212 EAETHQUAKES. 



M mg in miles the distances from M of the various stations 

 which have been shaken, and in equal divisions for minutes 

 lay down on M Tg the differences of time at which M, m^, 

 TTi^, &c. were shaken, then Mj Tj, MgTg, &c. are the 

 co-ordinates of points on an hyperbola. The degree of 

 exactness with which this hyperbola is in any given case 

 constructed is a check upon the accuracy of the time 

 observations and the position of the epicentriim. The 

 apex of the hyperbola is the epicentrum. 



The intersection of the asymptote with the ordinate 

 axis is the time point of the first shock, which, because 

 the scale for time and for space were taken as equal, gives 

 the absolute position of the centrum. This intersection 

 is shown by dotted lines. Knowing the position of the 

 centruTYi^ we can directly read from our diagram how far 

 the disturbance has been propagated in a given time. 



