The Law of Demand 



63 



FIGURE 15. The law of demand. 



subsequent work : The doctrine of the uniformity of the 

 demand function and the doctrine of the elasticity of 

 demand. The exposition of these two doctrines will be 

 facilitated by reference to Figure 15, in which, accord- 

 ing to the usual practice, quantities of commodity are 

 measured upon the axis 

 of abscissas, and the cor- 

 responding prices per 

 unit, upon the axis of 

 ordinates. 



The doctrine of the 

 uniformity of the demand 

 function, which is trace- 

 able to Cournot, 1 but is 

 especially stressed by 



Professor Marshall, has been put in these words: 

 " There is then one general law of demand viz., that 

 the greater the amount to be sold, the smaller will 

 be the price at which it will find purchasers ; or, in other 

 words, that the amount demanded increases with a 

 fall in price and diminishes with a rise in price." Re- 



1 Cournot: Recherches sur les principes math&matiques de la thtorie 

 des richesses, 21, 22. Assuming that the relation between price 

 and the amount demanded is represented by F(p), he says, p. 54: 

 " Si la fonction F(p) est continue, elle jouira de la proprie'te' commune 

 a toutes les fonctions de cette nature, et sur laquelle reposent tant 

 duplications importantes de 1'analyse mathe*matique: les varia- 

 tions de la demande seront sensiblement proportionelles aux varia- 

 tions du prix, tant que celles-ci seront de petites fractions du prix 

 originaire. D'ailleurs, ces variations seront de signes contraires, 

 c'est-a-dire qu'a une augmentation de prix correspondra une dimi- 

 nution de la demande." 



