In drawing the isoseismal lines (represented by the continuous curves in Fig. 25), Major Dutton made use of the well-known Rossi-Forel scale of seismic intensity, a translation of which is given below.[39] The curves range from the highest degree, 10, corresponding to disastrous effects on buildings, down to the lowest but one, 2, which would be applied to a shock felt only by a small number of persons at rest. It is evident, I think, that these lines cannot be regarded as drawn with great accuracy. The number of records (nearly 4000, from about 1,600 places), great as it is, is hardly sufficient for the purpose; and many were collected from newspapers. The circulars of inquiry also contained no distinct questions corresponding to the different degrees of the scale employed, and therefore it is not always certain that the intensity recorded was the maximum observed. But, if the curves might have varied in detail with a larger and more accurate series of observations, they must represent in their main features the distribution of seismic intensity throughout the disturbed area. One point of importance is the partial earthquake-shadow in the region of the Appalachian Mountains shown by the southward incurving of the isoseismals 4, 5, and 6, and especially by the first two of these lines. Another is the close grouping of the isoseismals in the State of Mississippi, illustrating a rapid fading of intensity as the earth-waves crossed the unconsolidated materials of the Mississippi delta. [Illustration: FIG. 25.--Isoseismal lines of Charleston earthquake. (_Dutton, etc._)] Owing to the short distance between the epicentre and the sea-coast, it is impossible to make more than a rough estimate of the extent of the disturbed area. Even when the boundary lies on land, it traverses some districts which are thinly populated and others where the inhabitants are unobservant, and unlikely to notice the slow oscillations which were alone perceptible at great distances. The shock was, however, felt at Boston (800 miles from the epicentre), La Crosse on the upper Mississippi (950 miles to the north-west), at several places in Cuba (between 700 and 710 miles), and in Bermuda (950 miles). To the south, the limits are unknown, there being no report from Yucatan, the nearest point of which is distant about 930 miles. If we assume the disturbed area to have a mean radius of 950 miles, then it must have covered no less than 2,800,000 square miles. And, that this estimate is not excessive, will be evident from the fact that the land-area disturbed (including parts of the great lakes and inlets in the sea-coast) amounted to about 920,000 square miles. PREPARATION FOR THE EARTHQUAKE.