100 



Adams and Colter — Elastic Constants of Rocks. 



The linear strain perpendicular to the length of the speci- 

 men was measured by an instrument which had been designed 

 by E. G. Coker some time previously for experiments on the 

 lateral strains developed in metals.* This is shown in figure 

 2, and consists of a pair of brass tubes, B, B ', provided with 

 set screws, A, A\ for attachment to the specimen, and con- 

 nected together by a flexible steel plate, Jf, forming the f ul- 



Fig. 2. Perspective view of lateral extensometer. 



crum. The ends of the tubes near the fulcrum plate are 

 pressed apart by an adjustable spring, S, to insure a uniform 

 pressure on the screw points gripping the specimen. On the 

 opposite end of one of the tubes is a spring, D, of ebony press- 

 ing against a double knife-edge, K, seated in a shallow V- 

 notch cut in the end of the other arm. The knife-edge car- 

 ries an adjustable mirror, J/, so that if any change in the 

 diameter of the specimen occurs the two tubes move relatively 

 to one another in a horizontal plane, thereby causing the knife- 

 edge mirror to rotate ; the rotation of this latter is observed 

 and measured by a telescope and scale placed at a suitable 

 distance. 



For convenience in adjustment, there is a screw, Z, for 

 tilting the apparatus about the axis of the gripping screws, 

 and the tubes, B, B', are trussed to prevent vibration. This 

 instrument was calibrated by aid of a Whitworth measuring 



* See Proceedings Koyal Soc. Edinburgh. Session 1904-5, vol. xxv, pt. vi. 



