DE. EVEEETT ON THE EIGIDITT OF GLASS. 191 



3. KiECHHOFP determined the value of only one constant <r, whereas we determine by 

 experiment two independent constants M and n, from which two others, a and k, are 

 computed. 



On the other hand, our method has the disadvantage of involving the unclamping and 

 reclamping of the mirrors between experiments on torsion and those of flexure, an 

 operation which introduces risk of bringing slightly different portions of the rod under 

 observation in the two cases. In future experiments it is intended to remove this 

 defect. 



No notice has been taken of possible effects due to the flexure of the arms which 

 support the mirrors. It is assumed that these effects are the same when the rod is bent 

 or twisted as when it is free, and on this assumption they will not affect the results. 



Again, the arms of the applied couples are not strictly equal to the distances E C, 

 F D (figs. 2, 3), unless the brass arms E C, F D are horizontal, and the further they 

 depart from horizontality the shorter do the arms of the couples become. In some of 

 the preUminary experiments, this deviation was measured, and when appreciable, allowed 

 for. In the final experiments it was so small and so nearly the same in torsion and 

 flexure that it is assumed to produce no appreciable error. 



Similar remarks apply to deviation from horizontality in the rays of light reflected 

 from the mirrors to the screen. 



The following values of a for different substances have been found by other experi- 

 menters : — 



KiECHHOFP, by the method above alluded to, foxmd for steel "294, brass -387. 



Wertheim, by a different method, glass (crystal) about -33. 



Professor J. Clekk Maxwell, by experiments in 1850, glass '332, iron '267. 



It will be observed that our own value for flint glass, •258, is smaller than any of these. 



