How did Charles Sanders Peirce contribute to measuring the shape of Earth?

Charles Sanders Peirce contributed to the determination of the Earth's shape primarily through his work with the U.S. Coast and Geodetic Survey. In 1871, he was tasked with supervising a project aimed at a more precise determination of the Earth’s ellipticity, which was necessitated by a cross-continental triangulation project connecting the Atlantic and Pacific coasts.1

To achieve this, Peirce advanced the theory and practice of pendulum swinging as a method for measuring the force of gravity. Between 1873 and 1886, he conducted extensive pendulum experiments at approximately 20 stations across the United States and Europe, as well as in the Canadian Arctic. His research required highly accurate measurements of length, which led him to perform a pioneering determination of the length of the metre based on a wavelength of light.1