142 3fr. John Milne [March 20, 



time. In Central Japan earthquake frequency had a maximum in 

 the ninth century, and since that time century after century, violent 

 shaking's have become less and less. In January 1)^44, at Comrie, 

 in Perthshire, twelve earthquakes were recorded. Now there may 

 not be one per annum. At the present time, in consequence of the 

 destruction of several large cities, the popular idea is that earthquakes 

 are on the increase. As a matter of fact, the world as an earth- 

 quake-producing machine has a steady output. On the average 

 about GO very large disturbances are recorded, and the greater number 

 of these, fortunately for humanity, have their origins beneath ocean- 

 beds or in sparsely inhabited regions. In addition to these mega- 

 seismic efforts, it is estimated that about 30,000 small earthquakes 

 take place per year, England's annual contribution to this number 

 being al)out half a dozen. If we had records like these extending 

 backwards through several ages, we might readily estimate the time 

 when seismic activity would cease. When this ceases, rock folding 

 will also cease, and the degrading processes resultiug in surface denu- 

 dation will be unopposed. Bit by bit land areas will be reduced to 

 sea level, and the habitable surfaces, as we now see them, Avill be no 

 more. 



An interesting observation bearing upon megaseismic frequency 

 is found in the analyses of registers relating to the North Pacific. On 

 the west side of that ocean seismic frequency is greatest in the summer, 

 while on the east side it is greatest in the winter. An explanation 

 for this is sought for in the seasonal alteration in the flow of ocean 

 currents, the oscillations of sea level and changes in the direction of 

 barometric gradients, which phenomena are inter-related. In summer 

 off the coast of Japan, the Black Stream runs perhaps 500 miles 

 farther north than it does in winter, while Dr. Omori points out that 

 although barometric pressure may on the Japan side of the Pacific be 

 low in summer, this decrease in load is more than compensated for by 

 the increased height of ocean level ; the inference is that the pressure 

 on the ocean bed is greater in summer than in winter, and this is the 

 time of the greatest seismic frequency. 



Another factor bearing upon earthquake frequency may perhaps be 

 found in the change in position of the earth's pole. A chart showing 

 the path of the earth's north pole indicates that its movements are by 

 no means always uniform. Although at times these may be nearly 

 circular, it also shows sharp changes in its direction of its motion. It 

 has even been retrograde. If on a chart showing these pole displace- 

 ments we mark^the time positions of world-shaking earthquakes, it is 

 seen that these?are grouped round the sharper bends of the pole-path. 

 World-shaking earthquakes have in fact been most numerous when 

 the pole path has deviated farthest from its mean position. The 

 observations embrace a period of 13 years during which 750 large 

 earthquakes were recorded. Although these earthquakes represent 

 large mass displacements, it is not supposed that they would be suffi- 

 cient to produce the observed pole movement. The pole movement, 



