12. OMORI, F.--"On the After-shocks of Earthquakes." _Journ. Coll. Sci. Imp. Univ. Japan_, vol. vii., 1894, pp. 111-200; abstract in _Japan Seismol. Journ._, vol. iii., 1894, pp. 71-80. 13. ---- "A Note on the Great Mino-Owari Earthquake of October 28th, 1891." _Pub. Earthquakes Inves. Com. in Foreign Languages_, No. 4, Tokio, 1900, pp. 13-24. 14. ---- "Sulla velocità di propagazione e sulla lunghezza delle onde sismiche." _Ital. Soc. Sismol. Boll._, vol. i., 1895, pp. 52-60--especially pp. 52-57. 15. ---- "Sull' intensità e sull' ampiezza del movimento nel gran terremoto giapponese del 28 ottobre 1891." _Ital. Soc. Sismol. Boll._, vol. ii., 1896, pp. 189-200. 16. ---- "Note on the After-shocks of the Mino-Owari Earthquake of Oct. 28th, 1891." _Pub. Earthquakes Inves. Com. in Foreign Languages_, No. 7, Tokio, 1902, pp. 27-32. 17. ---- "Note on the relation between Earthquake Frequency and the Atmospheric Pressure." _Tokyo Phys.-Math. Soc. Reports_, vol. ii., 1904, No. 8. 18. TANAKADATE, A., and H. NAGAOKA. "The Disturbance of Isomagnetics attending the Mino-Owari Earthquake of 1891." _Journ. Coll. Sci. Imp. Univ. Japan_, vol. v., 1893, pp. 149-192. FOOTNOTES: [54] I have not referred to the results of this survey, for, though changes in all the magnetic elements (especially in horizontal intensity) have taken place between 1887 and 1891-92, these changes cannot be ascribed with confidence to the earthquake in the absence of a thorough knowledge of the secular variation. [55] From the formula a=x*g/y, where _a_ is the maximum horizontal acceleration, _g_ the acceleration due to gravity, _y_ the height of the centre of gravity, and _x_ its horizontal distance from the edge about which the body was overturned. [56] These estimates are made, on the supposition of simple harmonic motion, from the formula 2*a=alpha*t^2/(2*pi^2), where 2_a_ is the total range or double amplitude, _a_ the maximum acceleration, and _t_ the period of the vibration. [57] Professor Omori finds that the mean daily number of earthquakes _y_ during the month _x_ (reckoned from November 1891) may be approximately represented by the equation-- y = 16.9 / (x + 0.397); or, taking the semi-daily earthquake numbers during the five days between October 29th and November 2nd, 1891, by the equation-- y = 440.7 / (x + 2.314), where _y_ denotes the number of earthquakes observed during the twelve hours denoted by _x_, the time being measured from the first half of October 29th. It is interesting to notice that, taking account of the mean annual frequency of earthquakes in ordinary years, the number of shocks observed at Gifu during the two years 1898-99 should, according to the latter formula, be 163; the actual number recorded was 160.