MODULUS OF ELASTICITY OF ROCKS. 



TABLE I. 



That is to say, provided a and ß are taken as algebraic quantities 

 having proper signs, we have simply 



d=a-}-ß 

 cij=ß — a 

 Let t' be the last position of the telescope, and put 



R.U. =The deviation of the right upper image, 

 Il.L.= ,, „ ,, ,, right lower image, 



L.U.= ,, ,, ,, ,, left upper image, 



L.L.= ,, ,, „ ,, left lower image. 



Then, from simple geometry, it may be easily proved that 

 L.L.=o^-f (D-fc + d)}', 

 L.V. = d+(D + c + d)r-2dw, 

 'R.h.=^d+(D + G + Q\)r+2{c + d)d, 

 E.U. = o + (D + c + d)j'-2d.^+2(c + d)^, 

 where ss' = c , sa=d , ts' = D. 



If there is no external disturbance, evidently we have 



ß = , /' = , = 0, 

 so that L.L. = o 



L.U. = 2d«, 

 R.L. = 2(c-fd)a, 

 R.U.=2(c-fd)« + 2d«, 



= 4{d-f|}a, 



the last of which is a well known form. 



