French physicist (1819-1868) best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation; he also made an early measurement of the speed of light, discovered eddy currents, and is credited with naming the gyroscope. ALS in French, signed "L. Foucault," one page, 5 x 7.5, July 23, 1850. In part (translated): "It seems to me that the weather is sufficiently firm so that we can fix to Thursday the 25th of July the meeting you made me the honor of asking for you and for the pupils of the Ecole Normale. If you see the sun shining the day after tomorrow around 2 you can come to my home on Rue d’Assas no. 34 with the certitude of confirming the experience that Mr. Pouillet has described so well to you." In fine condition, with a bit of light toning beneath the signature. An interesting letter from the year before he proved the Earth's diurnal motion with his famous pendulum, asking a fellow scientist to confirm experimental observations in good weather conditions.
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