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Translated for the Journal of the Franklin Institute

Physical Demonstration of the Rotation of the Earth by Means of the Pendulum

By M.L. Foucault

I owe to the kindness of M. Arago, and the intelligent zeal of our skilful instrument maker, M. Froment, who actively assisted me in the execution of this work, the means of being able already to reproduce the experiment on a larger scale. Profiting by the height of the Salle de la Meridienne, at the Observatory, I have been able to give my pendulum a length of 11 metre, (36 feet.) The oscillations become more slow and larger, so that between two consecutive returns of the pendulum to the fixed mark, a deviation to the left was very manifest.^

Foucault's Paper
In conclusion I will present on further remark:
It is, that the facts observed under these circumstances, accord perfectly with the results announced by Poisson in a very remarkable memoir, read by him before the Academy, 14th November, 1837. In this memoir, Poisoon, treating of the motion of projectiles in the air, and taking into consideration the diurnal movement of the earth shows, by calculation that in our latitude, projectiles thrown towards any point, experience a deviation which takes place constantly towards the right of the observer, standing at the point of departure and looking towards the trajectory. It appears to me that the mass of the pendulum may be compared to the projectile, which deviates towards the right while departing from the observer, and necessarily in the opposite direction in returning towards its mean plane of oscillation, and indicates its direction.. But the pendulum possesses the advantage of accumulating the effects, and allowing them to pass from the domain of theory into that observation.

Comptes Rendus de l'Acad. de Sciences de Paris. 3 Fevrier, 1851.

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