10. What is the shape of the knives on a lawn mower that makes it cut the same as a pair of scissors? 11. What may be the reasons for scissors not cutting as they should? 12. What are the essential features of a good incubator? 13. What is a thermostat? How does it work? Are thermostats of any use to the housewife on any other device than the incubator? 14. What mechanical factors are embodied in a typewriter? Find the pulley, the levers, the springs, etc. 15. What are the differences in a hectograph, a mimeograph and multigraph? PART X MOTORS, FUELS, AND GAS PLANTS CHAPTER XXXVII TREADLES AND WATER MOTORS =262. Definition of Motor.= A motor is a device for utilizing the power stored in gasoline, electricity or elevated water for doing work. The structure of the motor depends upon the source of its power, as does its name. Besides the motor, there is a treadle, or foot-power motor, used in the home. [Illustration: FIG. 149. Water motor.] =263. The Treadle.= The treadle is a small platform, which rocks on two pivots. As the treadle is rocked, it moves a rod attached to its outer edge, upward and downward. This rod is then attached to a wheel a short distance from the hub, so that the upward and downward motion of the shaft turns the wheel. When a belt is attached to the wheel, it will run a sewing machine or other small device. =264. Water Motors.= Water motors are commonly used in the household on washing machines and pumps (Figs. 149 and 149-_a_.) At least twenty-five pounds of water pressure is required to run an average-size washer. More pressure is advantageous. The motor may be, and often is, attached to tanks in which water is held under pressure, and used to pump water from a cistern or well. [Illustration: FIG. 149-_a_. "Reliable" water motor.] =265. Selecting a Water Motor.= Before purchasing any device to be operated by a water motor, ascertain how much water pressure you have available. Under enough pressure, the water from a faucet will give power enough to a small-sized water motor to run a washing machine, sewing machine or small feed grinders. These motors are usually less than one-half horse power. [Illustration: FIG. 150. Sectional view of water motor.] [Illustration: FIG. 150-_a_. Water motor assembled and in parts.] =266. Two Types of Water Motors.= One type of water motor is made up of a piston and valves in a cylinder (Fig. 150). The water pushes the piston to a certain point when a valve opens and lets out the water. The piston then moves backward until it automatically opens another valve, letting in more water, which, in turn, pushes the piston forward and again to the point where the first valve opens. The motion of the piston must be strong enough to do the work. About twenty-five pounds of water pressure is required in moving the piston forward when attached to a machine which might be operated by hand by a woman.