The first papr offered here was presented by Arago at the meeting of the Acadey of Scieces on February 3, 1851. In the third paper, "Sur les phénoménes d'orientation des corps tournant entraînés par un axe fixe...", Foucault presents his invention of the gyroscope, a freely spinning flywheel, which constitutes a different method of demonstrating the rotation of the Earth; he furthermore correctly predicts the use of the gyroscope as a compass and coins the word "gyroscope" (on p. 427), taken from the Greek, meaning "to look at the rotation".

Ever since Léon Foucault's public demonstration of his pendulum experiment, it has played a prominent role in physics, physics education, and the history of science. The Foucault pendulum is a long pendulum suspended high above the ground and carefully set into planar motion. The phenomenon described by Foucault concerns the orientation of the plane of oscillation of the pendulum.

"The experiment (with the pendulum) caused great excitement at the time. Heracleides had first suggested twenty-two centuries before that the earth was rotating and Copernicus had renewed the suggestion three centuries before. Since the time of Galileo two and a half centuries before, the world of scholarship had not doubted the matter. Nevertheless, all evidence as to that rotation had been indirect, and not until Foucault's experiment could the earth's rotation actually be said to have been demonstrated rather that deduced."

"Continuing to experiment on the mechanics of the earth's rotation, Foucault in 1852 invented the gyroscope, which, he showed, gave a clearer demonstration than the pendulum of the earth's rotation and had the property, similar to that of the magnetic needle, of maintaining a fixed direction. Foucault's pendulum and gyroscope had more than a popular significance (which continues to this day). First, they stimulated the development of theoretical mechanics, making relative motion and the theories of the pendulum and the gyroscope standard topics for study and investigation. Second, prior to Foucault's demonstrations the study of those motions on the earth's surface in which the deflecting force of rotation plays a prominent part (especially winds and ocean currents) was dominated by unphysical notions of how this force acted. Foucault's demonstrations and the theoretical treatments they inspired showed conclusively that this deflecting force acts in all horizontal directions, thus providing the sound physical insight on which Buys Ballot, Ferrel, Ulrich Vettin, and others could build. (DSB).

PMM: 330 lists the offprint with the title "Sur Divers Signes Sensibles du Mouvement Diurne de la Terre" - Barchas Collection, 738 (the periodical version, but only the first paper) - Dibner, No. 17 (offprint version).