96 Adams and Coker — Elastic Constants of Rocks. 



color process photographs and microphotographs of each of 

 the rocks employed. Before the investigation was completed, 

 Dr. Coker was called to the Professorship of Mechanical 

 Engineering in the Finsbury Technical Institute of London, 

 England, and was accordingly obliged to give up the work of 

 the research. His place was taken by Mr. Charles McKergow, 

 Lecturer in Mechanical Engineering in McGill University, 

 but who, immediately on the completion of the work, was 

 appointed to the Professorship of Mechanical Engineering in 

 the University of Virginia. A large number of the very care- 

 ful measurements of the elastic constants which are pre- 

 sented in the paper were made by Professor McKergow. 



Methods which may he used in the deterinination of the 

 Elastic Constants of Materials. 



The determination of the cubic compressibility of solid 

 substances is, as above mentioned, beset with serious difficulties. 

 On one hand every direct method which has been suggested 

 presents experimental difficulties which tend to impair its 

 accuracy, while on the other hand the indirect methods are 

 based on assumptions as to the isotropy of the materials, 

 which are not warranted in the case of certain rocks. The 

 indirect methods, however, depending on the theory of elas- 

 ticity, are capable of considerable variation, and it is of inter- 

 est to examine them in some detail in order to see whether 

 certain of them at least may not be depended upon to give 

 reliable and satisfactory results. 



The determination of the elastic constants of metals has 

 engaged the attention of many physicists and at the present 

 time a large amount of information exists as to the values of 

 these constants for various metals. 



It is well known that in homogeneous elastic substances, a 

 simple compression stress causes a lateral strain, which bears a 

 fixed ratio to the compression strain for any particular sub- 

 stance within the limit of elasticity. If then* we call^ the 

 stress on a plane perpendicular to x in the direction x, and e x 

 the corresponding strain, then for a direct compression stress 

 jj x there will be a strain in the direction of this stress of 

 amount p x /E, where E is Young's modulus, and lateral strain 

 <of magnitude jp x /mE, where m is the ratio of the longi- 

 tudinal compression to the lateral extension per unit of length. 



If we suppose further that a body is subjected to cubical 

 stress of intensity^, Ave easily see that for small and there- 

 fore superposable strains, the cubical strain e c is 



* See Ewing's Strength of Materials, Chapters I and II. 



