396 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



XL Earth Tremors and Teleseismic Disturbances. 



§ 56. Slight movements of the earth. 



A great many questions recently have been discussed relating to 

 slight slow oscillations shown by horizontal pendulums and other 

 instruments designed to detect delicate changes of level. No doubt 

 very many of these changes are correctly ascribed to the yielding 

 of the solid earth, due to variable loading of the soil by tidal, sea- 

 sonal and meteorological influences. But may not others be due 

 to the slow but steadily varying influence of subterranean forces 

 as discussed in this paper? Some of the strains thus constantly 

 arising would be released by microseismic disturbances which show 

 no periodicity or regularity ; while others would be cumulative and 

 have at length a small secular effect. The fact that the ground in 

 most places is comparatively so stable under the test of astronomical 

 observations seems to show that these effects usually are slight, 

 except in the neighborhood of the sea; but many small irregular 

 disturbances occur, and it is not improbable that a considerable 

 mumber of them may have their origin hidden deeper in the earth 

 ithan heretofore has been suspected. If great earthquakes originate 

 at depths of from eight to twenty miles, we may be sure that the 

 forces there at work produce some surface changes of level even if 

 no violent outbreaks occur. 



§ 57. Humboldt's views on earthquakes. 



In the fifth volume of the Cosmos (p. 288) Humboldt justly 

 remarks how Charles Darwin, " with his peculiar generalizing view, 

 has grasped the connection of the phenomena of earthquakes and 

 eruptions of volcanoes under one point of view." 



In his "Views of Nature," Vol. I, p. 361, Humboldt alludes to 

 the nearly simultaneous occurrence of volcanic and seismic phe- 

 i.omena in places widely separated, and says : 



" All these phenomena prove that subterranean forces are manifested 

 either dynamically, explosively, and attended by commotion, in earthquakes; 

 or possess the property of producing, or of chemically modifying a substance 

 in volcanoes ; and they further show, that these forces are not seated near 

 the surface in the thin crust of the earth, but deep in the interior of our 

 planet, whence through fissures and unfilled veins they act simultaneously at 

 widely distant points of the earth's surface." 



And in his " Travels," Vol. I, p. 172-3, he adds: 



