Another form of governor used extensively on stationary engines controls the speed by increasing or decreasing the cut-off. Thus with a heavy load the cut-off may occur at ½ the stroke while with a very light load it may be at 1/10 stroke. This is by far the most sensitive and economical form of governor, but on account of the reverse gear it is difficult to apply it on a tractor. (151) Reverse Gear. As explained under “Cylinders” the travel of the valve bears a definite relation to the piston position so that the ports may be opened and closed at the proper times. It may be shown by a rather complicated diagram that this relation of the valve together with that of the eccentric that drives it is only correct for one direction of rotation. For any other direction of rotation the relation of the valve and piston position must be changed. This may be done in several ways but the most common types are the Stevenson Link and the Wolff slotted yoke. The Stevenson link motion used on the majority of engines, consists of two independent eccentrics, one being fixed in the relation for forward motion and the other for the reverse direction. The ends of the eccentric rods leading from these eccentrics are connected by a slotted bar or link, in which a block is placed that is connected with the valve rod. The block is free to slide in the slot of the links, that is, it may be moved from one end of the slot to the other. When it is desired to have the engine rotate in a right handed direction, for example, the link is lowered so that the rod from the forward eccentric is brought directly in line with the block so that this eccentric alone acts directly on the valve through the valve stem. When the reverse is desired the link is raised until the rod from the reverse eccentric is brought in line with the block and valve stem, drive being by the reverse eccentric.