Pendulums of 1-meter and of 1/2-meter distance between the knife edges were constructed from Defforges' design by Brunner Brothers in Paris (fig. 21). These Defforges pendulums were cylindrical in form with hemispherical ends like the Peirce pendulums, and were hung on knives that projected from the sides of the pendulum, as in some unfinished Gautier pendulums designed by Peirce in 1883 in Paris. [Illustration: Figure 21.--REVERSIBLE PENDULUM APPARATUS of Defforges, as constructed by Brunner, Paris, about 1887. The clock and telescope used to observe coincidences are not shown. The telescope shown is part of an interferometer used to measure flexure of the support. One mirror of the interferometer is attached to the pendulum support; the other to the separate masonry pillar at the left.] [Illustration: Figure 22.--BECAUSE OF THE GREATER SIMPLICITY of its use, the invariable pendulum superseded the convertible pendulum towards the end of the 19th century, except at various national base stations (Kew, Paris, Potsdam, Washington, D.C., etc.). Shown here are, right to left, a pendulum of the type used by Peirce at the Hoosac Tunnel in 1873-74, the Mendenhall 1/2-second pendulum of 1890, and the pendulum designed by Peirce in 1881-1882.] [Illustration: Figure 23.--THE OVERALL SIZE of portable pendulum apparatus was greatly reduced with the introduction of this 1/2-second apparatus in 1887, by the Austrian military officer, Robert von Sterneck. Used with a vacuum chamber not shown here, the apparatus is only about 2 feet high. Coincidences are observed by the reflection of a periodic electric spark in two mirrors, one on the support and the other on the pendulum itself.] [Illustration: Figure 24.--THOMAS C. MENDENHALL (1841-1924). Although largely self-educated, he became the first professor of physics and mechanics at the Ohio Agricultural and Mechanical College (later Ohio State University), and was subsequently connected with several other universities. In 1878, while teaching at the Tokyo Imperial University in Japan, he made gravity measurements between Tokyo and Fujiyama from which he calculated the mean density of the earth. While superintendent of the U.S. Coast and Geodetic Survey, 1889-94, he developed the pendulum apparatus which bears his name.] Von Sterneck and Mendenhall Pendulums