An instrument to determine from the basket whether a balloon is ascending of descending relative to the surrounding air.

<instrument> An instrument which shows the direction of the wind; a wind vane; a weathercock; usually applied to a contrivance consisting of a vane above, connected in the building with a dial or index with pointers to show the changes of the wind. Origin: Gr. Wind + -scope: cf. F. Anemoscope. (01 Mar 1998)