Fakr. — Milne Earthquake Diagram. 417 



lected in other cases. It is thus quite an erroneous proceed- 

 ing to take the most marked phase of an earthquake diagram 

 and to assume that this corresponds to the waves of largest 

 amplitude in the earth ; and it is also quite erroneous to 

 derive the amplitude of the earth-wave by multiplying the 

 amplitude of the trace by the conversion factor from milli- 

 meters to seconds of arc. Eegard must be paid to the period 

 of the earth- wave and of the free boom- vibration. In the 

 tabulation of records of Milne seismographs no regard is at 

 present paid to the period of the earth-wave, and hardly more 

 to the period of free boom-vibration, and no information is 

 supplied of the value of the constant K, which, though often 

 negligible (i.e., when p and n are not very nearly equal), is 

 occasionally of paramount importance. This constant can be 

 deduced from the fact that the free vibration dies away 

 according to an expression of the form 



u = Ae - i kt cos 



Turning now to the result (1), that the forced vibration 

 takes place in the period of the disturbing cause, this enables 

 us to derive a value both of the periods and wave-lengths of 

 the various sets of waves acting on the boom ; and the record 

 is thus capable of supplying the information absolutely essen- 

 tial for the correct valuation of the amplitude E of the dis- 

 turbing cause, from which can be easily derived the total 

 movements and accelerations of the earth-particles at any 

 phase of the earthquake. But to do this the tape must be 

 driven at a much higher rate than is at present the case. It 

 is difficult to see the individual vibration, and it is impossible to 

 estimate the time of vibration to within an accuracy of a second, 

 as is certainly necessary for the correct determination of E, 

 with the tape moving at the rate of only 1 mm. per minute. 



The phenomenon of interference I have many examples of 

 in earthquakes I have recorded ; and what Professor Milne* 

 describes as "earthquake echoes" may be explained in thid 

 way, combined very often with increasing or decreasing 

 amplitude of the disturbing cause. 



To give ocular demonstration of the truth of these theo- 

 retical conclusions, I decided to attempt to imitate a series of 

 waves acting on the pillar, and of known period. For this 

 purpose I had two boxes attached, one to the east and the 

 other to the west of the pillar, and in these I placed sawdust. 

 Depending from the roof-ceiling of the room, by a rope pass- 

 ing over two pulleys fixed vertically above the centres of the 

 two boxes, were two chains. The length of the rope was 

 such that the two chains just touched the two beds of sawdust 

 together. By pulling these chains up and down at definite 



* B.A. Reports, 1899, p. 288. 

 27 



