=138. Force Pumps.= Force pumps are used on deep wells and in forcing water into storage tanks. They should be kept oiled; they should be operated with an even stroke, and the packing in them should be renewed if they leak air. In force pumps, the valves differ in their arrangement from suction pumps (Fig. 69). =139. Compressed-Air Pumps.= Compressed-air pumps consist of a tank for storing the compressed air--a pump to force air into the tank and cylinders equipped with valves. These act automatically. Whenever an outlet pipe is opened, the extra pressure of air from the storage tank raises the water from the well or cistern (Fig. 70). Air should be kept in the pressure tank. [Illustration: FIG. 70. Compressed-air pump system.] When this arrangement is used, open and close faucets slowly, not with a jerk. Fig. 70-_a_ shows plumbing where such a system is used. =140. Water Filters.= Water filters are devices for straining minute particles out of water. They are made of sand, charcoal or porcelain, kisselguhr and other materials. They are without value unless they are kept clean. A dirty filter is worse than none. Almost the only way to clean them is to sterilize them or put new material in them. Only with expert care can filters be made effective for removing disease germs. A dirty filter may prove a menace. Filters are valuable for removing coarse dirt from the water. [Illustration: FIG. 70-_a_. System of plumbing with compressed-air tank.] Filters on faucets should be cleaned or renewed every day. Large filters for rain water should be renewed every few months. CHAPTER XX PRESSURE TANKS; PLUMBING FIXTURES =141. Pressure Tanks.= A pressure tank is a device for storing water under pressure. It is usually placed in the basement of dwelling houses. =142. Construction of the Pressure Tank.= The tank is tight and strong, so that it will hold air and water under pressure. The tank originally has some air in it. When the water is pumped in, the air not being able to escape, is compressed. When there is a chance for water to escape from the tank which is connected to water pipes, the pressure of the compressed air on the water forces it to upstairs rooms and other points. To this tank is attached a pressure gage which indicates the amount of pressure; or, in other words, the amount of water in the tank, for when the water gets low, the pressure is reduced unless the air has escaped. A glass gage shows the height of water. Provision is made to let some air into the tank, for otherwise it may in time be all forced out of the tank or absorbed by the water. The water in a pressure tank may be used to pump water from a cistern into another tank. =143. Care of Pressure Tanks.= A pressure tank must not be pumped up to the extent that the pressure becomes greater than the strength of the tank. A safety valve is used in controlling the pressure.