Every now and then a mill is listed on a tax assessment return by the name of tub mill. A tub mill was a form of grist mill. The difference between a tub mill and an ordinary grist mill was in the method of operation. An ordinary grist mill was powered by a large wheel consisting of a series of vanes or paddles. Water fed by a wooden trough either struck the paddles at the top of the wheel (i.e. overshot) or at the bottom of the wheel (i.e. undershot). The water striking the paddles caused the wheel to turn. The turning waterwheel, by a series of gears, caused a vertical shaft to turn. The vertical shaft was connected to the grind stones, which likewise turned on each other, grinding the grain between them.
In a tub mill, the vanes were attached to the bottom of a tub which held the grindstones. The tub would be positioned at the base of a waterfall. The splashing water would hit the vanes and turn the tub and therefore the grindstones directly. Tub mills tended to be very small constructions due to the manner of their operating. Although there was no need for gearing in a tub mill, it meant that the grindstones turned only as fast as the speed of the water hitting the vanes. In an ordinary grist mill, the gearing could by so constructed as to increase the speed of the vertical shaft.
