120 LOUIS. VESSOT KING 
purposes of ordinary engineering practice, a cube of the material 
is put into a testing-machine and the stress required to crush the 
cube is measured. ‘The value of this stress per unit area gives the 
value of the limiting stress-difference for the material as long as 
the boundary conditions are similar to those existing during the test. 
Although this condition generally holds in engineering construc- 
tion work and the criterion is valid, this may by no means be the 
case when the boundary conditions are very different from those 
existing in the test, as for instance in the case of rupture in the 
neighborhood of a cavity or in the case of rupture due to stress- 
difference deep down in the earth’s crust. It has been customary 
up to the present to employ the crushing test criterion in applica- 
tions to stresses in the earth’s interior.‘ A mathematical analysis 
of the experiments of Dr. Adams,’ described in the accompanying 
paper, throws a great deal of light on the question and can be made 
to give valuable data on the limiting strength of rocks under con- 
ditions of extreme pressure. 
It will appear that the crushing test criterion of limiting stress- 
difference (e.g., 27,000 lbs. per sq. in. for a cubical specimen of 
Westerly granite of two inches side) is much too low when conditions 
of stress approach those existing in the earth’s interior. 
For the purposes of geodynamics, however, ‘‘we require to know 
what is the limiting stress-difference under which a material takes 
a permanent set or begins to flow rather than the stress-difference 
under which it breaks; for if the materials of the earth were to 
begin to flow, the continents would sink down and the sea-bottoms 
rise up.’ 
A mathematical analysis of tests described in § 4 afford a valu- 
able means of obtaining information on this question. 
§ 2. CONDITIONS OF TEST 
The tests described by Dr. Adams on the compression of cylin- 
drical rock specimens containing cylindrical cavities and inclosed 
© Darwin, loc. cit. 
2 Adams, ‘‘An Experimental Contribution to the Question of the Depth of the 
Zone of Flow,” see this journal, p. 97. 
3 Darwin, loc. cit. 
