174 



REPORT — 1897. 



A glance at the above table, or the diagrammatic representation of the 

 same (tig. 15), shows that either thei-e have been great differences in the 

 velocities with which movements have been propagated to points equally 

 distant from given origins, which is unlikely, or that there have been 

 laraer errors in the determination of the time at which motion commenced 

 at different stations. 



Possible causes for these errors are easily found. 



Fig. 15.— Velocities of Earth-waves round or through the Earth. 



180 170 160 ;fO "W ISO 120 HO 100 30 (30 70 60 60 tO 30 20 W 



Degrees, 1° = 111 Km. 



1. Different instruments; some being horizontal pendulums recording 

 photographically, others being pendulums varying in length and in the 

 frictional resistance of pointers recording on smoked surfaces, may have 

 unequal degrees of sensibility. 



2. Similar instruments may be differently adjusted. 



3. When a record is received on a sui-face moving at a rate of about 

 20mm. per hour, the error in determining the time at which a disturbance 

 commenced may be 1 minute. 



4. A local shock may be mistaken for one arriving from a distance. 



