110 Adams and Goker — Elastic Constants of Hocks. 



It was fonnd that these precautions being observed, the expan- 

 sion at the ends of the column was practically as great as at 

 the center where the measurement was taken ; the difference 

 being so small that no serious discrepancy was introduced. 



In a number of cases accurate measurements were taken 

 during the successive cycles of loading and unloading to which 

 the specimen was subjected in order to bring it to a state of 

 rest. These were recorded in the case of the Baveno granite 

 and the Stanstead granite and serve to show how the hyster- 

 esis of the rock may be reduced to a minimum by subjecting 

 the test-piece to this treatment. The measurements of each 

 cycle usually occupied from ten to fifteen minutes. 



In the case of a majority of the rocks investigated, a num- 

 ber 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 differ- 

 ent specimens were no greater than those which were obtained 

 by measuring the same specimens with the instrument attached 

 to different sides. 



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 columns of rock, nineteen of glass, and two of 

 iron were employed in this investigation and every 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. 

 o-^Poisson's Ratio. This is the reciprocal of m. 



1 / 71% \ 



D— Modulus of Cubic Compression = — w)E. The reciprocal 



o \ 11% — 2 / 



of this gives the decrease in volume of a cubic inch of the 



material for a pressure of 1 pound per square inch applied 



on every side. 



