Adams and Coke?' — Elastic Constants of Rooks. Ill 



(7= Modulus of Shears — ( ) E, which is the quotient of 



2 \m + 1 / ^ 



torsional 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 calcu- 

 lated from them. 



These values in the case of each rock are given in inch and 

 pound units, and the results are summarized in a general table 

 on page 121. The measurements were made in these units on 

 account of the fact that the testing machine employed was 

 graduated to read pounds. 



For purposes of comparison, however, this latter table has 

 been recalculated in C.Gr.S. units, and the results are set forth 

 in the second table, to be found on page 121. 



In the case of metal, Poisson's ratio is generally arrived at 

 by stretching the bar and determining the value of the longi- 

 tudinal 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 each table the first transverse line designates the spec- 

 imen employed as a, b, <?, 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 three inches, but this is not 

 stated in the 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 *one 

 square inch in the case of a square prism and three-quarters of 

 a square inch in the case of a round column. 



The fourth transverse line contains the letters U or P, 

 wdiich 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. 



In the four succeeding lines the four elastic constants E, <r, 

 D and C are given as determined by each measurement. 



Am. Jour. Sci.— Fourth Series, Vol. XXII, No. 128.— August, 1906. 



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