Water faucets are so common that we usually do not think about them unless they stop working. But we are able to have water piped into our homes only because men have learned how water and other liquids behave. The study of the way liquids act when they are moving or standing still is called hydraulics.

   One simple fact about water that everyone knows is that water runs downhill. It can also run uphill. Imagine a container of water on a stepladder. A hose fitted in the bottom of the container runs down to the floor and then curves upward. Water will flow into the upward curve of the hose just as high as the level of water in the container. The force the water in the container exerts pushes the water up into the hose.

   Engineers look for ways to get a high water level when they are planning water systems for cities. They build reservoirs on high hills so that the level of water will be higher than the tallest buildings in town. If necessary, artificial "hills" are made by building water storage tanks on high towers. Often the water is pumped up into these reservoirs or water towers and allowed to flow down from there into the city water mains.

   Another fact about water and other liquids is that they easily transfer pressure. They can also be used to increase pressure. A pipe filled with water and with rubber stoppers in each end will show how liquids transfer pressure. If one stopper is forced inward, the pressure on the water will force out the stopper at the other end. If one end of the pipe were larger and had a stopper ten times as big as the stopper on the other end, the pipe would then be a machine for increasing pressure. A force of five pounds exerted on the small stopper would be increased to ten times as much, or 50 pounds, on the large stopper.

   The ability of liquids to transfer pressure is used in automobile jacks. These jacks contain tanks of oil with a small stopper— called a piston—and a large one. When a man works the small piston, his force is greatly multiplied at the large piston. This piston raises the car. Hydraulic brake systems on cars are tubes filled with liquids and fitted with pistons.

   Canals, flood control systems, and hydro¬electric power systems are other uses of man's knowledge of hydraulics. This ancient science is being put to many useful tasks in modern life.