English: Pendulums used in Mendenhall gravimeter apparatus, from 1897 scientific journal. The portable Mendenhall apparatus, developed in 1890 by Thomas C. Mendenhall, provided the most accurate relative measurements of the local gravitational field of the Earth. The gravimeters were used at 340 "stations" in the US, and also in Germany, Holland, Java, the Philippine Islands, Cuba, Haiti, the Bahamas, the Dominican Republic, Panama, Canada, and Mexico. The swing of the pendulum was more regular than the most accurate clocks of the era, and as the "world's best clock" it was used in the 1920s by A. A. Michaelson to time his measurements of the speed of light at Mt. Wilson, CA. The pendulums are made of a copper-aluminum alloy, weighing 1200 g, 24.8 cm long from the agate suspension surface to center of bob. In use they were suspended from an agate knife edge in a vacuum tank. The period was picked off by reflecting a light beam from a small mirror at the top of the pendulum. The lines in the image are aliasing artifacts from scanning the original halftone image. Captions:

18.Dummy temperature pendulum with integral thermometer,
19.Pendulum and knife-edge plate, showing mode of suspension,
20.Leveling pendulum,
21.Half-second pendulum, front view,

22.Half-second pendulum, side view.