11 AN INVESTIGATION INTO THE 
to. The differences between the measurements thus made on rocks composed 
of several minerals were no greater than those found in the case of the lime- 
stones, 'which were composed of the single mineral calcite, or on glass. 
In the case of the majority of the rocks investigated, a number of prisms or 
columns cut from the same block of rock were measured in order to ascertain 
whether different test pieces would give identical readings. It was found 
as a result of these investigations that the differences between the different 
specimens were no greater than those which were obtained by measuring the 
same specimens with the instrument attached at different places. In the 
case, however, of the Quincy granite, test pieces from two different blocks of 
the rock were prepared, and it was found that while the several measurements 
made on each test piece agreed among themselves, there was a distinct 
divergence in the elastic constants of the two specimens of the rock. This 
was probably due to a difference in composition, as the two rocks differed 
somewhat in appearance. 
In the case of the green gabbro from New Glasgow, the results obtained by 
measurements made upon different parts of the same prism were discordant, 
for reasons which will be pointed out and which were dependent upon the 
structure of the rock. 
Fifty-five specimens of rock, nineteen of glass, and two of iron were employed 
in this investigation and every conceivable precaution was taken to insure the 
attainment of accurate results . The rocks in all cases were air dry, having been 
allowed to remain in the laboratory for several weeks after they had been 
cut, before the measurements were made. 
In the accompanying tables the following elastic constants are given: 
E = Young's modulus, i.e., the quotient of the longitudinal stress by 
the longitudinal compression. 
ff = Poisson 's Ratio ; this is the reciprocal of m. 
D = Modulus of Cubic Compression =- ( ) E. The reciprocal 
3 \m-2/ 
of this gives the decrease in volume of a cubic inch of the 
material for a pressure of i pound per square inch applied on 
every side. 
C = Modulus of Shear = ~ ( ) E. which is the quotient of tor- 
2 V W + I- 7 
sional stress to torsional strain. 
m = The ratio of longitudinal compression to lateral extension per 
unit of length. 
E and m are measured directly; the other values are calculated 
from them. 
