ON SEISMOLOGICAL INVESTIGATION. 



221 



their proper position on a curve of velocities, it may be assumed that they 

 are the result of fairly accurate observations. This also applies to the 

 two or three records on wave paths exceeding the 85° limit. In the 

 original diagram (' Report ' for 1897, p. 174) those observations which by 

 reference to orginal records are found untrustworthy are surrounded 

 by circles. 



The general results arrived at are easily remembered. If it is assumed 

 that motion is propagated round the earth, then the velocities over arcs 

 of 20°, 30°, 40° up to about 100°, which have lengths of 2,200, 3,300, 

 4,000, and 11,100 kilometres, are about 2, 3, 4, and 11 kilometres per 

 second. Along wave paths less than 20° the velocity of 2 kilometres per 

 second remains constant. For arcs greater than 100° the velocity 

 apparently increases at a rate somewhat less than the rate at which the 

 length of the arc increases. 



With the hypothesis that the vibrations travel along paths approxi- 

 mating to chords through the earth, then the above velocities must 

 be reduced. The actual velocities obtained as mean values from a 

 number of observations are given in columns 9 and 10 of the following 

 table : — 



Table shoiving the Helationskip hefAoeen the Apparent Velocities with ivJnch 

 Preliminary Treynors are propagated round or through the Earth, and dimen- 

 sions of the same, S)-c. The first four Velocities are deiived from Observations. 

 The last two are inferred} 



It will be observed that the quantities given in the eighth column are 

 approximately four times those in the ninth column. 



In questions relating to the direction taken by a wave in passing 

 through the earth, it must be remembered that this may not necessarily 

 be along a chord, but in consequence of refraction follow a path that is 

 curved. 



Apparent Duration of Freliminary Tremois. 



The following table gives the time intervals by which preliminary 

 tremors have outraced the longer period and larger waves constituting 

 the main portion of various earthquake disturbances. Beneath these 

 time records, inclosed in brackets, the distances of the various observing 

 stations from epifocal areas are given in geographical degrees or kilo- 

 metres : — 



' See British A'sociation Report, 1897, p. 174. 



