If the meizoseismal area had been a thickly populated one, the evidence of ruined and damaged houses would have provided materials for the construction of isoseismal lines surrounding the two epicentres. It is difficult, as it is, to gauge the equality of the effects on objects so different as railway-lines and buildings; and the isoseismals shown in Figs. 28 and 29 can therefore lay no claim to accuracy. Fig. 28 shows the epicentral isoseismals as they are drawn by Mr. Earle Sloan. They do not correspond to the degrees of any definite scale of seismic intensity; but they may be taken as representing the impressions of a very careful observer, who traversed the district immediately after the occurrence of the earthquake, and who, when drawing these lines, was biassed by no preconceived theory. Major Dutton, relying chiefly on Mr. Sloan's written notes, interprets the evidence differently, and obtains the series of curves shown in Fig. 29. In this case, also, the isoseismals correspond to no expressed standard of intensity. They are intended merely to represent the forms of the curves, and, by their less or greater distance apart, the more or less rapid rate at which the intensity varied. The chief difference between the two maps concerns the form of the Woodstock isoseismals. Major Dutton draws them approximately circular, leaving the map blank towards the north, where hardly any evidence was forthcoming. Mr. Sloan attributes the scantiness of effects here to a diminution of intensity, and makes the lines curve in towards the epicentre. They certainly must do so in crossing the North-Eastern Railway; and the somewhat southerly trend of Mr. Sloan's curves to the east of this railway seems to me to furnish the better representation of the distinctly greater intensity in that region. [Illustration: FIG. 28.--Epicentral isoseismal lines of Charleston earthquake according to Mr. Sloan. (_Dutton._)] [Illustration: FIG. 29.--Epicentral isoseismal lines of Charleston earthquake according to Major Dutton. (_Dutton._)] More important, however, than this divergence of opinion is the agreement in one respect between the two sets of curves. Both show a marked expansion around the points known as the Woodstock and Rantowles epicentres, especially about the former, and a contraction in the intermediate region. The evidence of these isoseismals therefore confirms that of the damaged railway lines, and establishes Major Dutton's inference that there were two distinct foci, the epicentres of which were about thirteen miles apart. ORIGIN OF THE DOUBLE SHOCK.