ON SE1SMOLOGICAL 1NVKSTIUATIONS. 33 



immediately after the earthquake and could therefore be regarded as a 

 consequence of the same, the companies were free from liability. The 

 result was that a careful inquiry was instituted as to the exact time 

 of the earthquake. This involved consultations with observatories near 

 to and at great distances from the stricken district as to the exact time 

 at which the earthquake had taken place. The result of these investi- 

 gations was that 2.22 p.m. in G.M.T. was adopted as the time of the 

 disaster, but from information received since this inquiry, I am led to 

 think that a safer estimate is 2.21 ± 1 minute. 



The records taken at stations all over the world, if we only consider 

 the times at which the first motion or P, was noted, lead with certain 

 variations to the same conclusion. Observatories in all the continents 

 rightly concluded that their records referred to the Guatemala earth- 

 quake, but the idea that these records might also refer to several other 

 earthquakes does not appear to have been considered. 



We expect maxima to recur at regularly spaced intervals when the 

 period of the pendulum approximates to that of the ground. Recur- 

 rences of maxima at varying intervals which we have here to consider 

 suggest a variable period in the movement of the ground. Although 

 this supposition may be true, it does not preclude the idea that accre- 

 tions of activity may arise from the generation of secondary disturbances. 



Mr. R. D. Oldham, who has made a careful study of this earth- 

 quake (see Proceedings of the Royal Society, vol. 76) writes to me as 

 follows : ' It seemed as if the well-defined maximum at 90° to 100° 

 was due to the combined effect of a group of waves, the faster travelling 

 having caught up the slower at about that distance, and these as they 

 travelled on separated again giving a long drawn out seismogram with 

 no defined maximum but a series of bulges, due partly to interference 

 of the waves travelling at different rates and partly to interference 

 between these and the swing of the pendulum.' 



With regard to this earthquake we know that it originated about 

 2.21, and records from stations on the American continent, whatever 

 phase of motion we consider, " support this conclusion. Very distant 

 stations from Guatemala, however, only fall in line with this so far as 

 Pj is concerned. This first maximum recorded at Capetown 2h. 58m., 

 Calcutta 3h. 13m., Bombay 3h. 8m., Kodaikanal 3h. 6m., and Perth 

 oh. 4m., apparently refers to an epicentre in the Indian Ocean, which 

 lies about 60° East and 35° South, and not to Guatemala. This dis- 

 turbance originated at about 2h. 34m. The second maximum at Cape- 

 town 3h. 26m., Bombay 3h. 43m., Kodaikanal 3h. 52m., and Perth 

 3h. 4m., approximately accords with Guatemala. The difference in time 

 between the Guatemala shock and the one in the Indian Ocean is about 

 fourteen minutes. The time taken for a compressional wave to travel 

 between these two origins, or 146°, would be about 21m. This being 

 so, unless we admit an error of six or seven minutes in one of these time 

 determinations, which might easily be the case, we cannot say that the 

 second earthquake was brought about by compressional waves from 

 Guatemala. A relationship is not proven, it is only suggested. 

 First of all it may be noticed for this earthquake, and also for others, 

 that the ground moved for a longer time at very distant stations from 

 the epicentre than it did at stations which were comparatively near 

 to the same. At Baltimore, Toronto, Victoria, Cordova, Edinburgh, 

 1911. d 



