304 CHARLES DAVISON 
By means of an arbitrary scale, for which we are indebted to the 
joint labors of Professors M. S. de Rossi and F. A. Forel, the 
intensity at any place may be expressed according to the 
mechanical effects produced by the earthquake. A series of iso- 
seismal lines may then be drawn, each surrounding the places 
where the shock was of a given intensity and excluding those 
where it was distinctly less; and if the series is complete, the 
innermost isoseismal enables us to determine the position of the 
epicenter, generally with a close approach to accuracy. 
But the method of intensities does more than this. When the 
isoseismal lines are carefully drawn—and this is only possible 
roughly elliptical in form; their longer. axes are parallel or 
nearly so, but they are not coincident. In my report on the 
Hereford earthquake (pp. 216-218), it is shown that this must 
be the case when the earthquake is due to the friction generated 
by a fault-slip; for the focus is then a surface inclined to the 
horizon. Moreover, the focus and relative positions of the iso- 
seismal lines are indices of the direction and slope of the fault- 
plane. The longer axes of the curves are parallel to the fault-line 
or strike of the fault; and, on the side toward which the fault 
slopes, the isoseismal lines are further apart than on the other 
side of the fault-line, except at great distances in the case of a 
strong earthquake, when the inequality is reversed. 
I can give no better example of a slight earthquake than that 
which occurred on April 1, 1898, in the south of Cornwall. The 
positions of the principal places where it was felt are shown in 
Fig. 1, but the coast-line is omitted in order to simplify the 
diagram. The continuous curves represent the isoseismal lines 
of intensities 4 and 3 of the Rossi-Forel scale; and their forms 
and relative position show that the fault-line must run from 
Be 23° Whe wo WW 38° Sy ancl Wmat wine tiault mmst Ince to tle 
southeast. 
The latter inference is corroborated by the study of the 
sound-phenomena, to which the two dotted lines relate. The 
outer of these lines represents the boundary of the sound-area, 
* Quart. Jour. Geol. Soc., Vol. LX VI, 1900, pp. 1-7. 
