ELASTIC CONSTANTS OF ROCKS- 
II 
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 carries a micrometer microscope, 
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, 7, while the weights of the microscope and 
projecting arms are balanced by lead cylinders, /. 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. 
FIG. 2. Perspective view of lateral extensometer. 
The proportions of this instrument were so adjusted that one division on 
the micrometer scale corresponded to ^^ 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. This instrument enabled us 
to determine the modulus of simple compression with great accuracy. 
The linear strain perpendicular to the length of the specimen 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.* Figure 2 is 
*See Proceedings Royal Soc., Edinburgh, Session 1904-5, Vol. xxv, pt. vi. 
