Part I. The loss and reconstruction of scientific sources with a particular reference to mechanics and theory of materials: The loss and reconstruction of the article by Hellinger in the Encyclopaedia of athematics.- The loss of the works by Archimedes and their reconstruction by Heiberg.- The loss of the works by Piola and the Italian tradition of Mechanics.- Part II. The recovery of the works by Hellinger, Archimedes, Piola and Heiberg: The monumental translation by Heiberg and his (nearly lost) commentaries written in Latin.- The masterpiece by Hellinger and its influences on modern continuum mechanics.- The forgotten Piola.-Part III. The recovery of the work by Archimedes and the role of Heiberg.- Part IV. The different ways of evaluating the importance of sources: why bibliometric indices?