B etween the Derby earthquakes of March 24th, 1903, and July 3rd, 1904, there exist several points of intimate resemblance. The isoseismal lines of the two shocks, when drawn on one map, are so closely related that they might be the isoseismals of a single earthquake; both earthquakes were twins; the two epicentres in 1904 were nearly or quite coincident with those in 1903; and both earthquakes were followed by an after-shock, originating for the most part in the interfocal region of the fault. The points in which they differ are of little consequence. The intensity of the shock of 1903 was greater than that of the shock of 1904; the impulses at the two foci were approximately of the same strength in 1903, and of unequal strength in 1904; they occurred at the same instant on the former occasion, while, on the latter, they were separated by a brief interval of time. The principal shock of 1903 occurred on a weekday at 1.30 P.M. , that of 1904 at 3.21 on a Sunday afternoon. Thus, although of less intensity, the recent earthquake was felt and heard over a much wider area than its predecessor. There was at the time practically no traffic in the streets, and many of the observers (especially of those at a great distance from the centre) were lying down—conditions that favoured the observation of the earthquake in places at which, on an ordinary occasion, it would have passed totally unperceived. There were at least three earthquakes in