REID— INTERIOR OF THE EARTH. 



291 



seismograms, but it was not until 1900 that Oldham succeeded in 

 showing definitely that the seismograms of a number of Milne 

 instruments gave clear evidence of the existence of three groups of 

 waves. Oldham also published a diagram, which was an extension 

 of Seebach's so-called " hodograph," showing the relation between 

 the time of transmission of each group and the distance from the 

 earthquake origin, measured along the surface of the earth. Milne 

 soon improved these curves by adding observations of a large num- 

 ber of recorded shocks." The general forms of the transmission 

 curves are shown in the diagram. It will be seen that the curves 



0° 



20° 



40° 



60° 



80° 



.100° 



of the first and second " preliminary tremors," as Milne called the 

 first two groups of waves, are curved, indicating that the velocity of 

 transmission increases with the distance from the origin ; a con- 

 clusion which had already been drawn from earlier, but less ac- 

 curate, observations. Milne attempted to explain this by assuming 

 that the path of the seismic disturbance lay along the chord and not 

 along the earth's surface; this practically shortens the distance to 

 the observing stations, and if the curves are plotted, with distances 



3 Rep. of the Com. on Seismol. Investig., B. A. A. S., 1902, p. 7. 



