The origin and diffusion of the cross-bar wheel?

Abstract

The cross-bar wheel has never been completely legitimized in the history of draught, being alternately declared in direct line of descent or merely collateral. This wheel (PL. IXa and FIG. 7) is characterized by a single diametric bar, thick enough to accommodate the nave, and by two slender ‘cross-bars’ on either side of the nave which traverse the central bar at right angles to it, and the ends of which are mortised into the felloe. There may alternatively be four (or more) shorter cross-bars in similar relation to the central bar, but with their inner ends mortised into it.