THE EARTH, A FAILING STRUCTURE. 
71 
quakes are more frequent when the earth and moon are in 
certain relative positions than at other times, the unusual 
stresses being produced by the weight of the water concerned 
in tidal movements. More earthquakes occur in years when 
the range of variation of latitude is great than in years when 
the range is small, and Omori states that “all of the destruc- 
tive earthquakes of recent years in Japan occurred exactly 
or very nearly when the latitude [at Tokyo ] was at a maxi- 
mum or minimum.” The variation of the latitude from its 
mean value — that is, the variation of the pole of the earth 
from its mean position — causes small unusual stresses in the 
earth. 
Such relations of earthquakes to other phenomena indi- 
cate that the earth is now a failing structure. 
The action of the earth under the tide-producing forces 
furnishes no information in regard to the strength of the 
earth. The stresses thus produced in the earth are much 
too small to serve as a test of strength. In Darwin’s in- 
vestigation, from which he concluded that the stress-differ- 
ences in the earth due to the weight of the continents might 
be as great as 4 tons per square inch, he also concluded that 
the greatest stress-difference produced in the earth by the 
moon’s tide-generating influence was less than 2 pounds per 
square inch (128 grams per square centimeter).* Such 
feeble stresses, far within the elastic limit, produce measur- 
able elastic deformation, and thus furnish a test of rigidity, 
of stiffness, but not of strength. 
It is important for various reasons to know whether the 
earth is a failing structure and to what extent it is failing. 
Curiously enough, in so far as our personal safety is con- 
cerned, the weaker the structure the safer we are. On a 
very weak earth continuously failing the changes in eleva- 
tion are so slow 7 that w 7 e have plenty of time, as a rule, to 
move back w T hen the sea encroaches upon the land, and w 7 e 
* See page 205 of Darwin’s paper. 
