Adams and Coker — Elastic Constants of Rocks. 99 



with this method, as well as the facts with regard to Nagaoka's 

 figures just mentioned, it was decided to adopt a somwhat dif- 

 ferent method and one which avoided both torsion and flexure 

 and depended simply on strain produced by simple compressive 

 stress. This will be termed the " Method of Simple Compres- 

 sion." Among the possible indirect methods, this seems to be 

 the most satisfactory, since the assumptions necessary in the 

 calculation of compressibility are reduced to a minimum, and 

 the range of stress for which the ratio of stress to strain is 

 practically constant is great. 



It was found possible to measure the strain obtained very 

 accurately by means of an apparatus forming part of the equip- 

 ment of the testing laboratory of McGill University, for the 



Fig. 1. Instrument for determining the modulus of a simple strain. 



use of which we are indebted to Professor H. T. Bov ey. This is 

 an instrument designed by Professor Ewing, and of which a 

 sketch is given in figure 1. In this, A is a specimen of the 

 rock gripped by screws passing through a pair of collars, B, 

 which are 1*25 inch apart, to which latter metal rods, C, are 

 attached. The upper rod carries a glass plate, D, with a fine 

 line scratched upon it, the position of which can be adjusted 

 by a screw, E, while the lower rod carried a micrometer-micro- 

 scope, F. The upper and lower collars, B, are connected by 

 a stud, G, the upper one engaging with the conical hole of the 

 swivel piece, H, in the lower, and contact is maintained by a 

 spring, Z, while the weights of the microscoj)e and projecting 

 arms are balanced by lead cylinders, J. A buzzer was attached 

 to the upper lead cylinder which when operated caused a slight 

 vibration in the instrument, producing a perfect adjustment 

 as the pressure was applied. 



The proportions of this instrument were so adjusted that 

 one division on the micrometer scale corresponded to stoWo" 

 of an inch, and before using it the instrument was calibrated 

 by aid of a Whitworth measuring machine and was found to 

 be in correct adjustment. 



