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chimney cap, bonnet
1.An abacus or cornice forming a crowning termination of a chimney.
2. A rotary device, moved by the wind, which facilitates the escape of smoke by turning the exit aperture away from the wind, preventing the entry of rain or snow and improving the draft.
3. A chimney hood.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.