5 oo POPULAR SCIENCE MONTHLY 



For each of the eight periods of the sun-spot cycle, the percentage 

 of years of seismic and volcanic activity has been calculated from the 

 data of Fig. 1, as shown in the table at the end of this article, and has 

 been plotted in the frequency curves of Figs. 10 and 11. For Sayles, 13 

 complete sunspot cycles are available, and for Jensen, K). In the 

 same way the average intensity of the phenomena of the years of each 

 period has been calculated and plotted in Figs. 12 and 13. In order 

 to obtain the mean curve it is necessary to combine the six shown in 

 Figs. 10 to 13 into one. The first process is the combination of 

 Jensen's four curves into two which shall be comparable to the two of 

 Sayles. This is done in Figs. 11 and 15. In Fig. 14 the solid 

 line reproduces Sayles's line of Fig. 10 directly, while the dotted line 

 gives the mean between Jensen's two curves of Fig. 11. Thus we 'have 

 two mean seismo-volcanic frequency curves. In the same way, in Fig. 

 15 the solid line reproduces Fig. 12, and the dotted line gives the 

 mean of the two lines of Fig. 13 ; and we have two mean seismo-volcanic 

 intensity curves. In these curves, as in the others, percentages are 

 used, so that when diverse phenomena such as frequency and intensity 

 come to lie compared and averages computed, each receives the same 

 weight. In Fig. 16 the frequency curves of Sayles and Jensen shown 

 in Fig. 14 are combined into the solid line and the intensity curves of 

 Fig. 15 into the dotted line. In order to make the two curves com- 

 parable the maximum in each case has been reckoned as a hundred. As 

 a result of the combination of the data of our two authorities, the 

 personal equation is largely eliminated. It will be noticed that the 

 curves of Fig. 16 are much smoother than those of preceding figures. 

 This is especially true of the frequency curve where there is least lia- 

 bility to errors of judgment. Finally, in Fig. 17 the two curves of 

 Fig. 16 are combined into one, shown by the solid line. This represents 

 the net result of Sayles's data as to the combined seismic and volcanic 

 frequency and intensity of 13 complete sun-spot cycles, and of Jensen's 

 independent data as to the uncombined seismic and volcanic frequency 

 and intensity of 10 complete sun-spot cycles. The dotted line in 

 Fig. 17 is the mean sun-spot curve derived from Fig. 1, and calculated 

 and plotted in precisely the same manner as the six curves of Figs. 

 10 to 13 from which the mean seismo-volcanic curve is derived. For 

 the sake of convenience in comparison, the sun-spot curve has been 

 plotted with the minimum at the top. . 



The resemblance between the mean sun-spot and mean seismo- 

 volcanic curves is extraordinary. The maximum of the one occurs 

 at the same time as the minimum of the other, and in both cases there 

 is a steady progress from maximum to minimum and back. If our 

 terrestrial data of earthquakes and volcanoes were as complete as our 



