The Assayer of Galileo Galilei is a classic of Italian literature The Assayer was written in the context of the discussion on comets, and responds, word by word, to the Libra astronomica ac philosophica firmed by Lotario Sarsi, but written by Orazio Grassi. From the formal (i.e. rhetorical) point of view the Assayer is an example of the judicial, defensive speech. Nevertheless, in the book one can notice the presence of the epideictic speech, as well. The epideictic speech prises the methodological values proper to the Copernican vision of the universe, and blaming these involved in the Aristotelian and Ptolemaic approaches. There is in the sixth paragraph of the Assayer a very famous group of four rhetorical periods in which one can notice the presence of the new interpretation of the antique metaphor of the book. The rhetorical exercise developed in the present essay tries to entangle the complex node of this metaphor and her new interpretation. It seems that for Galilei the metaphor in question is not a trope, but passes to the topica. In the end some considerations on dialectics in Galilei are also proposed.