148 



Dr. Hopkinson on Torsional 



[Dec. 12, 



has at hand, and which are obtained from the cinders of a copper- 

 furnace. The cavities when thus filled up are well rubbed so as to 

 escape notice, but they may usually be detected by looking at the 

 mirror obliquely. 



It was perhaps the presence of these bits of copper in the mirror 

 which Ou-tseu-hing saw broken up in the 13th century, that misled 

 him into concluding that the phenomenon of the magic mirror was 

 produced by the inlaying of denser copper in a portion of the face 

 exactly corresponding with the design on the back. 



When the face of the mirror has been made quite smooth, an 

 amalgam consisting, according to the Tokio makers, of half tin and 

 half mercury, with perhaps a trace of lead, or of 



Tin 69*36 per cent., 

 Mercury 30 „ 

 Lead 0*64 „ 



according to the analysis of MM. Champion and Pellet (" Industries 

 de l'Empire Chinois") is rubbed over the surface with a stiff straw 

 brush or with the hand. The mirror is finally wiped clean with a soft 

 kind of paper, mino-gami, " paper from the province Mino," which is con- 

 sidered to scratch the surface less than silk. Leather was formerly 

 never employed in polishing, as it would have been considered im- 

 pious to pollute so holy a thing as a mirror by touching it with 

 the skin of an animal; for under the old feudal system in Japan, 

 workers in skins, saddlers, and others, belonged to the Eta or pariah 

 class. 



When mirrors possessed by private people require brightening up, 

 in consequence of the surface tarnishing, the paste produced when 

 razors are sharpened on a hone is usually rubbed over the face of the 

 mirror. 



III. " On the Torsional Strain which remains in a Glass Fibre 

 after release from Twisting Stress." By J. HOPKINSON, 

 D.Sc, F.R.S. Received October 4, 1878. 



It has long been known that if a wire of metal or fibre of glass be 

 for a time twisted, and be then released, it will not at once return to 

 its initial position, but will exhibit a gradually decreasing torsion in 

 the direction of the impressed twist. The subject has undergone a 

 good deal of investigation, especially in Germany. The best method 

 of approximating to an expression of the facts has been given by 

 Boltzmann ("Akad. der Wissensch. Wien," 1874). He rests his 

 theory upon the assumption that a stress acting for a short time will 



