\ 



220 LECTURES. 



(119.) The application of the torsion pendulum to the measurement 

 of small fo]'ces, — Coulomb's balance of torsion, — Cavendish's experi- 

 ment of weighing the earth. The hair spring of a watch — new clock. 



Torsion is a means of exhibiting the elasticity of some bodies which 

 ordinarily appear inelastic. The elasticity of a lead wire may be 

 shown by torsion ; also of a rope of moistened clay. 



The degree of elasticity of some solids depends on a peculiar ar- 

 rangement of the molecules of the surface, called temper. Steel, 

 heated to a cherry red, and then plunged into cold water, has its 

 elastic force much increased — it becomes as hard and as brittle as 

 glass. If it be again heated until it exhibits a blue color, and is 

 again plunged into water, a "^ spring temper" is produced, or the 

 metal assumes a much wider range of elasticity. 



A tempered bar of steel is larger than one of the same weight 

 which has been suffered to cool gradually ; also on breaking the bar 

 the temper is found to be superficial. Probable explanation of tem- 

 per. The outer crust is suddenly cooled over a heated and dilated 

 nucleus — the latter shrinks in coolings anil leaves the crust in a state 

 of tension. Cast iron may also be tempered by the solicRfying pro- 

 cess called chill-casting. 



Grlass also possesses the property of receiving a temper. Large 

 drops of this substance let fall into water suddenly solidify at the 

 surface, and thus the molecules assume a state of tension analogous 

 to that of tempered steel. Pieces of glass of this kind are called 

 Prince Eupert's drops ; they will bear a considerable blow on the end, 

 but if the tail of the drop be broken, the whole explodes into a fine 

 powder. 



The molecular force developed in this explosion is astonishingly great 

 — a thick tumbler broken by it. 



The drops lose their peculiar property by being heated and gradu- 

 ally cooled. 



The existence of a state of tension in the unannealed drop shown 

 by polarized light. 



The method of annealing glass for domestic and other uses ex- 

 plained. 



The Chinese gong metal, called tam-tam, which consists of four 

 parts of copper and one of tin, possesses the remarkable property of 

 becoming hard and brittle by slow cooling. 



(to be continued in the next report.) 



