Another window pulley is made of metal like that in a clock spring (Fig. 127). The spring is drawn out when the window is lowered, and the weight of the window is just enough to hold it, so very little force is needed to raise the window, as the spring is pulling on it, too. =229. Check Valves.= Check valves are made to prevent doors from slamming. They are used in offices and public buildings, and, occasionally, in homes (Fig. 128). One kind contains glycerine and castor oil, which move from one compartment to another as the door is opened and slowly flow back as a spring pulls the door shut. [Illustration: FIG. 128. Check valve.] The other kind is operated by compressed air and a spring. The air causes the steady action of the door stop. Another type of pneumatic hinge is attached to a door which is hung so that it would naturally swing shut. When the door is opened, the air is exhausted from part of the hinge. After it has been opened, the slow equalization of the air inside the door stop and outside allows the door to close slowly without slamming. [Illustration: FIG. 129. Door holder.] =230. Door Fastener.= A door fastener (Fig. 129) is a small device which has a strong spring on the inside. When the spring is released, it pushes down on a rod which is capped with rubber. When down, this comes in contact with the floor and holds the door in place. To change the position of the door, a small lever is used to lift the rod and compress the spring, thus releasing the door stop from contact with the floor. =231. Window Shades.= Window shades are equipped with a spring in one end of the roller to aid in raising it. At the end of the spring is a flat bar which is held in position by the bracket on which the shade is hung. Small catches hold the curtain when it is at the desired position (Fig. 130). If the spring becomes weak, draw the curtain down. This compresses the spring. Stop so that the clamps always fall into place to hold it. Then remove the curtain from the brackets and roll it up by hand. Place it back on the brackets. It can then be raised or lowered as wanted, and will work with more power. Take care when doing this not to wind the spring so tight that it will draw the curtain clear around the roller, thus letting the spring unwind or breaking the spring. [Illustration: FIG. 130. Spring in curtain roller.] [Illustration: FIG. 131. Hinge.] =232. Hinges.= There are some hinges which should be of interest to women. These are the ones for doors which swing only one way, and for those which swing both out and in (Fig. 131).