Actually, the use of the concept of equilibrium (in the Newtonian sense)  
is much older than Marshall. The Italian Montanari (1683) in discussing  
foreign trade argues that there is a close connection between the  
equilibrium of fluids and international trade.  
in Galiani (Della Moneta, 1751) the concept is clearly present (p. 91),  
and in his later Dialogues sur le commerce du bl=E9s (1770, pp. 236-37) =  
we  
have a very interesting application of the idea of equilibrium of bodies  
to the equilibrium of prices in the free market: =93rien n=92est si vrai =  
que  
les prix des bleds laiss=E9s en libert=E9 se mettent en =E9quilibre=94.  
  
Luigino