The name is derived from the use of a torsion pendulum, which consists of a heavy bob suspended by a thin strip of metal or wire, having the property of combining a long period of vibration with a low energy requirement for maintenance of oscillation when the bob is rotated about the axis of the supporting wire. Robert Leslie of London patented a torsional pendulum in 1793 (patent no. 1970, dated 13 December). However, the torsion clock is generally attributed to the American Aaron D. Crane, who in 1829 and 1841 received patents for a single-ball torsion-pendulum weight-operated timepiece. This was made for eight-day, 30-day and one-year durations from a single winding and was produced in relatively small quantities, c. 1845, by the J. R. Mills Co. , whose New York office was known as The Year Clock Co. In 1855 Crane was granted another patent for a torsion pendulum having 'two or more weights or balls', which was said to be temperature-compensated. After 1857 Crane moved to Boston where the one-year clock may have been made in limited quantities until his death in 1860. Silas Burnham Terry received a patent for a torsion balance in 1852, probably for a marine clock. This was adapted to a cheap 30-hour spring movement in either a miniature sharp Gothic or marine wall octagon case. The escapement was a conventional recoil attached to a thin ribbon placed in torsion through a cross-bar connecting rod, but the movement was only manufactured for a short period. Anton Harder in Germany appears to have devised his torsion pendulum clock independently c. 1879. It went for 400 days, but some models are only of one month duration. The final development of the torsion pendulum clock is the Atmos clock, deriving its power from temperature changes and therefore of unlimited duration. The earliest torsion pendulum clocks are very rare, but modern ones have been manufactured in hundreds of thousands.