394 



Royal Society. 



I. Dynamical Contraction. — The opposite of dynamical expan- 

 sion. 

 II. Permanent Expansion of cooling. — (Ci\ystalline expansion.) 

 III. Temporary Expansion. — The thermal expansion clue to the 

 reglowing of the wire daring cooling. This is the proximate cause 

 of the stops aud kiclvs which occur in cooling. It is evanescent as 

 an opposing force, and must not be confounded with the permanent 

 or crystalline expansion of cooling, of which it is simply the effect. 

 We have, then, in cooling : — 



(1) Dj^namical contraction ; 



(2) Permanent expansion of cooling ; 



(3) Temporary expansion (cooling-kicks) ; and to these may 



be added 



(4) Influence of oxide. 



Fig. 1, Oommercial steel wire. First heating. 

 Fig. 2. Do. do. Second heating. 



Fig. 3. Air-hardened wire. 

 Fig. 4. Water-hardened wire. 

 Fig. 5. Annealed steel wire, 

 returns. 



N.B. — In eacli figure the upper curve refers to the heati 

 cooling effect. 



High-temperature contraction. Contracti 



the lower to the 



