Claude Louis Marie Henri Navier, 1785-1836.

French engineer, mathematician and economist.

Claude Louis Marie Henri Navier is a towering figure in engineering history, considered the father of structural analysis.  His importance in economics lies in his development of cost-benefit analysis, as a predecessor of Jules Dupuit. 

Originating from Dijon, Burgundy, Henri Navier graduated from the École Polytechnique in 1804 and the École Nationale des Ponts et Chaussées in 1806.  While following a successful career as a civil engineer, Henri Navier remained involved as a teacher at the ENPC, becoming full professor by 1830.  Navier was a mentor for a critical generation of engineer-economists (notably Dupuit).

Navier's principal contribution to economics is his 1830 article on the evaluation of public works (roads, bridges, etc.)   In typical engineering fashion,  public works were traditionally evaluated in terms of their costs to produce, and choice of projects ranked accordingly.  For example, if there are two possible routes to lay a road from Paris to Lille, then the route chosen will be the one that is costs less to produce. However, Navier pointed out this was evidently unsatisfactory for public works, where the objective was not to benefit the producer (the government), but rather to benefit the community at large.  Appealing to the cost-oriented classical economics, Navier proposed to measure the benefit to the community in terms of "costs saved" by consumers.  A particular route might be more expensive to build than another, but it might lower transportation costs to users of the road by more.  Navier proposed a decision rule that projects should only be undertaken if the total community benefit exceeded total recurring costs, e.g. if annual cost savings to users of a road exceeded the interest on capital plus maintenance costs of the road.

Navier's method of cost-benefit analysis was the starting point of  Dupuit's 1844 breakthrough paper.  In particular, Dupuit made two important corrections. He first criticized Navier's narrow focus on transportation costs to measure cost savings, noting that there were other types of savings that need to be included (e.g. access to new sources of cheaper raw materials).  Dupuit's second point was more revolutionary: Navier had recognized that cheaper transportation costs not only benefited existing traffic, but might induce an increased volume of traffic. Navier proposed to measure the total benefits by applying the same costs per unit saved by old traffic to estimate the costs saved by the new traffic. Dupuit noted that this was an error that led to overestimation - as evidently the new traffic did not think it worthwhile to travel before, so they are not saving the same costs per unit as the old traffic.  From this kernel, Dupuit went onto propose to calculate consumers' surplus instead. 

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