54 Prof. H. Nagaoka on the Elastic Constants of 



on the one hand the wants of physicists, whose aim is to apply 

 dynamics to the study of the geological phenomena, and on 

 the other to meet the needs of seismologists, engaged in 

 solving the problems touching the propagation of seismic 

 waves. 



Preparation of the Specimens. — The present experiments 

 deal principally with the determination of Young's modulus 

 and the modulus of rigidity, made on specimens of rocks 

 which were easily accessible. 



The number of rocks examined amounted to about eighty 

 different specimens collected from various localities. These 

 rocks were first cut in the shape of a rectangular parallele- 

 piped, and afterwards carefully polished into prisms of nearly 

 1 centim. square cross-section and 15 centim. length. It 

 was at first proposed to experiment with much larger speci- 

 mens, but it was generally found impossible to find a large 

 homogeneous piece with no trace of cleavage ; in addition to 

 this, the apparatus with which the elastic constants were to 

 be measured w r ould become cumbrously large, and require 

 great solidity, increasing at the same time the difficulties of 

 experiment. 



Most of the specimens were apparently isotropic, but on 

 close examination it was found that the isotropy was only 

 superficial. Rocks as slates with distinct sedimentation planes 

 were generally cut parallel and perpendicular to them ; where 

 such planes of symmetry were not easily discernible, the 

 specimen was conveniently cut into prisms. 



The thickness of these prisms was measured by a contact 

 micrometer reading, by means of a vernier, to ^-jto mimm - 

 at three different places in the middle line of two opposite 

 faces ; namely, one at the middle and two at one quarter 

 distance from the ends. The mean density of the prism was 

 measured by dividing the mass by the volume, which was 

 calculated from the known length and thickness. The density 

 of several prisms cut from the same sample did not generally 

 agree, showing that the material was only roughly homo- 

 geneous. 



Modulus of Elasticity.— -Young's modulus was measured 

 hj flexure experiment. The specimen to be tested was placed 

 on two steel wedges, which served as fulcrums. The edge of 

 the wedge was slightly rounded in order to prevent cutting 

 on applying heavy weights. The flexure due to the weight 

 hung at the middle of the prism was measured by means of 

 a scale and telescope. By a special arrangement, a plane 

 mirror was attached to the prism at the place where it rested 

 on the wedges. The mirror was nearly vertical and the image 



