68 
HAYFORD. 
acts as a highly elastic body. The observed rate of propaga- 
tion of the wave, combined with the known density of the 
material through which the wave is propagated, enables one 
to compute the rigidity. Such computations have been made. 
But the rigidity is simply the relation between the applied 
force and the distortion produced. It is the stiffness. Such 
computations furnish no information in regard to strength. 
As soon as one considers the stresses involved, at points re- 
mote from the source of an earthquake, it is evident that 
they are insignificant as compared with the elastic limit of 
the material. 
I conceive the earth to be a failing structure, slowly yield- 
ing to the forces acting upon it. I conceive that, at the 
source of an earthquake, stresses having accumulated to the 
breaking limit, a fracture occurs, and there is sudden move- 
ment, causing, as a rule, various other fractures at near 
points. From all these points of fracture elastic vibrations 
start out in every direction, and progress until they die out 
on account of internal friction ; and this may not be until 
the vibrations have traversed the whole earth. This con- 
ception of elastic vibrations traversing a failing earth, which 
is at the same time slowly yielding in a non-elastic manner 
to other stresses, contains an apparent contradiction which 
is, however, not real. 
It is not uncommon nor mysterious for a mass of ma- 
%J 
terial to respond with elastic vibrations to suddenly applied 
forces, while at the same instant it is yielding in a non- 
elastic manner to forces applied steadily without change of 
direction. A large mass of cold pitch will resist a light blow 
of a hammer, while at the same instant it is slowly yielding 
and flowing under the comparatively small stresses due to 
its own weight. 
In the deep copper mines of the northern peninsula of 
Michigan the behavior of the whole earth, with respect to 
earthquakes and stresses due to other causes, is well illus- 
trated on a small scale. At certain times during each day 
blasts are set off in the solid rock at various places in each 
mine. Each battery of blasts is a miniature earthquake. In 
