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FROM TECHNICAL OBJECT TO SOCIAL UTOPIA The motivating forces of engineers' technological imagination in the XIXth century This paper highlights a number of underlying and structuring factors in the nineteenth century technological imagination of the civil engineer - a creative, inventive and speculative engineer who had often cast off all academic orthodoxy and, in a competitive environment, was devoted to the promotion of intellectual projects and material achievements. A representative sample of six engineers was selected for their careers and inventions. The study of their works (books, brochures, etc.) reveals their tendency, in the name of 'the law of irrepressible progress'? and by virtue of their fruitful inventiveness, to undo everything that had been done before them and to rebuild a better society for all. From better in the best of worlds, it is a small step towards the new ideal promised society. Although the construction of a utopia is not always explicitly recognized or achieved, it does constitute the underlying tendency, the asymptotic horizon of French engineers' speculations.