68 



REPORT — 1900. 



coming round the surface of the world in opposite directions. Further, the 

 repetition at a given station as a reflection of a disturbance at the antipodal 

 point of its origin might occur at an interval of time after the first move- 

 ment not very different from that separatmg the two surface trains. 



Such a possibility indicates that seismic repetitions cannot be ex- 

 clusively used to support the hypothesis of surface radiation. Examples 

 of earthquake recurrences are given in the following table. The tirst 

 column gives the numbers of the earthquakes in the British Association 

 registers and their origins. Where the position of an origin is not 

 known from observations made in its vicinity its latitude and longitude 

 are determined by one of the methods described in the succeeding sections 

 of this report (see pp. 79-80). Such determinations must only be re- 

 garded as approximations. The second column gives the arcual degree- 

 distances of the origins from the observing stations referred to in the 

 third column by their initial letters. In this third column there is also 

 noted the number of minutes' interval between the maximum motion and 

 its apparent repetition. The fourth column gives the calculated distance 

 of the observing station from the origin, and the nearness to which it 

 approximates to the corresponding figures in the second column is evi- 

 dently an indication of the value of these observations in determining 

 seismic foci. The basis for these calculations is that a surface-wave 

 travels 180 degrees in 105 minutes. In the last column the letters G, I, 

 and B (good, indifferent, and bad) indicate that the determinations in the 

 fourth column lie within 10°, 20°, or more than 20° from those in the 

 second column, which latter figures, however, it must be remembered, are 

 themselves but approximations. 



