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Grasshopper escapement
The grasshopper escapement is an unusual, low-friction escapement for pendulum clocks invented by British clockmaker John Harrison around 1722. An escapement, part of every mechanical clock, is the mechanism that gives the clock's pendulum periodic pushes to keep it swinging, and each swing releases the clock's gears to move forward by a fixed amount, thus moving the hands forward at a steady rate. The grasshopper escapement was used in a few regulator clocks built during Harrison's time, and a few others over the years, but has never seen wide use. The term "grasshopper" in this connection, apparently from the kicking action of the pallets, first appears in ''The Horological Journal'' in the late 19th century.
==History==
The grasshopper escapement was invented by John Harrison who used it in his regulator clocks, and also in the first three of his marine timekeepers, H1 - H3. Determining longitudinal position was a major problem in marine navigation; Newton argued that astronomical positioning could be used, but an easier theoretical possibility was to use accurate knowledge of the time at a specific base location. The difference in time between local time, which was easy to measure, and the time at base gives the difference in longitude between the base and the ship, since 24 hours of time is equivalent to 360 degrees of longitude. A large prize was offered for a solution to the problem and Harrison devoted his life to devising and building highly accurate timekeepers. Precision and friction were the main problems. Two advantages of the grasshopper escapement are the repeatability of its operation and its freedom from the need for lubrication. The repeatability of its operation is inherent in its design. One pallet is released only by the engagement of the other; the impulse given to the pendulum is thus completely regular in its timing. The lubricants available to Harrison were poor, messy and short-lived. This meant that conventional clocks had to be stopped frequently for cleaning and oiling. Using his clean and absolutely stable grasshopper escapement Harrison began a series of long-term investigations into the performance of clocks, leading to his invention of the gridiron pendulum which nullified the effects of thermal expansion and contraction due to temperature changes. The performance of his improved clocks in turn gave him an accurate, convenient standard against which to test his marine timekeepers.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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