CHANGE OF ELASTIC CONSTANTS OF FERROMAGNETICS. 31 



to a stand, cavrios the scale pan tor the weight W. Near the 

 upper and lower ends, the spechiien touches the horizontal axes 

 carrying two mirrors M and 3f respectively. The axes are so 

 pivoted at their ends, that they rotate with almost no friction. 

 ^S' is a vertical glass scale illuminated from ])ehind ; its image 

 formed by successive reflexions in M and 31' is observed by a 

 elescope T. If the load on the pan causes the elongations o/' 

 and ol respectively at 3r and M, the rotations of the axes are 

 given by 



<P'=— and <P=—y 



where r is the common thickness of the two axes. Again, if 71 

 and 71 be the deflections of the scale due to separate rotations <p' 

 and <f respectively, we have 



^ = 27 and <p = -^^^^^-^^ 



where d=31'S and a = 3[31'. The actual deflection observed is 

 s=n—n. 



Now ' dl-ul'=r((p-(f') 



=^('-{;[i+(t-7)(i- :;)]}• 



In our experiments, 



J=624.5cm., a=6.5.83cm., r=0.795rara., 



and 4- =0-1054. 



a 



n' 

 Since n was very small compared with s, and — appears as 



a small correction, it was sufficient for our purposes to determine 

 ?i' roughly only once for a specimen throughout the sets of obser- 

 vations. This was easily done by directing the telescope to 31' and 



