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Jules Dupuit (18 May 1804 - 5 September 1866)  was
a France|French civil engineer and economist.  

He was born in Fossano, Italy then under the rule
of Napoleon Bonaparte.  At the age of ten he
emigrated to France with his family where he
studied in Versailles - winning a Physics prize at
graduation.  He then studied in the Ecole
Polytechnic as a civil engineer.  He gradually
took on more responsibility in various regional
posts.  He received a Légion d'honneur in 1843
for his work on the French road system, and
shortly after moved to Paris.  He also studied
flood management in 1848 and supervised the
construction of the Paris sewer system.  He died
in Paris. 

Engineering questions led to his interest in
economics, a subject in which he was self-taught. 
His 1844 article was concerned with deciding the
optimum toll for a bridge.  It was here that he
introduced his curve of diminishing marginal
utility.  As the quantity of a good consumes
rises, the marginal utility of the good declines
for the user.  So the lower the toll (lower
marginal utility), the more people who would use
the bridge (higher consumption).  Conversely as
the quantity rises (people allowed on the bridge),
the willingness of a person to pay for that good
(the price) declines. 

Thus, the concept of diminishing marginal utility
should translate itself into a downward-sloping
demand function.  In this way he identified the
demand curve as the marginal utility curve. This
was the first time an economist had put forward a
theory of demand derived from marginal utility. 
Although not the first time that the demand curve
had been drawn, it was the first time that it had
been proved rather than asserted.

Dupuit's logic was suspect in that it skipped over
the fact that marginal utility is particular to an
individual, while market demand is an aggregate. 
Nothing was said on the interpersonal
comparability of utility in order to connect
personal marginal utility to aggregate demand. 
Dupuit, however, did not draw a supply curve and
thus did not get price-determination into his
story.

Dupuit went on to define "relative utility" as the
area under the demand/marginal utility curve above
the price and used it as a measure of the welfare
effects of different prices -- concluding that
public welfare is maximized when the price (or
bridge toll) is zero.  This was later known as
Alfred Marshall|Marshall's "consumer surplus".

Dupuit's reputation as an economist does not rest
on his advocacy of laissez-faire economics (he
wrote "Commercial Freedom" in 1861) but on
frequent contributions to periodicals.  Wanting to
evaluate the net economic benefit of public
services, Dupuit analysed capacities for economic
development, and attempted to construct framework
for marginal utility|utility theory and measuring
the prosperity derived with public works. He also
wrote on monopoly and price discrimination.

== External links ==
*
http://cepa.newschool.edu/het/profiles/dupuit.htm
Biography of Jules Dupuit