200 EARTHQUAKES. 



origin of the disturbance, it is probable that errors of this 

 description are small and will not make material differences 

 in the general results. 



2. We have reasons for believing that the transit 

 velocity of an earthquake is dependent on the nature of 

 the rocks through which it is propagated. Errors which 

 arise from causes of this description will, however, be 

 practically eliminated if our observation points are 

 situated on an area sufficiently large, so that the dis- 

 tribution of the causes tending to alter the velocity of a 

 shock balance each other. It must be remarked, that 

 causes of this description may also produce an alteration 

 in the direction of our shock. 



Other errors which may sometimes enter into our 

 results, when determining the origin of shocks by means 

 of observations on velocities, are the assumptions that the 

 disturbance has travelled along the surface, from the 

 epicentruTYi and not in a direct line from the centrum. 

 Again, it is assumed that the origin is a point, whereas 

 it may possibly be a cavity or a fissure. Lastly, if we 

 desire extreme accuracy, we must make due allowance for 

 the sphericity of the earth and the differences of elevation 

 of the observing stations. 



I. The method of straight lines. — Criven a number of 

 pairs of points Aq, Aj, Bq, b^, Cq, Cj, &c., at each of which 

 the shock was felt simultaneously, to determine the origin. 



Theoretically if we bisect the line which oins Aq and 

 Aj by a line at right angles to Aq, a,, and similarly bisect 

 the lines B^, Bj, Cq, Cj, all these bisecting lines a^, a,, h^, 6p 

 Cq, Cj, &c., ought to intersect in a point, which point will 

 be the epicentrum, or the point above the origin. 



This method will fail, first, if A^, Aj, Bq, Bj, Cq, c, form a 

 continuous straight line, or if they form a series of parallel 

 lines. 



