Fare. — Milne Earthquake Diagram. 419 



In every case conclusion 1 — viz., that the boom no longer 

 vibrates in its natural free period, but adopts the period of the 

 forcing cause — is exactly verified. The amplitude of the 

 resulting swings is in every case larger than that of the static 

 displacement of -^-mm., as with the period it theoretically 

 should be, and as the synchronism becomes more perfect the 

 boom- swing becomes larger; but I want to emphasize that, 

 whatever has been the period of the forcing cause within the 

 limits adopted, synchronism more or less complete is apparent. 

 In no case, however, is the swing of the boom so great as it 

 theoretically should be, supposing B to be 0-8 mm. This 

 may be in part due to the imperfect imitation of a siDe curve 

 which resulted from the arrangement adopted, and there is no 

 reason to conclude that because the tilt of the pillar due to 

 the static alteration of a given disposal of weight is E, the 

 displacement of the pillar due to a periodic alteration of this 

 disposal will also be B. 



Interference effects are seen in most of the artificial seismo- 

 grams. Where they are absent the periods are so close that 

 the free vibration has been damped down to comparative 

 insignificance before opposition of place would occur. It 

 appears to be probable, although the period of free vibration 

 was determined as 16-5 sec, that its accurate value was more 

 nearly 166 sec. It is difficult to determine the quantity 

 accurately to 0*1 sec, and yet the result shows that it is 

 important to do so. 



As a result of this examination it appears — (1) that strict at- 

 tention should be paid to accurately recording the period of free 

 vibration ; (2) that the tape should be driven at such a speed 

 as will enable the period of forced vibration to be determined ; 

 (3) that the value of the constant K should be recorded. 



We might then hope, by determining the amplitude of the 

 earth-movement of any particular waves of given wave-length 

 at different stations, to ascertain the law which governed the 

 decrease of intensity with distance, and to determine how the 

 velocity of the waves varied with their length. 



As I am unacquainted with the practical details of 



Dr. Omori's instrument I have refrained from discussing the 

 acceleration of the earth-particles given by him for certain 

 earthquakes ; ::: but if the considerations above set forth apply 

 to his instrument, and if also they have been omitted in 

 arriving at the results he gives, then by taking them into 

 account the values of the accelerations would be so much 

 reduced that it is improbable we should have felt the earth- 

 quakes he discusses. 



* Publication of the Earthquakes Investigation Committee, No. 5, 

 Tokyo, 1901, p. 45 et seq. 



