Earthquakes and other earth movements . earthquakesother00miln Year: 1886 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
Earthquakes and other earth movements . earthquakesother00miln Year: 1886 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 1
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