DETERMINATION OF EAKTHQUAKE OEIGINS. 209 



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 ad- 

 vanced appears to be much simplified. The determina- 

 tion 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 earth- 

 quake shock at three places, to determine its horizontal 

 velocity and coseismal line. 



The solution of this is contained in the formula 



tan 6 = a (t,-t,) sin ^ 



c{t.,-t,) + a (t^ - t,) cos ^' 



When A, B, and C are three stations at which a shock is 

 observed at the times t^, t^, and ^3 ; a, 6, and c are the 

 distances between A, b, and c, and cf) is the angle made 

 by the coseismal lines x ax, yBy, 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 pro- 

 blem may be solved more readily by geometrical methods. 



