412 Dr. C. Davison—British Earthquakes. 
The connexion between the intensity of the shocks and the areas 
disturbed by them is shown in the following table :— 
Tape I. 
Disturbed Area in Square Miles. 
Intensity. 
Maximum. | Minimum. Average. 
8 98000 33000 65900 
7 63600 1000 24500 
6 3100 74 1200) wl 
5 3000 90 $50 
4 1130 28 260 
3 219 183 126 
The intensity at the epicentre is not, however, proportional to the 
energy expended in producing an earthquake, for it depends to ~ 
a great extent on the depth of the focus and the nature of the surface 
rocks. A more satisfactory though still a rough test is the area 
included within a given isoseismal, say that of intensity 4.1 If an 
earthquake be regarded as strong when this area exceeds 5000 square 
miles, as moderate when it lies between 1000 and 5000 square miles, 
and as slight when it is less than 1000 square miles, then Great 
Britain has been visited during the last twenty-one years by 9 strong, 
7 moderate, and 228 slight earthquakes, and 11 earth-sounds.? 
PERIODICITY. 
The monthly distribution of earthquakes is given in the next 
Table (II), the upper figure denoting the number of earthquakes 
during the first 14 days in February and the first 15 days in each 
of the other months, the lower figure the number ose ed during the 
remainder of each month. 
Tasie II. 
Jan. | Feb. | Mar. | Apr. | May | June | July 
Aug. Sept Oct. | Nov.| Dee. 
Applying the method of overlapping means’ to determine the annual 
and semi-annual periods, it appears that there is a well-marked 
annual period with its maximum in October, the amplitude being 
‘ The disturbed area is an unsatisfactory test, for the Pembroke earthquake of 
1893 was felt over a larger area than the stronger shock of 1892, and the Derby 
Sculie cele of 1904 than the stronger earthquake ‘of 1903. 
The strong earthquakes are those of Pembroke in 1892 and 1893, Hereford in — 
1896, Inverness in 1901, Derby in 1903 and 1904, Carnarvon in 1903, Doncaster in — 
1905, and'Swansea in 1906. ‘The moderate earthquakes are those of Bolton in 1889, 
Inverness in 1890, Leicester in 1893, Carlisle in 1901, Strontian in 1902, Derby in 
1906, and Oban in 1907. 
3 Phil. Trans., 1893 A, pp. 1108-15; Boll. della Soc. Sismol. Ital., vol. iv 
1898, pp. 89-100. 
