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Mi;. TJ. D. OLDHAM ON TIIK PROPAGATION OF 



Third Phase. Commencement. 



Here we get a result different from that obtained in the case of the first and 

 second phases, in that the time intervals increase in practically the same ratio as 

 the distances, and there is no indication of an increase of apparent velocity with 

 the distance. The irregularity in the values of the rate of propagation is easily 

 explicable by the difficulty of obtaining an accurate record of the commencement of 

 this phase of motion. Whatever may be the manner of propagation of this form of 

 wave motion, it seems to manifest itself in much the same manner as the ripples 

 which radiate over the surface of a pond. There is a band of larger and distinctly 

 visible ripples, and as these radiate they come to be preceded and followed by an 

 ever widening belt of longer and flatter wavelets whose limits cannot be determined 

 on account of the gradual manner of their decrease in height. In a similar manner 

 the surface undulations which form the last phase of a distant earthquake commence 

 as very long and flat waves, gradually increasing in height, and the exact moment at 

 which these will begin to influence an instrument may be materially delayed or 

 advanced by very slight differences in its sensitiveness. 



This may be illustrated by taking the maximum and minimum values for the rate 

 of travel given by the records averaged in the table given above. They are 



