ON SEISMOLOGICAL INVESTIGATION. 



69 



We have here twenty-four determinations, out of which thirteen are 

 considered as being good, four as indifferent, and seven as bad. The three 

 bad determinations for earthquake No. 337 may be explained by the 

 assumption that we have here been dealing with markings due to second- 

 ary shocks which simulated seismic repetitions, a view that is strengthened 

 when we refer to the seismograms of this earthquake. It must also be 

 noted that No. 337 was less than Nos. 333 or 338, from which it may bn 

 inferred that the original impulse was not sufficiently great to give rise to 

 duplications. The fact that the Cape of Good Hope records for 309 and 

 338 are bad may arise from the circumstance that this station was within a 

 comparatively short distance of the antipodes of these shocks, and there- 

 fore any wave coming from that point would be eclipsed in the records of 

 the main disturbance. The remaining two bad determinations may be 

 explained in the same manner that those for No. 337 have been explained. 



The Victorian record for No. 333 is of particular interest as indicating 

 that the time taken for an earthquake to travel round the world or to 

 traverse two diameters slightly exceeds 210 minutes. 



When considering whether these repetitions are to be regarded as 

 surface waves or as mass waves reflected from an antipodes, a feature not 

 to be overlooked is their smallness. To illustrate this I here give a table 

 for the thirteen good observations showing the amplitudes in millimetres 

 of the primary disturbances and those of their repetitions, together with 

 the arcual distance each may be supposed to have travelled. 



It is satisfactory to note that the magnitude of these repetition ampli- 

 tudes fairly accords with what might be anticipated (see p. 70). 



8. Amplitude in relation to Distance from an Oriiiin. 



In the following table amplitudes are expressed in millimetres and 

 occupy a position corresponding to the numerator of a fraction, whilst in 

 the position of a denominator distances from origins are expressed in 

 degrees. Observing stations are indicated by their initial letter or letters. 



Inasmuch as there are reasons for believing that the instruments 

 giving the subjoined records have not in all cases been adjusted to have 

 the same frictional resistances and as these records are few, the result 

 to which they point must bq received with caution. When they are 



