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The tourbillon is a mechanical device that was invented to improve the precision of old-fashioned pocket watches. These devices continually rotate the escapement, balance wheel, and spring to compensate for bias due to gravity. A high-quality modern mechanical watch can achieve a precision, referred to as rate error in the horological community, of 1 s per day, while a less expensive mechanical watch may attain a precision of 5 s per day. To place these figures in perspective, the precision of an inexpensive quartz watch is less than .5 s per day. Measuring the precision of a mechanical timepiece is not quite as obvious at it may seem because the variation in watch speed depends on various factors, such as temperature and spatial orientation. Temperature compensation has been the subject of much thought and experimentation over the centuries. Consequently, the construction of a modern mechanical watch is such that, over a limited temperature range, the watch is insensitive to variations in ambient temperature. Variation in both accuracy and precision, that is, in watch speed and rate error, due to the orientation of the gravitational field is a characteristic of oldfashioned pocket watches.