SOME time ago, an attempt was made to develop a practical method for the appraisal of projects of road construction.1 The proposed method is one of comparative statics and compares the national product before and after the construction of a road. The essential elements of the method are the following. The whole economy is divided into a number of geographically separated centers. The movement of products from one center to another gives rise to transportation costs and, consequently, the price of the product of center i in center k depends on the costs of transportation from i to k. Supply and demand equations are assumed for each center and each product. The supply of each product i depends on the price of product i and the prices in i of all other products as cost elements. Two alternative assumptions are made with regard to the reactions of demand to price changes:

Edited by L.H. Klaassen, L.M. Koyck and H.J. Witteveen, Presented on the occasion of his 25 years jubilee as a professor at the Netherlands School of Economics at Rotterdam. (With a bibliography of professor Tinbergen's work).