Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/297

236 Upon the seismic Map A, I have laid down, the outline of the coasts, the positions of all the principal towns and villages, distinguishing all those at which seismic observations were made. The isoseismal curves—first, second, third, &c.—are marked respectively, by broken lines, in which the number of consecutive dots, fixes that of the curve it designates. The chief mountain ridges, or summits of chief elevation, are lightly marked in, and lastly, the whole of the wave-paths, observed in direction upon the surface, have been marked with red lines from each town or place of observation respectively.

It became necessary in the protracting of these lines of wave-path, which were all observed by the prismatic compass, to have regard to the magnetic declination. The observations made, upon the local declination at various points, and given in Part II., showed that over the whole observed area of country, it did not differ anywhere more than one or two degrees, from the mean declination at Naples; except at one or perhaps two spots, where there appeared to be great local disturbance.

As the most convenient arrangement, therefore, and one