any small causes of local disturbance); so that the forms of the isoseismal curves, indicate truly to the eye the relative distances in all directions around the seismic vertical, to which the same decree of overthrowing force, has extended horizontally.
If the impulse were the same, in all horizontal or emergent directions, and the earth were perfectly homogeneous, all radii for equal overthrow must be equal, and the isoseismal curves must all be circles. If the impulse be the same in all directions, but these curves be found, ellipses or ovals, then the longer radii indicate the directions, in which the conditions of the medium (the earth), have permitted it to transmit the wave furthest, with least loss.
If the impulse be greater, towards one or both opposite directions in some given azimuth, then even in an homogeneous medium, the isoseismals must be elliptic; and if both conditions concur—i.e., preponderance of original impulse in one direction, and heterogeneity of medium,—then from both causes the curves will become distorted; and although still closed curves, and of the elliptic order, they may assume almost any form.
