Another contrivance for the same purpose as the former, but acting not by a weight, but by means of the smoke or rarefied air passing up the chimney, which striking against a set of oblique vanes, fixed to a vertical spindle, causes it to revolve with great rapidity. Upon the spindle is fixed a small bevilled wheel working into another small wheel placed upon a horizontal axis, which has a screw cut upon the other end of it; this screw works into a wheel on the axis of the pulley that drives the spit: the latter is thus carried slowly round. The vanes should be placed in the narrow part of the chimney, where the motion of the smoke is swiftest, and should occupy nearly the whole space, so as to intercept the greatest part of the current.