windsail

noun

1. A wide tube or funnel of canvas, used to convey a stream of air for ventilation into the lower compartments of a vessel.

   “We were ventilated by a big canvas windsail, opening between the fore and main masts, and running down into the hold.”

2. The sail or vane of a windmill.

   “The dome in turning carries with it the windsails, M N, and their axle, P Q ; and thus the latter may be made to coincide with the direction of the wind, or the plane in which the radii of the sials turn may be made perpendicular to that direction.”