1919] 



on The Propagation of Earthquake Waves 



443 



two sets of wave-fronts shows that the differences between the forms 

 of the rajs of the two types of wave are comparatively slight. 



In addition to the different velocities which characterise the two 

 types of wave there is another point of difference which seems 



SEISMIC RAYS and 

 I50CHR0NIC LINES OR WAVE FRONTS 



worthy of attention. AVhen the speeds of propagation are plotted 

 against the corresponding distances from the earth's centre, the 

 curves representing the two types of wave are similar in form, rising 

 rapidly to a maximum and then becoming practically parallel to the 



