MODULUS OF ELASTICITY OF ROCKS. 3 



A rough sketch of the arrangement is shown in Fig. 1. The 

 specimen is placed horizontally and, when it is bent, its plane 

 of curvature is also horizontal. There are necessarily one scale, 

 four fulcrums and four mirrors, of which two mirrors Mj and Mo 

 are attached to the specimen as in König's method, while the 

 others M3 and M4 are rigidly fixed to the support. Four different 

 images of one and the same scale S are to be seen in the field 

 of the telescope T, Fig. 2. They are all reflected twice by the 

 following mirrors respectively : — 



Right upper image reflected by the mirrors Mj and Mo, 



right lower „ ,, „ ,, Mi and M4, 



left upper „ „ ,, „ M3 and Mo, 



left lower „ ,, „ „ Mg and M4. 



The apparatus is shewn in Figs. 3, 4 and 5, in its front- 



and side-views as well as in its plan. The two mirrors Mi and 



M2 rotate as the specimen is bent, while the other mirrors M3 



and M4 are fixed unless the apparatus itself is displaced. The 



fulcrums Fi, Fo, F3 and F4 are so adjusted that the edges of any 



two of them lie in a vertical plane. A small framework F, which 



is shown in Fig. 6, serves to apply bending force to the specimen. 



The frame- work consists of two wedges, one fixed (W,) and the 



other movable (W2) inside a proper case. After placing a proper 



specimen between the two wedges, the movable wedge W.. may 



be pushed firmly against the specimen by the fixed screw S. 



At the extremities of the strings Si and So, which run over small 



pulleys Pi, P2 etc. towards the observer, some weights are hung 



which supply the bending force. The support of the fulcrums 



is made of soft iron, which is rigidly screwed on a wooden block. 



From what has been just described, it may be easily seen 



that, when equal weights are hung on both Si and S2, no bending 



