Adams and Coker — Elastic Constants of Mocks. 109 



0*08 inch in diameter and 0*125 inch deep and 1'25 inches 

 apart, lying at equal distances above and below the center of 

 the prisms. These holes were chamfered at the outer end as 

 shown in figure 4, and were found to afford the most perfect 

 attachment which could be secured for the points of the 

 instrument. 



By means of these prisms two sets of measurements of the 

 vertical compression could be made with each prism, by attach- 

 ing the instrument first to one pair of opposite faces and then 

 to the other. 



In some cases round columns were used (see fig. 5). These 

 were approximately 1 inch in diameter and 3 inches in length. 

 With these it was possible to make four sets of measurements 

 in compression with each column, by drilling eight pairs of 

 holes as above described, whose plains intersected at angles of 

 45° instead of 90° as in the square prisms. 



It was of course necessary in every case, whether prisms or 

 columns were employed, to exercise great care to have the end 

 of the test pieces very carefully faced and absolutely parallel 

 to one another. Before the actual measurements were made, 

 the rock in every case was brought to a " state of ease " in 

 the manner already described. 



The pressure was applied in most cases by a 100 -ton Wick- 

 stead testing machine, which was so carefully adjusted that it 

 was sensitive to a load of 4 pounds. 



The specimen having been placed in the testing machine and 

 brought to a state of ease was then after careful adjustment sub- 

 mitted to loads increasing in successive stages of 1,000 pounds 

 until the limit of safety had been reached, when the load was 

 reduced successively by the same amounts, accurate readings 

 being taken at each increment and decrement of load. The 

 maximum load employed in the case of most rocks was 9,000 

 pounds, equivalent to from 9,000 pounds to about 11,500 

 pounds per square inch according to whether a square or round 

 prism was employed. In the case, however, of some of the 

 stronger rocks a load of as much as 15,000 pounds per square 

 inch was employed. 



In the determination of the lateral strain, which w T as made 

 upon the same columns as those used for measuring the ver- 

 tical compression, care was taken that the theoretical conditions 

 were realized, and that the material was free to expand later- 

 ally, as otherwise the values obtained for the lateral extension 

 would be inaccurate. In all cases, therefore, the measuring 

 apparatus was set as nearly as possible upon the central section 

 of the test piece, and the ends of the specimen, after being 

 ground smooth, were coated with a thin film of oil, so that the 

 polished pressure plates of the machine would have as little 

 tendency as possible to prevent freedom of lateral expansion. 



