Adams and Coker — Elastic Constants of Rocks. 101 



machine and the scale adjusted so that one division corre- 

 sponded to one millionth of an inch. 



Application of the Method of Simple Compression to the De- 

 termination of the Cubic Compressibility of Metals. 



The behavior of such metals as wrought iron and steel over 

 a wide range of stress shows that these metals may be con- 

 sidered as almost perfectly elastic. The results of the theory 

 of elastic bodies may therefore be applied in their cases with 

 great confidence. 



As a typical example of the behavior of such materials we 

 may consider the deportment of a specimen of wrought iron 

 when subjected to a cycle of compression stresses, commencing 

 at 1,000 pounds and rising to 9,000 pounds, afterwards re- 

 turning to the original load. The readings obtained for the 

 longitudinal and lateral strains will show in such a case that 

 equal increments or decrements of load produce strains which 

 are very exactly proportional thereto. This is clearly shown 

 in a plot of these readings, where the ordinates represent the 

 total load and the abscissae represent strains. In both cases 

 the relation of stress to strain is represented by a straight line 

 returning upon itself. Traces which vary but very little from 

 the ideal straight line are given by black Belgian marble, as 

 will be seen on page 114. 



Such results afford an arbitrary standard by which can be 

 judged the degree of approximation to perfect elasticity ex- 

 hibited by other metals and by rocks under similar conditions. 



If we now calculate the value of the modulus JE iov simple 

 compression, since this is the relation of the compression 

 stress j? to the strain <?, we have 



p=JEJe. 



If we call A the cross-sectional area of the specimen when 

 stressed by a load P, and x the decrease of length over a 

 measured length Z, gripped between the screw points of the 

 measuring apparatus, we obtain 



xA 



which in the case of a specimen of wrought iron examined for 

 a range of 8,000 pounds, gave a value of 28,100,000, the units 

 being pounds and inches. 



The ratio m of the longitudinal strain to the lateral strain in 

 the same case was 3'65, and using the formula 



3 in -2 



