‘In the month of September 1852, Mr. Foucault presented to the Academy of Sciences another physical demonstration of the Earth's rotational momentum. Founded not only on the fixity of the rotation plane of a body freely suspended by its center of gravity and turning around one of its principal axes, Mr. Foucault called this apparatus a gyroscope. In this apparatus, there is a perfectly defined fixed plane, under which the Earth rotates normally: thus, the observer, moving with the Earth, can see, as in the pendulum experiment, the plane we speak of moving from east to west……’

Provenance: according to the vendor the letter came from an archive that belonged to an English family in the mid-19th century. Francois Arago published a book called 'Astronomie Populaire' in 1856, and used engravings of Foucault's drawings in the text. A member of this family acquired the drawings and sent them with a letter and transcription of the relevant part of Arago's book

Foucaults Gyroscope - The Foucault gyroscope was a gyroscope created in 1852 by French physicist Léon Foucault, it was conceived as a follow-up experiment to his pendulum in order to further demonstrate the Earth's rotation

Following the perceived misinterpretation of his pendulum experiment, Foucault sought to develop an apparatus featuring a "body freely suspended by its center of gravity and rotating around one of its principal axes." This design allowed for the examination of a plane with "absolute directional stability." The remarkable mechanical precision of Foucault's gyroscope unequivocally demonstrated this concept to the scientific community, garnering widespread recognition for the instrument.

Collaborating with Paul-Gustave Froment, Foucault constructed an apparatus characterized by an inner gimbal balanced on knife-edge bearings within an outer gimbal. The outer gimbal, in turn, was suspended by a fine, torsion-free thread, ensuring that the lower pivot point bore minimal weight.

Before positioning the gyroscope, it was spun at a speed of 9,000–12,000 revolutions per minute using a gear arrangement. This provided sufficient time to balance the gyroscope and conduct 10 minutes of experimentation. Observations could be made using either a microscope equipped with a tenth of a degree scale or a long pointer.