204 EAETHQUAKES. 



jection being near to the centre of the map. This was 

 done so that the measurements which were made upon 

 the map might be more correct than those we should 

 obtain from an ordinary chart where this portion of the 

 world was not the centre of projection. Next, a number 

 was taken as equal to the velocity with which the sea wave 

 had travelled. The first velocity taken was about 400 

 feet per second — this being about the velocity with which, 

 theoretically, it must have travelled in an ocean having 

 a depth equal to that indicated upon the charts — also it 

 seemed to have travelled at this rate from the various 

 times of arrival as recorded at places along the coast. 

 Circles were then drawn round Tocopilla, Cobija, Iquique, 

 and Mejillones with radii equal to 2, 8, 10, and 15, each 

 multiplied by (60 x 400). It was then seen by trial that 

 it was impossible to draw a single circle which should 

 touch four circles and also pass through Huanillos. These 

 four circles were, in fact, too large. Four new but smaller 

 circles, which are shown in the map, were next drawn, their 

 radii being respectively equal to the numbers 2, 8, 10, 

 and 16, each multiplied by (60 x 350), and it was found 

 that a circle, with a centre c, could be drawn which would 

 practically touch the four circles, and at the same time 

 would pass through Huanillos. 



III. The method of hyperbolas, — The method which I 

 call that of hyperbolas is only another form of the method 

 of circles. It is, however, useful in special cases, as, for 

 instance, where we have the times of arrival of earthquakes 

 at only two stations. Between Tokio and Yokohama, at 

 which places I frequently obtain tolerably accurate time 

 records, the method has been applied on several occasions 

 with advantage. In the preceding example let us suppose 

 that the only time records which we had were for Huan- 

 illos and Mejillones, and that the wave was felt at the 



