English: Drawing of the use of a Kater's pendulum, a reversible freeswinging pendulum invented in 1817 by British captain Henry Kater as a gravimeter instrument to measure the local acceleration of gravity. The image shows Kater's original pendulum, and is probably from Kater's original 1818 paper. The pendulum had two opposing pivots near either end of the rod, consisting of short knife blades, so it could be hung from either end, with the knife blades resting on agate plates. As shown, the pendulum was swung in front of a precision pendulum clock that had been adjusted to keep correct time by the passage of stars overhead. The precise times when the two pendulums swung in synchronism were determined by looking through the sight on the left, to eliminate parallax error. The adjustment weight on the rod was adjusted until the pendulum had the same period of swing when hung from either pivot. From this period and the distance between the pivots the strength of gravity could be calculated precisely. Alterations to image: removed frame, converted to 32 greyscale PNG.