MODULUS OF ELASTICITY OF ROCKS. 5 



middle part of the specimen. The pLanes passing through the 

 edges of the fulcrams standing face to ftice shouhl be normal to 

 the length of the specimen. 



3. The four mirrors are so clamped in their proper positions 

 that the images of the scale reflected by them stand side by side 

 within the telescope-field. To adjust them properly requires much 

 practice. 



4. The constants of the micrometer-screw for all images are 

 determined. They are nearly equal to each other but not strictl'y 

 so. One mm. of the scale division is equal to about 23 divisions 

 of the micrometer-screw, which is again equivalent to a rotation 

 of 0-176x10-' rad. i.e. l"-068. 



5. Zero-readings are taken for all images in a fixed order ; 

 i.e. (i) right upper image, (ii) right lower image, (iii) left upper 

 image, (iv) left lower image. 



6. The suspended weights consist of some forty pieces of 

 equal weight. A definite number of pieces, say i m., are taken 

 off from one string and added on the other. The bending force 

 due to this is obviously mg, where y represent the value of gravity. 

 The time-record corresponding to this transposition of weights is 

 taken. 



7. After a certain time, the readings are noted for all images 

 in the same order as in the case of the zero-reading. 



8. Second transposition of weights ; the time recorded ; 

 scale-readings noted : and so on till a definite amount of bending 

 force is reached. 



9. The weight is then transposed in the opposite way so 

 that the force diminishes gradually and ultimately becomes op- 

 positely directed. In this way, a series of observations is made 

 to complete the cyclic process several times. 



