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  1. A device used in experimental meteorology for viewing the relative motions in a rotating fluid system.

    It consists of a Dove prism (Hecht 1974) mounted in a rotating barrel and aligned along the axis of rotation of the vessel containing the fluid. Since the image in the prism rotates twice during each revolution of the prism itself, the rotating vessel will appear stationary if the rate of rotation of the barrel is one-half the rotation of the vessel, and in the correct sense.

  2. An instrument that measures the flow velocity of a gas through a tube.

    It consists of a lightweight piston, externally threaded, that fits loosely inside a vertically mounted glass tube. The gas flows in at the bottom of the tube, lifting the piston and imparting a rotary motion to it as air flows through the threaded section. The height to which the piston rises is a measure of the flow velocity of the particular gas.

    Hecht, E. 1974. Geometric Optics. Addison–Wesley Publishing Co., . 167–168.

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