This chapter focuses on the audience’s attention to the correspondence of Gottfried Wilhelm Leibniz and Christiaan Huygens with Denis Papin, which E. Gerland recently edited on behalf of the Royal Academy in Berlin. It examines how the general knowledge of the correct principles of mechanics still was in those days from the fact that Papin, in calculating the efficiency of a machine, makes an error, which Huygens has to correct by reference to the principle that a system’s center of gravity cannot rise on its own. The age’s ideas about chemistry are signaled by the fact that Leibniz thought that the ethyl alcohol flame could be used for air replacement in diving bells, which Papin refutes by means of experiments. Papin is opposed to the use of strong drugs and expects much of the curative power of the organism supported by diet. He regarded the invention of the steam engine as more important than that of the transformation of metals.