[58] The last violent shock before the end of 1893 occurred on September 7th, 1892, and its effects on the frequency of after-shocks is shown by the daily numbers recorded at Gifu during the first fortnight in September. These are--2, 2, 2, 3, 5, 5, 28 (on September 7th), 8, 8, 5, 4, 3, 2, 4, 3. [59] The periodicity of after-shocks is discussed in the papers numbered 4, 12, 16, and 17 at the end of this chapter. In these, the existence of diurnal and other periods is clearly established. Professor Omori also shows that the mean daily barometric pressure is subject to fluctuations with maxima occurring on an average every 5-1/2 days, and that earthquakes are least frequent on the days of the barometric maxima and minima, and more frequent in the days immediately preceding and following them. [60] Of the Japanese earthquakes of 1885-92 originating beneath the land, twenty-six per cent. were accompanied by a recorded sound; but less than one per cent. of those originating beneath the sea and not more than ten miles from the coast. CHAPTER VIII. THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896, AND THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901. Among the earthquakes described in this volume, the Hereford and Inverness earthquakes hold but a minor place. The damage to buildings, though unusual for this country, was slight when compared with that caused by the preceding shocks; there was no loss of life, not a single person was injured by falling masonry. The interest of the earthquakes lies entirely in the detailed study rendered possible by numerous observations of the shock and sound,[61] and in the bearing of this evidence on the general theory of the origin of earthquakes. THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896. The principal earthquake of this series occurred at 5.32 A.M. on December 17th, and was preceded by at least nine minor shocks (the first of which was felt at about 11 or 11.30 P.M. on December 16th), and followed by two others on the same day, and by a third and last on July 19th, 1897. The accounts of these preliminary movements will be found on a later page, as their bearing will be more fully apparent after the discussion of the principal shock. [Illustration: FIG. 60.--Isoseismal and Isacoustic lines of Hereford earthquake. (_Davison._)] ISOSEISMAL LINES AND DISTURBED AREA. On the map in Fig. 60, the continuous curves represent isoseismal lines corresponding to the degrees 8, 7, 6, 5, and 4 of the Rossi-Forel scale. The isoseismal 8, which is the most accurately drawn of the series, is an elongated oval, 40 miles long, 23 miles broad, and containing an area of 724 square miles. The longer axis is directed W. 44° N. and E. 44° S. Within this curve, there are 73 places where buildings are known to have been damaged, 55 places being in Herefordshire, 17 in Gloucestershire, and one in Worcestershire.