AFTER-SHOCKS AX I) SrACE-DISTRir.lTroN OF SEISMIC WAVES. 17 



where the seismic centre is outside the region which we are con- 

 sidering. Any OIK' avIh) Ikis any knowledge of astronomy knows 

 that the ratio of the niimhers of eclipses of the moon and of 

 tlie sun theoretically observable at a given observatory differs 

 from the actual number of eclipses known to occur. 



Similarly, though of a wholly difierent nature, the actual 

 frequency at a given station may be different from that indicated 

 in the above sections of this paper. A certain region, notwith- 

 standing the scarcity of the quakes originating in it, may be 

 frequently visited by seismic waves ]iroceeding from the sur- 

 rounding districts. Again, one region may be shaken so slightly 

 {js to escape attention, while the other weaker region is violently 

 damaged by the same seismic energy. 



A complicated case such as the above, may be most clearly 



illustrated by analogy taking the case 



of heat. Let four thick pieces of 

 copper {A, C) and lead {B, D) be 

 arranged side by side as in the an- 

 nexed figure. Suppose we heat the 

 system at a point p to a high 

 temperature, sufiicient to melt the 

 greater part of the lead ])iece B, 

 but little of the copper piece C. 

 Then, taking into account the heat 

 conductivity and melting j)oint of 



the two metals, we may easily so adjust the breadths of the 

 pieces B and (■, that the lead piece 1) also melts in the part 

 where it is in contact with the copper piece C, though the latter 

 <loes not participate in the melting. The remaining piece Ay 

 tlîougli it is nearly as distant from the source of heat as the 



