THE EARTH, A FAILING STRUCTURE. 
John F. Hayford. 
(Address of the retiring President, delivered before the Society, 
Saturday evening, December 7, 1907.) 
The earth is a failing structure. Stresses are set up within 
it by many forces. It is yielding to those forces and is being 
deformed beyond its elastic limit. The yielding certainly 
occurs every year, and probably every day and every hour. 
This is the proposition which I lay before you to-night. 
Some of the reasons for believing this proposition will be 
sketched briefly. 
A competent structure may be defined as one which bears 
the stresses brought to bear upon it without permanent 
injury to itself. When a competent structure is unloaded, 
it returns to its original shape and size by virtue of its elas- 
ticity. Its deformation while loaded is fixed by the elastic 
constants of the material. A steel bridge under normal con- 
ditions carrying safe loads is a good illustration of a compe- 
tent structure. Under every additional pound of load applied 
it moves to a new position. It responds to every shifting of 
the loads upon it. Every motion is governed by the laws of 
elasticity. When the loads are removed, the bridge goes 
back accurately to its former shape and position and is with- 
out any discoverable difference in any respect from its state 
before being loaded. 
The characteristics of a failing structure may be best illus- 
trated by the well-known behavior of a piece of bridge steel 
being tested to destruction, under tension, in a testing ma- 
chine. Suppose that successively larger loads are applied 
and removed. In the earlier stages of the test, as long as 
the loads are within the elastic limit for that material, the 
9— Bull. Phil. Soc., Wash., Vol. 15. (57) 
