210 EAETHQUAKES. 



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 ad : 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. 

 The difference in time of the arrival of two disturb- 

 ances. — In the various calculations which have been made 

 to determine an origin based on the assumption of a 

 known or of a constant velocity, we have only dealt with 

 a single wave, which may have been a disturbance in the 

 earth or in the water. A factor which has not yet been 

 employed in this investigation is the difference in time 

 between the arrival of two disturbances ; one propagated, 

 for instance, through the earth, and the other, for 

 example, through the ocean. The difference in the times 

 of the arrival . of two waves of this description is a 

 quantity which is so often recorded that it is well not to 

 pass it by unnoticed. To the waves mentioned we might 

 also add sound waves, which so frequently accompany 

 destructive earthquakes, and, in some localities, as, for 

 instance, in Kameishi, in North Japan, are also commonly 

 associated with earthquakes of but small intensity. It 

 was by observing the difference in time between the 

 shaking and the sound in different localities that Signor 

 Abella was enabled to come to definite conclusions regard- 

 ing the origin of the disturbances which affected the 

 province of Neuva Viscoya in the Philippines, in 1881 ; 

 the places where the interval of time was short, or the 

 places where the two phenomena were almost simul- 

 taneous, being, in all probability, nearer to the origin 

 than when the intervals were comparatively large. I 

 myself applied the method with considerable success 



