70 



REPORT — 1900. 



plotted as curves it is seen that each has the same general character. 

 The rate at which amplitude at first decreases is about '2 mm. per 

 degree of travel. Earthquakes like Nos. 343 and 347 from whatever 

 may have been their amplitude in the epifocal district, are reduced 

 to an amplitude of 4 mm. after about 50° of travel. Larger earth- 

 quakes, like Nos. 337, 344, and 345, travelled 80° or 90° before their 

 amplitude sank to this quantity ; whilst the largest of all, Nos. 333 and 338, 

 show an amplitude of more than 4 mm. after travelling nearly halfway 

 round the world. From an amplitude of 4 or 5 mm. the rate of decrease 

 becomes less and less. For example, the amplitude of No. 337 between 

 77° and 105° falls from 5 mm. to 3 mm., or at the rate of "07 mm. per 

 degree ; whilst from 105° to 165° the rate at which amplitude decreases 

 has been "01 mm. per degree at travel. 



These slow rates of decrease indicate that it is reasonable to suppose 

 that the large waves of earthquakes may reach distant stations by 

 travelling in opposite directions round the world. 



If the large waves of earthquakes are merely surface waves, it would be 

 expected that oceans would exert a marked damping effect upon their ampli- 

 tude. Indications of this apparently exist in the records for earthquakes 

 Nos. 337, 338, and 347 (also see eai'thquake 263, p. 81). In the first the 

 amplitude for Toronto is greater than that observed in Mexico, the path 

 to the former being across North America, and the latter being sub- 

 oceanic. In No. 338 the record for Mauritius is less than that for the 

 Cape of Good Hope. In No. 333 this condition is, however, reversed. 

 Lastly, in No. 347 the Shide record, which refers to a comparatively long 

 continental path, is greater than the records for Victoria, the Cape of 

 Good Hope, Bombay, or Mauritius, the shorter paths to which are beneath 

 oceans. Although, for reasons already stated, stress cannot be laid upon 

 these observations, the latter at least suggests that we are dealing with 

 surface waves rather than with mass waves. 



9. Arcual Velocity in relation to Surface Configuration of the Earth, 



With the object of determining whether large waves are propagated 

 more quickly over continents than over ocean beds, whether the rate of 

 transmission along mountain axes is greater than in directions transverse 

 to the same, and generally to determine whether there are directions over 

 or through our globe in which motion is transmitted more rapidly than in 

 others, the following table has been prepared. The apparent surface 



