BJGIDITY OF ROCKS AND BYSTERESI8 FUNCTION. 3 



be inexact by the later researches of Saint- Venant. (1) Vicat (2) prob- 

 ably was the first elastician who published a well-considered experi- 

 ment on yielding. In a paper " De fili Bombycini vi elastica," 



Göttingen 18-11, AVeber (3) wrote " deuplex tensionis effectus 



alter primarius seu momentaneus ac subitus, alter secundarius seu 

 subsequens et continuatus " It is then noted that this "pro- 

 longate continuata " is not permanent. An experimental discus- 

 sion of set und yielding is found in a memoir entitled " Sur la 



torsion des fils métaliques et " written by Ignace Giulio. (4) 



His conclusion is instructive : '* L'altération de forme produite par 

 l'action d'une force extérieure sur un corps élastique se compose de 

 deux parties : l'une indépendente de la durée de cette action et 

 sensiblement proportionnelle à son intensité ; l'autre, croissant plus 

 rapidement que la force qui la produit, et suivant une fonction de la 

 durée de son action." Then he says that when the external force 

 ceases to act, the first part of the deformation disappears instantane- 

 ously, but the second part persists, diminishing, however, continu- 

 ously with the lapse of time. 



G. Wiedemann, (5) in his paper ' : Ueber die Torsion," concludes 

 that the temporary torsions of a wire twisted for the first time by 

 increasing loads increase more rapidly than these loads, and that the 

 torsional sets or torts of the wire increase still more rapidly. In a 

 paper entitled " Memoire sur la torsion," G. Wertheini 00 also divides 



(1) Saint-Venant. Memoire sur la torsion des prismes ; Comptes Rendus, Tom. XXXVI. 

 1853. 



(2) Vicat. Annales des ponts et chaussés, 1 er semestre, 18.34. 



(3) W. Weber. W. Weber's Werke, herausgegeben v. d. König. Gesel. d. Wiss. zu Göt- 

 tingen. Bd. I. pp. 447-474. 



(4) Ignace Giulio. Memorie della reale Accademie delle Scienze di Torino. Serie II. 

 Torao. IV. 1842. 



(5) G. Wiedemann. Pogg. Annalen. Bd. CVI. 1859. 



(6) G. Wertheim. Annales de chemie et de physique. Tom. 50. 1857. 



