340 EARTHQUAKES. 



As examples of these actions I will quote the following. 



The diagram of the disturbance of March 1, 1882, 

 taken at Yokohama, shows that the vibrations at the 

 commencement of the disturbance had a period of about 

 three per second, near the middle of the disturbance the 

 period is about 1*1, whilst near the end the period has 

 decreased to -46. That is to say, the backward and forward 

 motion of the ground at the commencement of the 

 earthquake was six times as great as it was near the end, 

 when to make one complete oscillation it took between 

 two and three seconds. Probably the period became still 

 less, but was not recorded owing to the insensibility of 

 the instruments to such slow motions.* 



We have not yet the means of comparing together 

 diagrams of two or more earthquakes, one having been 

 taken near to the origin, and the other at a distance. 

 The only comparisons which I have been enabled to make 

 have been those of diagrams taken of the same earth- 

 quake, one in Tokio and the other in Yokohama. As 

 this base is only sixteen miles, and the earthquake 

 may have originated at a distance of several hundreds 

 of miles, comparisons like these can be of but little 

 value. 



Other diagrams illustrating the same point are those 

 obtained at three stations in a straight line, but at differ- 

 ent distances from the origin of a disturbance produced 

 by exploding a charge of dj-namite in a bore-hole. A 

 simple inspection of these diagrams shows that at the 

 near station the disturbance consisted of backward and 

 forward motions, which, as compared with the same dis- 

 tiu-bance as recorded at a more distant station, were very 

 rapid. Further, by examining the diagram of the motions, 



' Trans. Sets. Soc. of Japan, vol. iv. Milne, Systematic Olservation 

 of Earthquakes, 



