242 



EAETHQUAKES. 



joined by a free curve. The lower points are similarly 

 joined. The points of bisection of ordinates drawn 



between these two curves 

 are taken as points in a 

 curve to show the true 

 secular change in seismic 

 energy. 



By looking at this wavy 

 line it will be seen that the 

 intervals between maxima 

 and minima are closer to- 

 gether in early times than 

 they are later on. 



Thus, between the 

 eighth century and the 

 ninth century, points of 

 maximum and minimum 

 seismic efforts occurred at 

 times a century apart, 

 whilst later on, from the 

 eleventh to the fifteenth 

 century, they were at in- 

 tervals of 300 years apart. 

 By inspecting either 

 the wavy line or the re- 

 sultant curve, it will be 

 seen that since the ninth 

 century down to the pre- 

 sent time there has been 

 a decided decrease in seis- 

 mic energy. From the 

 ninth century down to the 

 fifteenth century this de- 

 crease is represented by a 



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