The Perpetuum Mobile invented by Petrus Peregrinus: A Reconstruction
At the conclusion of his Epistula de magnete of 1269, Petrus Peregrinus de Maricourt described the construction of a continually moving toothed wheel that is powered by a magnet. The effectiveness of the device is enhanced by a small rounded weight made of brass or silver that is located between two of the theeth. According to the inventor, this metallic ball "seeks the center of the earth in virtue of its own weight, thereby aiding the motion of the teeth." In the secondary literature and in nearly all translations and reprints of the Peregrinus letter, this perpetuum mobile is illustrated by a picture that stems from the printed edition published by Achilles Gasser in 1558. This drawing is in evident contradiction with the text, and it is different from a variety of drawings of the same device that can be found in the manuscripts. In this paper, the different drawings of Petrus' machine are compared to the text as it is given in the recently published critical edition of the Peregrinus letter by Loris Sturlese (1995). With reference to other medieval perpetua mobilia, it is shown how the surviving illustrations of this construction must be slightly modified in order to represent a perpetual motion that is in accordance with the text and with the physical ideas of the 13th century.