Barker's milln.

A mechanical contrivance for producing rotary motion, consisting of a hollow vertical revolving axis with two (or more) horizontal arms fitted into its lower end; water, admitted at the top of the central tube, fills that and the arms, and by its discharge through lateral apertures near the ends of the latter causes the whole machine to revolve in the direction opposite to that of the discharge, the moving power being the excess of hydrostatic pressure on the sides of the arms opposite to the openings, aided by centrifugal force.