ELASTIC CONSTANTS otf ROCKS. 23 
These values in the case of each rock are given in the respective tables, 
expressed in inch and pound units, and the results are summarized in 
a general table on page 69. 
The measurements were made in these units on account of the fact of the 
testing machine employed was graduated to read pounds. 
For purposes of comparison, however, this latter table has been recalculated 
in C. G. S. units, and the results are set forth in the second table to be found 
on page 69. 
In the case of metal, Poisson's ratio is generally arrived at by stretching 
the bar and determining the value of the longitudinal extension divided by 
the lateral contraction. In case of rocks the tensile strength being low and the 
materials being brittle, it is more convenient and more accurate to make the 
determination by compressing a short bar or column, and determining the 
value of the longitudinal compression divided by the lateral expansion. This 
gives the value designated as m, of which Poisson's ratio is the reciprocal. 
Theoretically one method is as accurate as the other. In actual practice it 
might be supposed that the short compression columns in question would not 
expand quite so much at the ends as in the middle because of the fric- 
tion against the compression plates. In order, however, to cause these to slip 
as easily as possible over the ends of the column, the surface of the rock in con- 
tact with them was always made very smooth and also was slightly oiled. .It 
was found that, these precautions being observed, the expansion at the ends 
of the column was practically as great as at the center, where the measure- 
ment was taken, the differences being so small that no serious discrepancy 
was introduced. 
In the tables the first transverse line designates the specimen employed 
as a, 6, c, or d. The second line gives the diameter of the specimen, which 
is often slightly different in the two directions. The length of the column 
in all cases was about 3 inches, but this is not stated in th'e table, as the 
compression is not measured on the total length of the column, but on 
the length of that portion of it which lies between the points of attachment 
of the instrument. 
The third line gives the area, which is approximately i square inch in the 1 
case of a square prism and three-quarters of a square inch in the case of a 
round column. 
In the four succeeding lines the four elastic constants E, <r, D, and C, are 
given, as determined by each measurement. 
Another transverse line contains the letters U or P, which designate the 
two diameters of the column when two measurements were made on the 
same square prism, these two directions being always at right angles to 
one another. In the case of round columns, on which measurements were 
frequently made in several planes, these are designated as "first holes," 
"second holes," etc. 
