In this example, as in the preceding ones, it will be observed that it has been sea waves that have been dealt with, rather than earth vibrations. It is evident, however, that these latter vibrations may be dealt with in a similar manner. In these determinations it will also have been observed that it is assumed that the disturbance has radiated from a centre, and, therefore, approached the various stations in different directions. If we assume that we have three stations very near to each other as compared with their distances from the origin, so that we can assume that the wave fronts at the various stations were parallel, the determination of the direction in which the wave advanced appears to be much simplified. The determination of the direction in which a wave has passed across three stations was first given by Professor Haughton. _Haughton’s method._—Given, the time of an earthquake shock at three places, to determine its horizontal velocity and coseismal line. The solution of this is contained in the formula _a_ (_t_{2}_ - _t_{1}_) sin β tan φ = ——————————————————————————————————————————————————————. _c_ (_t_{3}_ - _t_{2}_) + _a_ (_t_{2}_ - _t_{1}_ cos β When A, B, and C are three stations at which a shock is observed at the times _t_{1}_, _t_{2}_, and _t_{3}_; _a_, _b_, and _c_ are the distances between A, B, and C, and φ is the angle made by the coseismal lines _x_ A _x_, _y_ B _y_, and the line A B, which are assumed to be parallel. This I applied in the case of the Iquique earthquake, but owing to the smallness of the angles between the three stations A, B, and C, the result was unsatisfactory. The problem ought to be restricted, first, to places which are a long distance away from a centre, and, secondly, to places which are not nearly in a straight line. This problem may be solved more readily by geometrical methods. Plot the three stations A, B, and C on a map, join the two stations between which there was the greatest difference in the time observation. Let these, for example, be A and C. Divide the line A C at point D, so that A D : D C as the interval between the shock felt at A and B is to the interval between the shock felt at B and C. The line B D will be parallel to the direction in which the wave advanced.