1755, which is generally considered to hold the first place, because its disturbed area was not certainly exceeded by that of the latter. That disturbed area is, however, an untrustworthy measure of intensity will be evident from the following table, in which the earthquakes described in this volume (omitting those of Ischia) are arranged as nearly as may be in order of intensity, beginning with the strongest:-- Earthquake. Disturbed Area in Sq. Miles. Indian 1,750,000 Japanese 330,000 Neapolitan 39,200 Charleston 2,800,000 Riviera 219,000 Andalusian 174,000 Hereford 98,000 Inverness 33,000