192 EARTHQUAKES. 



work of this description it was found that a very im- 

 portant group of earthquakes might be studied by a line 

 of stations commencing at Saporo in the north, passing 

 through Hakodate, down the east coast of the main island, 

 to Tokio or Yokohama in the south. A further aid to 

 the study of this group, together with the study of an 

 important local group, might be effected with the help of 

 a few additional stations properly distributed on the plain 

 of Musashi, which surrounds Tokio. With this example 

 before us it will be recognised that the choice of sites for 

 a connected set of seismological observatories will often 

 be more or less a special problem. If earthquake stations 

 were to be placed in different directions around Tokio 

 without preliminary investigation, it is quite possible that 

 some of them might be so situated that they would seldom 

 if ever work in conjunction with the remaining observa- 

 tories, and therefore be of but little value. And this 

 remark must equally apply to districts in other portions 

 of the globe. The method is crude, and, so far as actual 

 earthquake origins are concerned, it only yields results 

 which are approximate. The crudeness and the want of 

 absoluteness in the results is, however, more than coun- 

 terbalanced by the certainty with which we are enabled to 

 express ourselves with regard to such results as are ob- 

 tained. Even when working with the best instruments 

 we have at our command, unless we are employing some 

 elaborate system, this method of working gives a most 

 valuable check upon our instrumental records, and enables 

 us to interpret them with greater confidence. 



Determination of earthquake origins from the direc- 

 tion of motion. — If we assume that an earthquake is pro- 

 pagated from a centre as a series of waves, in which 

 normal vibrations are conspicuous, and obtain at two 

 localities, not in the same straight line with the origin, 



