A spring brake to wear against the iron tire of the wheel is the most satisfactory. The brake may be tripped and set against the tire automatically by a small lever and weight attached to the underside of the wheel. When the speed is too fast the weight swings out and sets the brake. When the speed slackens the weight drops back towards the center and releases the brake. When the speed is about right the weight swings between the two spring catches. BULL TREADMILL On dairy farms it is common to see a valuable pure bred bull working a treadmill for exercise and to pump water. Sometimes he turns the cream-separator, but the motion is too unsteady for good results. Treadmills for this purpose are very simple. The mechanism turns a grooved pulley which propels a rope power conveyor. The rope belt may be carried across the yards in any direction and to almost any distance. Bull treadmills consist of a framework of wood which carries an endless apron supported on rollers. The apron link chains pass around and turn two drumhead sprocket-wheels at the upper end and an idler drum at the lower end. The sprocket-wheel drum shaft is geared to an auxiliary shaft which carries a grooved pulley. A rope belt power conveyor runs in this groove and carries power from the bull pen to the pump. Bull tread powers usually have smooth inclined lags, because a bull's steps on the tread power are naturally uneven and irregular. This construction gives an even straight tread to the travel surface. To prevent slipping, soft wooden strips are nailed onto the lags at the lower edges. Even incline tread blocks or lags are also recommended for horses that are not shod and for all animals with split hoofs. The traveling apron of the power is placed on an incline and the treads are carried around the two drums at the upper and lower ends of the frame by means of endless chains. There is a governor attachment which regulates the speed and prevents the machinery from "running away." [Illustration: Figure 107.--Bull Tread Power. Treadmills have gone out of fashion. Too much friction was the cause, but a mill like this is valuable to exercise a pure bred bull. Some dairymen make him pump water.]