198 EARTHQUAKES. 
deep in order to escape these artificial tremors, which 
affected the earth through a coarse, pebbly gravel to 
nearly this depth. 
The observations of M. d@ Abbadie and of George and 
Horace Darwin have abundantly demonstrated that 
natural earth vibrations are incessant and universal. It 
is also well known that at times there are large wave- 
like undulations which travel over the earth’s surface and 
mark their progress by the disturbance of the levels of 
lakes and seas. These movements or undulations are 
known as earth pulsations. 
That a body as dense as the earth and of its magni- 
tude should throb and pulsate as if it were a thing of 
life, seems hardly possible ; but, when we measure the 
forces which constantly operate within it, and act upon 
it from without, we learn that its magnitude and density 
are great or small only in a relative sense ; its rocks are 
more resisting than its water, but yet when the tidal 
forces of nature play upon the rocks they are twisted, 
contorted and broken as if they were glass toys in the 
hands of a giant. 
Barometrical pressure alone has been shown by Mr. 
George Darwin sufficient to depress a continent. On 
the assumption that the earth has a rigidity like steel 
a ‘*barometric rise of one inch over an area like 
Australia, gives a load sufficient to sink that continent 
two or three inches.” 
The tides have been shown sufficient to cause our 
shores to rise and fall with their ebb and flow. 
We feel assured, therefore, that known forces are suf- 
ficient to cause earth tremors and pulsations; and it 
seems but reasonable to ascribe to these same or kindred 
forces—when acting with greater intensity and power— 
the causes which, under favorable conditions, produce 
all the phenomena of earthquakes. Yet, as a matter of 
fact, it is found that what we may call external causes— 
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