Praying Machine, Praying Mill, or Praying Wheel,
an apparatus used in Tibet, and other parts of the East, as a mechanical aid to prayer. They are of various forms, the commonest being a cylinder or barrel of pasteboard fixed on an axle, and inscribed with prayers. The devout give the barrel a turn, and each revolution counts as an utterance of the prayer or prayers inscribed. It is common enough to see them fixed in the bed of a running stream, as they are then set in motion by the water, and go on praying night and day, to the special benefit of the person who has placed them there. The Tartars also suspend them over their domestic hearths, that they may be set in motion by the current of cool air from the opening in the tent, and so twirl for the peace and prosperity of the family.
Entry from Everybody's Cyclopedia, 1912.